Search

Seed retention, and ultimately seed shatter, are extremely important for the efficacy of harvest weed seed control (HWSC) and are likely influenced by various agroecological and environmental factors. Field studies investigated seed-shattering phenology of 22 weed species across three soybean [Glycine max (L.) Merr.]-producing regions in the United States. We further evaluated the potential drivers of seed shatter in terms of weather conditions, growing degree days, and plant biomass. Based on the results, weather conditions had no consistent impact on weed seed shatter. However, there was a positive correlation between individual weed plant biomass and delayed weed seed–shattering rates during harvest. This work demonstrates that HWSC can potentially reduce weed seedbank inputs of plants that have escaped early-season management practices and retained seed through harvest. However, smaller individuals of plants within the same population that shatter seed before harvest pose a risk of escaping early-season management and HWSC.

Potential effectiveness of harvest weed seed control (HWSC) systems depends upon seed shatter of the target weed species at crop maturity, enabling its collection and processing at crop harvest. However, seed retention likely is influenced by agroecological and environmental factors. In 2016 and 2017, we assessed seed-shatter phenology in 13 economically important broadleaf weed species in soybean [Glycine max (L.) Merr.] from crop physiological maturity to 4 wk after physiological maturity at multiple sites spread across 14 states in the southern, northern, and mid-Atlantic United States. Greater proportions of seeds were retained by weeds in southern latitudes and shatter rate increased at northern latitudes. Amaranthus spp. seed shatter was low (0% to 2%), whereas shatter varied widely in common ragweed (Ambrosia artemisiifolia L.) (2% to 90%) over the weeks following soybean physiological maturity. Overall, the broadleaf species studied shattered less than 10% of their seeds by soybean harvest. Our results suggest that some of the broadleaf species with greater seed retention rates in the weeks following soybean physiological maturity may be good candidates for HWSC. U.S. Department of Agriculture, Agricultural Research Service Areawide program; HATCH Program of the National Institute of Food and Agriculture; U.S. Department of AgricultureUnited States Department of Agriculture (USDA) Published version We would like to thank the staff and students at each university for helping conduct this research, specifically Kreshnik Bejleri, Sheri Heard, John Sanders, Barbara Scott, Annie Klodd, Zach Schaefer, Russ Garetson, Vitor Damiao, Matheus Martins, Camille Werner, Bruno Flaibam, and Camila Grassmann, and each institute's research and experimental stations. The authors would also like to thank the U.S. Department of Agriculture, Agricultural Research Service Areawide program for funding and the HATCH Program of the National Institute of Food and Agriculture and the U.S. Department of Agriculture for providing partial funding. No conflicts of interest have been declared.

Understanding how plants alter their growth in response to interplant competition is an overlooked but complex problem. Previous studies have characterized the effect of light and water stress on soybean or common ragweed growth in monoculture, but no study has characterized soybean and common ragweed growth in mixture. A field study was conducted in 2015 and 2016 at the University of Nebraska-Lincoln to characterize the growth response of soybean and common ragweed with different irrigation levels and intraspecific and interspecific interference. The experiment was arranged in a split-plot design with irrigation level (0, 50%, 100% replacement of simulated evapotranspiration) as the main plot and common ragweed density (0, 2, 6, 12 plants m-1 row) as the subplot. Crop- and weed-free controls and three mixture treatments were included as subplots. Periodic destructive samples of leaf area and biomass of different organ groups were collected, and leaf area index (LAI), aboveground biomass partitioning, specific leaf area (SLA), and leaf area ratio (LAR) were calculated. Additionally, soybean and common ragweed yield were harvested, and 100-seed weight and seed production were determined. Soybean did not alter biomass partitioning, SLA, or LAR in mixture with common ragweed. Soybean LAI, biomass, and seed size were affected by increasing common ragweed density. Conversely, common ragweed partitioned less new biomass to leaves and increased SLA in response to increased interference. Common ragweed LAI, biomass, and seed number were reduced by the presence of soybean and increasing common ragweed density; however, seed weight was not affected. Results show that adjustment in biomass partitioning, SLA, and LAR is not the method that soybean uses to remain plastic under competition for light. Common ragweed demonstrated plasticity in both biomass partitioning and SLA, indicating an ability to maintain productivity under intra- and inter-specific competition for light or soil resources.Nomenclature: Common ragweed; Ambrosia artemisiifolia L.; soybean; Glycine max L. Merr.

Although sorghum [Sorghum bicolor(L.) Moench ssp.bicolor] is the fifth most important grain crop in terms of global production, no commercial hybrids carry genetically engineered (GE) traits for resistance to insect pests or herbicides due to regulatory concerns about gene flow to weedy relatives. However, non-GE herbicide resistance currently is being developed in grain sorghum and will likely transfer to related weeds. Monitoring the impact of this new nuclear technology on the evolution and invasiveness of related weeds requires a baseline understanding of the population biology of grain sorghum genes once they transfer to in situ weed populations. We previously characterized the rate of gene flow from grain sorghum to shattercane [Sorghum bicolor(L.) Moench nothosubsp.drummondii(Steud.) de Wet ex. Davidse], a conspecific weed relatively common in North America; as well as the ecological fitness of an F1population whenS. bicolornothosubsp.drummondiiwas the maternal parent. Here we report the ecological fitness of aS. bicolornothosubsp.drummondii×S. bicolorssp.bicolorF2population relative to its crop and weed parents. Parental and F2populations were grown in two Nebraska environments in 2012 and 2013. Traits evaluated included overwinter survival, field emergence, biomass production and partitioning at anthesis, total seed production, and 100-seed weight. Results indicated that F2traits were generally intermediate between the parents, but more similar toS. bicolornothosubsp.drummondiithan to grain sorghum. The one exception was overwinter survival, which was nearly 0% for both the F2and the grain sorghum parent in these northern environments. Thus, the frequency of crop alleles stably introgressed intoS. bicolornothosubsp.drummondiipopulations appears to primarily depend on overwinter survival of the F2and which selective pressures are imposed upon it by the cropping system. These data provide needed baseline information about the environmental fate of nuclear genetic technologies deployed in this important global crop.

Conservation agriculture (CA) is characterized by zero or minimum tillage, permanent land cover with crop residue or plant growth, and crop rotation. Only a few studies have addressed the impact of soil texture on relative CA:CP productivity, and none have simultaneously evaluated soil texture interactions with optimal residue retention rate. We simulated the interaction effects of soil texture and residue retention on crop yield for CA and conventional practices (CP) for a monsoon-season maize [Zea mays L.] and dry-season wheat [Triticum aestivum L.] double-cropping system in the midhills of Nepal. The simulation used 35 years of historical weather data, fifteen soil profile descriptions representing a range of soil texture classes, and variable rates of residue retention. Crop models were calibrated with field data and simulation results evaluated against measured field data from other published studies. Optimal maize residue retention rate was 60 % and optimal wheat residue retention rate was 10 %, regardless of soil texture. At optimal residue retention, CA maize yielded 0.37 Mg ha−1 less than CP on clayey soils and 0.25 Mg ha−1 less than CP on loam soils, equivalent to 5 % and 4 % yield loss, respectively. However, mean wheat yield was greater with CA than CP for all soil textures, with an average yield increase of 0.17 Mg ha−1, or 18 % increase in wheat yield. The time required for CA to produce equivalent maize yields to CP increased by approximately 1.23 yr per % of soil clay. Possible non-linearities exist in the relationship between soil clay and time-to-equivalent maize yield. Wheat yield was more stable, but maize yield less stable, with CA compared with CP for all soil texture categories.

Cover crop (CC) species selection can contribute to reducing soil penetration resistance (brassica species), improved soil nitrogen (N) cycling (legume species), and suppression of weeds (grass species). However, one of the main concerns about including CCs in water-limited environments is soil water use and the consequences to subsequent crops. To determine the effects of individual CC species under water-limited environments, we evaluated fall and spring CC biomass produced, and soil water and N content, penetration resistance, weed density and biomass during the maize growing season, and maize grain yield. The experiment was conducted under a winter wheat-maize-fallow rotation at two locations (North Platte and Grant, NE) during 2016−2017 and 2017−2018 (four site-years). Treatments consisted of seven popular CC species plus a control (fallow), planted after winter wheat harvest. Spring oats, Siberian kale, and purple top turnip produced greater fall biomass, while cereal rye produced the greatest amount of spring biomass. However, cereal rye reduced soil volumetric water content in North Platte 2016−2017 and increased soil penetration resistance from 20–30 cm soil depth across site-years likely due to soil water use. Spring cover crop growth suppressed weeds early in the maize growing season. Due to its aboveground biomass production, cereal rye decreased weed density and biomass by 80 and 88 %, respectively, compared to the fallow treatment. On the other hand, except for brassicas, CCs decreased N levels in the soil during maize growing season, and all CC species reduced maize grain yield up to 30 % compared to fallow (except spring oats). Spring oats can be an alternative to cereal rye as CC species for semi-arid regions. However, since CCs did not promote any maize yield gain, our findings suggest that producers should use caution when incorporating CCs in their cropping systems in water-limited environments. This research provides valuable information on the potential impact of CCs on rainfed maize production, as well as help producers and agronomists develop better CC management programs for cropping systems in semi-arid regions.

Despite the potential to increase soil organic matter, cycle soil nutrients, and suppress weeds, there is a concern that cover crops (CCs) soil water use negatively impacts subsequent crops in water-limited environments. Cover crop management practices such as planting and termination timing may mitigate the detrimental impacts of CCs in semi-arid cropping systems. To determine the effects of CCs under water-limited environments, we evaluated the total CC biomass produced in the fall and spring, soil water content during the subsequent maize growing season, weed density and biomass, crop residue, and soil nutrients at the maize V6 development stage, and maize productivity. The experiment was conducted under a wheat-maize-fallow rotation at two sites (Grant and North Platte, NE) during 2016−2017 and 2017−2018. Treatments consisted of three planting timings after winter wheat harvest and three CC termination timings and fallow (no CCs) before maize establishment. Planting CCs shortly after winter wheat harvest increased CC biomass in the fall and early spring compared to late planting. Cover crops terminated early in the spring reduced weed biomass (−70 %), while CCs terminated late in the spring reduced both weed density (−56 %) and biomass (−82 %) compared to fallow. Due to above-normal precipitation during the experiment, soil water measured at 0−20 cm soil depth was impacted by CCs only at Grant 2016−2017. Cover crops terminated late in the spring increased crop residue biomass but decreased total nitrogen at 0−10 cm soil depth by 17 %, and decreased soil nitrate at the 10−20 cm soil depth up to 26 % compared to fallow. Cover crops planted late in the summer (August) and terminated late in the spring (May) had the most detrimental impact on maize grain yield. Late termination of CCs in the spring reduced corn grain yield by up to 20 % compared to fallow. Despite enhanced weed suppression and crop residue, CCs decreased soil nitrogen and maize grain yield, especially when terminated late in the spring. Producers in semi-arid regions of the Great Plains willing to incorporate CCs should use caution when selecting management strategies for their CCs to minimize maize grain yield and economic losses.

The DSSAT (Decision Support System for Agrotechnology Transfer) is the most widely used model package to characterize growth, development, yield, and N uptake of multiple crop species. The objectives of this study were: 1) to evaluate the performance of the maize (Zea mays L.) models CSM–CERES and CSM-IXIM available in DSSAT version 4.5, when simulating high yield conditions, 2) to test the IXIM model with an alternative approach to estimate crop N demand, based on Plenet and Lemaire (2000), and 3) with this alternative approach, examine some options to simulate grain N concentration. The two models were evaluated with data collected from two experimental fields in Almacelles, Spain, during three consecutive years under various N management treatments, combining fertilization and residue handling. Fertilization treatments included two doses of mineral fertilizer: 300 kg N ha−1 (N300), along with a N-free fertilized control (N0). Crop residues were either removed (R) or incorporated (I). The grain yields obtained in the fields (14% moisture) varied, depending on the N fertilization, from 11 to 20 Mg ha−1. In our high yielding irrigated maize conditions, both models were able to simulate grain yield and biomass accurately, with RRMSE of 9.5 and 11.4% for CERES and 14.9 and 14.3% for IXIM respectively. Estimations of N uptake were also accurate, with RRMSE of 12.0 and 8.6% for CERES and IXIM. The IXIM model with the alternative approach to estimate crop N demand simulated grain yield and crop N uptake better than the IXIM with the current approach based on Jones (1983). The RMSE was reduced by 22% for yield and by 55% for biomass, while the simulated N uptake reduced the RMSE by 12%. With this alternative approach, the best grain N simulations were obtained with a modification of the Ciampitti and Vyn (2013) function.

Spring tillage is a component of an integrated weed management strategy for control of early emerging glyphosate-resistant weeds such as common ragweed; however, the effect of tillage on common ragweed emergence pattern is unknown. The objectives of this study were to evaluate whether spring tillage during emergence would influence the emergence pattern or stimulate additional emergence of common ragweed and to characterize common ragweed emergence in southeast Nebraska. A field experiment was conducted for three years (2014 to 2016) in Gage County, Nebraska in a field naturally infested with glyphosate-resistant common ragweed. Treatments consisted of a no-tillage control and three spring tillage timings. The Soil Temperature and Moisture Model (STM2) software was used to estimate soil temperature and moisture at a 2-cm depth. The Weibull function was fit to total common ragweed emergence (%) with day of year (DOY), thermal time, and hydrothermal time as independent variables. Tillage treatments and year had no effect on total common ragweed emergence (P = 0.88 and 0.35, respectively) and time to 10, 25, 50, 75, and 90% emergence (P = 0.31). However, emergence pattern was affected by year (P =

Summary Glyphosate-resistant Ambrosia trifida is a competitive and difficult-to-control annual broad-leaved weed in several agronomic crops in the Midwestern United States and Ontario, Canada. The objectives of this study were to compare treatments for control of glyphosate-resistant A. trifida with tillage followed by pre-emergence (PRE) and/or post-emergence (POST) herbicides in glyphosate-resistant maize and to determine the impact of A. trifida escapes on maize yield. Field experiments were conducted in 2013 and 2014 in grower fields infested with glyphosate-resistant A. trifida. Tillage prior to maize sowing resulted in 80–85% control compared with no tillage. Tillage followed by PRE application of saflufenacil plus dimethenamid-P with or without atrazine resulted in 99% control compared with ≤86 and 96% control with PRE herbicides alone at 7 and 21 days after application respectively. Tillage or POST-only herbicides resulted in 4–14 A. trifida plants m−2, whereas a PRE and POST programme had

Traditional breeding has been used to develop grain sorghum germplasm that is tolerant to acetolactate synthase (ALS)-inhibiting herbicides (Inzen Technology, DuPont). Inzen sorghum carries a double mutation in the ALS gene (Val560Ile and Trp574Leu), which confers high level of tolerance to ALS-inhibiting herbicides. Overreliance on ALS-inhibiting herbicides for weed control during the 1990s resulted in the evolution of ALS inhibitor–resistant shattercane populations in Nebraska. According to a survey conducted in 2013, ALS inhibitor–resistant weedySorghumpopulations persist in Nebraska. The objectives of this research were to determine whether the ALS mutations present in Inzen sorghum were present in the ALS inhibitor–resistant shattercane and johnsongrass populations detected in Nebraska and northern Kansas, and whether these populations evolved ALS resistance independently. Primers specific to the Val560and Trp574region of the ALS gene were used to screen the populations with PCR. The Trp574Leu mutation was present in one ALS inhibitor–resistant johnsongrass population. The Val560Ile was detected in three ALS inhibitor–resistant shattercane, one susceptible shattercane, one ALS inhibitor–resistant johnsongrass, and one susceptible johnsongrass population. Moreover, Val560Ile was present in resistant and/or susceptible individuals within johnsongrass and shattercane populations that were segregating for ALS resistance, indicating that by itself the Val560Ile mutation does not confer resistance to ALS-inhibiting herbicides. None of the populations presented both mutations simultaneously, as does Inzen sorghum. A shattercane population containing the Ser653Thr mutation was also detected. This research indicates that the ALS mutations present in Inzen sorghum already exist individually in weedy sorghum populations. Moreover, our results present strong evidence that ALS resistance in these populations evolved independently. Thus, widespread overreliance on ALS-inhibiting herbicides prior to adoption of glyphosate-tolerant crops in the 1990s exerted sufficient selective pressure on shattercane and johnsongrass populations for resistance to evolve multiple times in the Midwest. Finally, a survey of the 5′ portion of the ALS gene in more diverse wild and weedySorghumspecies was hampered by limited coverage in genomic resequencing surveys, suggesting that refined PCR-based methods will be needed to assess SNP variation in this gene region, which includes the Ala122, Pro197, and Ala205codons known to confer ALS resistance in other species.

Traditional breeding technology is currently being used to develop grain sorghum [ Sorghum bicolor (L.) Moench ssp. bicolor ] germplasm that will be tolerant to acetolactate synthase (ALS)-inhibiting herbicides. This technology (Inzen™, DuPont™) has the potential to improve sorghum production by allowing for the postemergence control of traditionally hard-to-control grasses. However, grain sorghum and shattercane [weedy Sorghum species; Sorghum bicolor (L.) Moench ssp. drummondii (Nees ex Steud.) de Wet ex Davidse] can interbreed and introduced traits such as herbicide tolerance could increase the weediness of the weedy relative. Our objective was to develop a simulation model to assess management options to mitigate risks of ALS-resistance evolution in shattercane populations in US sorghum production areas. Assuming a single major gene confers resistance and gene frequencies change according to the Hardy-Weinberg ratios we constructed a stage-structured (seedbank, plants) matrix model with annual time steps. The model explicitly considered gene flow from Inzen plants to shattercane populations. The management strategies considered in the model were: a) continuous sorghum, b) sorghum followed by ( fb ) soybeans and c) sorghum fb fallow fb winter wheat, where postemergence ALS-inhibiting herbicides were only used in Inzen years. During sorghum years two options were tested: continuous Inzen and Inzen fb conventional sorghum, for a total of six management strategies. The parameter values used in the model were obtained from our research, the literature, and expert opinion. For each management strategy we ran deterministic and stochastic simulations (with stochastic levels of herbicide efficacy). The time for resistance evolution was predicted to decrease with increased cropping system complexity (more crop diversity than continuous production of Inzen). Evolution of resistance was predicted to occur rapidly if Inzen sorghum is planted continuously because of high selection pressure (ALS-inhibiting herbicide application) and crop-to-weed gene flow. Rotating Inzen with conventional sorghum did not assist with shattercane management. Rotating Inzen with non-sorghum crops where effective herbicide options are available assisted with keeping shattercane density at low levels while postponing resistance evolution to some extent. Crop and herbicide rotation will be key strategies for shattercane management in Inzen sorghum.

The supply of soil resources is critical for the establishment and long-term competitive success of a plant species. Although there is considerable research on the effects of water supply on crop growth and productivity, there is little published research on the comparative response of crops and weeds to limiting soil water supply. The objective of this research was to determine the growth and transpiration efficiency of corn and velvetleaf at three levels of water supply. One corn or velvetleaf plant was grown in a large pot lined with plastic bags. When seedlings reached 10 cm, bags were sealed around the base of the plant, so the only water loss was from transpiration. Daily transpiration was measured by weighing the pots at the same time each day. The experiment was conducted in the fall of 2007 and in the spring of 2008. Four replicates of each species–water treatment were harvested periodically to determine biomass accumulation and leaf area. The relationship between cumulative aboveground biomass and water transpired was described using a linear function in which the slope defined the transpiration efficiency (TE). Corn TE was greater than velvetleaf TE in all treatments during both trials. In the fall trial, corn TE was 6.3 g kg–1, 47% greater than that of velvetleaf TE. In the spring trial, TEs of both species were lower overall, and corn TE increased with declining water supply. Corn produced more biomass and leaf area than velvetleaf did at all water-supply levels. Velvetleaf partitioned more biomass to roots compared with shoots during early growth than corn did. The ability of corn to generate more leaf area and its investment in a greater proportion of biomass into root growth at all levels of water supply may enable it to more-effectively avoid velvetleaf interference under all levels of soil-water supply.

Common waterhemp is one of the most commonly encountered and troublesome weeds in the midwestern United States. It is well known that water stress adversely affects crop growth and yield; however, the effects of water stress on weed growth and seed production are poorly understood. The objective of this study was to determine the effects of degree and duration of water stress on growth, development, and fecundity of two common waterhemp biotypes in greenhouse experiments conducted at the University of Nebraska–Lincoln. No difference was observed in growth, development, and seed production between two biotypes in response to degree and duration of water stress; therefore, data were combined. The degree of water stress study included five treatments, where the amount of water applied to each pot at 2-d interval was equivalent to 100, 75, 50, 25, and 12.5% of pot (soil) water content. The highest plant height (163 cm), number of leaves (231 plant−1), and growth index (4.4 × 105cm3) were recorded at 100% of pot water content (no water stress). Similarly, aboveground biomass, total leaf area, and seed production reached their maximum at 100% of pot water content treatment, whereas they were reduced as degree of water stress increased. The study of water stress duration included five treatments, where amount of water applied to each pot at 2-, 4-, 6-, 8-, and 10-d intervals was equivalent to 100% of pot water content. The highest plant height (150 cm), number of leaves (210 plant−1), and growth index (3.8 × 105cm3) were observed at 2-d interval of water stress, whereas seed production was similar at 2-d (36,549 seeds plant−1) and 4-d (34,176 seeds plant−1) intervals. This study shows that common waterhemp has capacity to survive and reproduce even under a higher degree and duration of water stress.

The diversity-productivity, diversity-invasibility, and diversity-stability hypotheses propose that increasing species diversity should lead, respectively, to increased average biomass productivity, increased invasion resistance, and increased stability. We tested these three hypotheses in the context of cover crop mixtures, evaluating the effects of increasing cover crop mixture diversity on aboveground biomass, weed suppression, and biomass stability. Twenty to forty cover crop treatments were replicated three or four times at eleven sites using eighteen species representing three cover crop species each from six pre-defined functional groups: cool-season grasses, cool-season legumes, cool-season brassicas, warm-season grasses, warm-season legumes, and warm-season broadleaves. Each species was planted in monoculture, and the most diverse treatment contained all eighteen species. Remaining treatments included treatments representing intermediate levels of cover crop species and functional richness and a no cover crop control. Cover crop planting dates ranged from late July to late September with both cover crop and weed aboveground biomass being sampled prior to winterkill. Stability was assessed by evaluating the variability in cover crop biomass for each treatment across plots within each site. While increasing cover crop mixture diversity was associated with increased average aboveground biomass, this was the result of the average biomass of the monocultures being drawn down by low yielding species rather than due to niche complementarity or increased resource use efficiency. At no site did the highest yielding mixture out-yield the highest yielding monoculture. Furthermore, while increases in cover crop mixture diversity were correlated with increases in weed suppression and increases in biomass stability, we argue that this was largely the result of diversity co-varying with aboveground biomass, and that differences in aboveground biomass rather than differences in diversity drove the differences observed in weed suppression and stability. The results of this study contradict popular interpretations of the diversity-productivity, diversity-invasibility, and diversity-stability hypotheses.

321 Grain sorghum is ranked as the fi ft h most important cereal crop in the world aft er wheat (Triticum aestivum L.), rice (Oryza sativa L.), corn (Zea mays L.), and barley (Hordeum vulgare L.), and is the third most common cereal planted in the United States, trailing corn and wheat (Defelice, 2006; USDA-NASS, 2015). Sorghum is a warm season C4 grass species that is highly effi cient in the conversion of solar energy and use of water. Sorghums are cultivated throughout the world for food, fodder, syrup, and biofuel production. In the United States, the crop is primarily used for livestock feed and is ranked second aft er corn for ethanol production (Paterson, 2008). In spite of the agronomic potential and food value of grain sorghum, the land area of sorghum production has declined in many parts of the United States (USDA-NASS, 2015), in part because the number of options for weed management in sorghum is limited. Most post-emergence (POST) herbicides labeled for grain sorghum are eff ective on broadleaf weed species but have only limited activity on annual grasses. Consequently, pre-emergence (PRE) herbicides are the primary option for annual grass control in grain sorghum (Hennigh et al., 2010). However, grain sorghum is oft en grown in dry environments and the absence of adequate soil moisture oft en reduces the activation and effi cacy of PRE herbicide treatments (Hennigh et al., 2010). Acetolactate synthase-inhibiting herbicides, also known as acetohydroxyacid synthase (AHAS)-inhibitors, are commonly used to control grass weeds in certain broadleaf and grass crops (Hennigh et al., 2010). However, conventional grain sorghum is susceptible to ALS-herbicides that have grass activity. Th e ALS-inhibiting herbicides are eff ective in small quantities (grams of active ingredient per hectare), allow for wide application windows, have high safety margins on labeled crops, are eff ective on a broad spectrum of weed species, typically possess soil residual activity, and have low mammalian toxicities (Tranel and Wright, 2002). However, there are at least eight known mutation sites in the ALS gene that confer resistance Pest Interactions in Agronomic Systems

2449 Enhancing ecosystem services of current cropping systems is a priority for sustaining crop and livestock production, developing biofuel industries, and maintaining or improving soil and environmental quality. Integrating CCs with existing cropping systems has the potential to enhance ecosystem services such as: (i) food, feed, fi ber, and fuel production, (ii) C and other nutrient and water cycling, and (iii) soil, water, and air quality improvement. Th is is particularly important with increased concerns about the following challenges to agriculture: high production costs, environmental degradation, food security, and climate change. According to the Soil Science Society of America Glossary of Terms, CCs are defi ned as a “close-growing crop that provides soil protection, seeding protection, and soil improvement between periods of normal crop production, or between trees in orchards and vines in vineyards. When plowed under and incorporated into the soil, CCs may be referred to as green manure crops” (SSSA, 2008). While the use of CCs is not a new concept, the implications of their re-emerging importance and impacts on ecosystem services such as crop and livestock production and soil and environmental quality deserve further discussion. Historically, CCs have been used to meet a few specifi c needs (i.e., soil conservation, N2 fi xation, and weed and pest management), but now CC management questions increasingly revolve around the potential multi-functionality of CCs including soil C sequestration, mitigation of greenhouse gas emissions, benefi ts to “soil health,” feed for livestock, biofuel production, farm economics, and others. Th ere are many studies on CCs assessing soil and crop production, but few have attempted to discuss or integrate all the multiple ecosystem services that CCs provide (Dabney et al., 2001; Snapp et al., 2005). Th us, a summarization of the existing knowledge about potential multiple CC benefi ts is needed for a broader understanding of CC impacts on soil and agricultural production and identifi cation of knowledge gaps that deserve further research. Th is summarization will help answer the following question: Can CCs provide multiple ecosystem services to address the current challenges in soil and environmental quality, crop and livestock production, biofuel production, among others? review & interpretation

Soybean cyst nematode (SCN) is a major soybean yield–limiting disease in the United States. Henbit, a winter annual species common to no-till fields in the midwestern United States, is known to act as an alternative host for SCN. A simulation was performed to estimate how likely SCN was to reproduce on henbit roots during a 30-yr period in two important soybean production areas of Nebraska. Simulations were conducted using published information on henbit seedling emergence, SCN reproduction on henbit roots, and SCN response to soil temperature. Results indicate that SCN would be able to complete one generation on henbit roots under Nebraska conditions. The SCN reproductive cycle was not likely to be completed before the winter in south central Nebraska, but one SCN generation was predicted to be completed in the fall in 2 out of 30 simulation years (7% likelihood) in southeast Nebraska. Based on our predictions, to reduce the chances of SCN population build-up in the absence of its main host (soybean), weed management in fields infested with both henbit and SCN should be completed after crop harvest in the fall when most henbit seedlings have emerged and are growing but the SCN developing on henbit roots have not yet achieved full maturity in Nebraska.

Glyphosate-resistant common waterhemp is a difficult-to-control annual broadleaf weed that has become a serious management challenge for growers in Nebraska and other states in the United States. The objectives of this study were to confirm glyphosate-resistant common waterhemp in Nebraska by quantifying level of resistance in a dose-response study, and to determine the sensitivity and efficacy of POST soybean herbicides for controlling suspected glyphosate-resistant common waterhemp biotypes. Seeds of suspected glyphosate-resistant common waterhemp biotypes were collected from seven eastern Nebraska counties. Greenhouse dose-response experiments were conducted to evaluate the response of common waterhemp biotypes to nine rates of glyphosate (0 to 16×). Common waterhemp biotypes were 3- to 39-fold resistant to glyphosate depending on the biotype being investigated and the susceptible biotype used for comparison. Results of the POST soybean herbicides efficacy experiment suggested that glyphosate-resistant biotypes, except a biotype from Pawnee County, had reduced sensitivity to acetolactate synthase (ALS)–inhibiting herbicides (chlorimuron-ethyl, imazamox, imazaquin, imazethapyr, and thifensulfuron-methyl). Glufosinate and protoporphyrinogen oxidase (PPO)–inhibiting herbicides (acifluorfen, fluthiacet-methyl, fomesafen, and lactofen) provided ≥ 80% control of glyphosate-resistant common waterhemp at 21 d after treatment (DAT). This study confirmed the first occurrence of glyphosate-resistant common waterhemp in Nebraska, and also revealed reduced sensitivity to ALS-inhibiting herbicides in most of the biotypes tested in this study.

Common ragweed emerges early in the season in Nebraska, USA and is competitive with soybean; therefore, preplant herbicides are important for effective control. Glyphosate has been used as a preplant control option; however, confirmation of glyphosate-resistant (GR) common ragweed in Nebraska necessitates evaluating other herbicide options. The objectives of this study were to (1) evaluate the efficacy of preplant (PP) herbicides followed by (fb) glufosinate alone or in tank-mixture with imazethapyr, acetochlor, or S-metolachlor applied post-emergence (POST) for control of GR common ragweed in glufosinate-resistant soybean; (2) their effect on common ragweed density, biomass, and soybean yield; and (3) the partial economics of herbicide programs. A field experiment was conducted in a grower’s field infested with GR common ragweed in Gage County, Nebraska, USA in 2015 and 2016. Preplant herbicide programs containing glufosinate, paraquat, 2,4-D, dimethenamid-P, cloransulam-methyl, or high rates of flumioxazin plus chlorimuron-ethyl provided 90 to 99% control of common ragweed at 21 d after treatment (DAT). The aforementioned PP herbicides fb a POST application of glufosinate alone or in tank-mixture with imazethapyr, acetochlor, or S-metolachlor controlled GR common ragweed 84 to 98% at soybean harvest, reduced common ragweed density (≤ 20 plants m─2) and biomass by ≥ 93%, and secured soybean yield 1,819 to 2,158 kg ha─1. The PP fb POST herbicide programs resulted in the highest gross profit margins (US$373 to US$506) compared to PP alone (US$91) or PRE fb POST programs (US$158). The results of this study conclude that effective and economical control of GR common ragweed in glufosinate-resistant soybean is achievable with PP fb POST herbicide programs.

Glyphosate-resistant giant ragweed is one of the most competitive weeds of agronomic crops in the United States. Early emergence and rapid growth rate makes giant ragweed a competitive weed early in the season and reduces crop yields. Therefore, early spring control of giant ragweed using a preplant herbicide is critical. Glufosinate is an alternative POST herbicide for weed control in glufosinate-resistant soybean. Field experiments were conducted at David City, NE, in 2012 and 2013 to evaluate the efficacy of preplant herbicides followed by glufosinate applied alone or in tank mixes for control of glyphosate-resistant giant ragweed in glufosinate-resistant soybean. Preplant treatments containing 2,4-D, flumioxazin, glufosinate, paraquat, saflufenacil, and sulfentrazone provided 79 to 99% control of giant ragweed 21 d after treatment (DAT), and subsequent application of glufosinate alone or in tank mixes resulted in 90 to 99% control at 21 DAT. Preplant application ofS-metolachlor plus metribuzin or chlorimuron, flumioxazin plus thifensulfuron followed by glufosinate resulted in < 40% control of giant ragweed, and soybean yields were < 870 kg ha−1. Although statistically comparable to several other treatments, preplant application of 2,4-D or saflufenacil tank mixes followed by glufosinate resulted in the highest level of control (> 97%) and soybean yield (2,624 to 3,378 kg ha−1). This study confirms that preplant herbicide options are available for control of glyphosate-resistant giant ragweed, and a follow-up application of glufosinate will provide season-long control in glufosinate-resistant soybean.

First- and second-year seedbank emergence of 23 summer annual weed species common to U.S. corn production systems was studied. Field experiments were conducted between 1996 and 1999 at the Iowa State University Johnson Farm in Story County, Iowa. In the fall of 1996 and again in 1997, 1,000 seeds for most species were planted in plastic crates. Seedling emergence was counted weekly for a 2-yr period following seed burial (starting in early spring). Soil temperature at 2 cm depth was estimated using soil temperature and moisture model software (STM2). The Weibull function was fit to cumulative emergence (%) on cumulative thermal time (TT), hydrothermal time (HTT), and day of year (DOY). To identify optimum base temperature (Tbase) and base matric potential (ψbase) for calculating TT or HTT, Tbaseand ψbasevalues ranging from 2 to 17 C and −33 to −1,500 kPa, respectively, were evaluated for each species. The search for the optimal model for each species was based on the Akaike's Information Criterion (AIC), whereas an extra penalty cost was added to HTT models. In general, fewer seedlings emerged during the first year of the first experimental run (approximately 18% across all species) than during the second experimental run (approximately 30%). However, second-year seedbank emergence was similar for both experimental runs (approximately 6%). Environmental effects may be the cause of differences in total seedling emergence among years. Based on the AIC criterion, for 17 species, the best fit of the model occurred using Tbaseranging from 2 to 15 C with four species also responding to ψbase= −750 kPa. For six species, a simple model using DOY resulted in the best fit. Adding penalty costs to AIC calculation allowed us to compare TT and HTT when both models behaved similarly. Using a constant Tbase, species were plotted and classified as early-, middle-, and late-emerging species, resulting in a practical tool for forecasting time of emergence. The results of this research provide robust information on the prediction of the time of summer annual weed emergence, which can be used to schedule weed and crop management.

Predictions of weed emergence can be used by practitioners to schedule POST weed management operations. Common sunflower seed from Kansas was used at six Midwestern U.S. sites to examine the variability that 16 climates had on common sunflower emergence. Nonlinear mixed effects models, using a flexible sigmoidal Weibull function that included thermal time, hydrothermal time, and a modified hydrothermal time (with accumulation starting from January 1 of each year), were developed to describe the emergence data. An iterative method was used to select an optimal base temperature (Tb) and base and ceiling soil matric potentials (ψband ψc) that resulted in a best-fit regional model. The most parsimonious model, based on Akaike's information criterion (AIC), resulted when Tb= 4.4 C, and ψb= −20000 kPa. Deviations among model fits for individual site years indicated a negative relationship (r= −0.75; P < 0.001) between the duration of seedling emergence and growing degree days (Tb= 10 C) from October (fall planting) to March. Thus, seeds exposed to warmer conditions from fall burial to spring emergence had longer emergence periods.

Winter annual weeds are becoming prolific in agricultural fields in the midwestern United States. The objectives of this research were to understand the roles of soil temperature (daily average and fluctuation) and moisture on the emergence of nine winter annual weed species and dandelion and to develop predictive models for weed emergence based on the accumulation of modified thermal/hydrothermal time (mHTT). Experiments were established at Lincoln, NE; Mead, NE; and at two sites (irrigated and rainfed) near Clay Center, NE, in 2010 and 2011. In July of each year, 1,000 seeds of each species were planted in 15 by 20 by 6-cm mesh baskets installed between soybean rows. Soil temperature and water content were recorded at the 2-cm depth. Emerged seedlings were counted and removed from the baskets on a weekly basis until no additional emergence was observed in the fall, resumed in late winter, and continued until emergence ceased in late spring. Weather data were used to accumulatemHTT beginning on August 1. A Weibull function was selected to fit cumulative emergence (%) on cumulativemHTT (seven base temperature [Tbase] by six base water potential [Ψbase] by three base temperature fluctuation [Fbase] candidate threshold values = 126 models); it was also fit to days after August 1 (DAA1), for a total of 127 candidate models per species. The search for optimal base thresholds was based on the theoretic-model comparison approach (Akaike information criterion [AIC]). All three components (Tbase, Ψbase, andFbase) were only important for Virginia pepperweed. For downy brome and purslane speedwell, includingTbaseand Ψbaseresulted in the best fit, whereas for dandelion includingTbaseandFbaseresulted in the best fit. A model including onlyTbaseresulted in the best fit for most species included in this study (Carolina foxtail, shepherd's-purse, pinnate tansymustard, henbit, and field pansy). For field pennycress, the model based on DAA1 resulted in the best fit. Threshold values were species specific. Soil temperature was the major environmental factor influencing winter annual weed emergence. Even though soil moisture and often temperature fluctuation are essential for seed germination, ΨbaseandFbasewere not as critical in the predictive models as initially expected. Most seedlings (> 90%) of downy brome, pinnate tansymustard, Carolina foxtail, henbit, and field pansy emerged during the fall. Virginia pepperweed, purslane speedwell, dandelion, shepherd's-purse, and field pennycress seedlings emerged during both fall and spring. The results of this research provide robust information on the prediction of the time of winter annual weed emergence, which can help growers make better management decisions.

Cover crops have traditionally been used to reduce soil erosion and build soil quality, but more recently cover crops are being used as an effective tool in organic weed management. Many studies have demonstrated microbial community response to individual cover crop species, but the effects of mixed species cover crop communities have received less attention. Moreover, the relationship between arable weeds and soil microbial communities is not well understood. The objective of this study was to determine the relative influence of cover crop diversity, early-season weed communities, and tillage on soil microbial community structure in an organic cropping system through the extraction of fatty acid methyl esters (FAMEs). A field experiment was conducted between 2009 and 2011 near Mead, NE where spring-sown mixtures of zero (control), two, and eight cover crop species were included in a sunflower–soybean–corn crop rotation. A mixture of four weed species was planted in all experimental units (excluding the no-cover control), and also included as an individual treatment. Cover crops and weeds were planted in late-March, then terminated in late-May using a field disk or sweep plow undercutter, and main crops were planted within one week of termination. Three (2009) or four (2010–11) soil cores were taken to a depth of 20 cm in all experimental units at 45, 32, and 25 days following cover crop termination in 2009, 2010, and 2011, respectively. Total FAMEs pooled across 2009 and 2010 were greatest in the two species mixture–undercutter treatment combination (140.8 ± 3.9 nmol g −1 ) followed by the eight species mixture–undercutter treatment combination (132.4 ± 3.9 nmol g −1 ). Abundance of five (2009 and 2010) and seventeen (2011) FAME biomarkers was reduced in the weedy treatment relative to both cover-cropped treatments and the no-cover control. In 2009 and 2010, termination with the undercutter reduced abundance of most actinomycete biomarkers while termination with the field disk reduced abundance of C18:1( cis 11) and iC16:0. Canonical discriminant analysis of the microbial community successfully segregated most cover crop mixture by termination method treatment combinations in 2009 and 2010. Microbial communities were most strongly influenced by the presence and type of early-spring plant communities, as weeds exerted a strong negative influence on abundance of many key microbial biomarkers, including the AMF markers C16:1( cis 11) and C18:1( cis 11). Weeds may alter soil microbial community structure as a means of increasing competitive success in arable soils, but this relationship requires further investigation.

Robust predictions of weed seedling emergence from the soil seedbank are needed to aid weed management. A common seed accession (Illinois) of giant ragweed was buried in replicate experimental gardens over 18 site years in Illinois, Michigan, Kansas, Nebraska, Ohio, and South Dakota to examine the importance of site and climate variability by year on seedling emergence. In a nonlinear mixed-effects modeling approach, we used a flexible sigmoidal function (Weibull) to model giant ragweed cumulative seedling emergence in relation to hydrothermal time accumulated in each site-year. An iterative search method across a range of base temperature (Tb) and base and ceiling soil matric potentials (ψband ψc) for accumulation of hydrothermal time identified optima (Tb= 4.4 C, ψb= −2,500 kPa, ψc= 0 kPa) that resulted in a parsimonious regional model. Deviations between the fits for individual site-years and the fixed effects regional model were characterized by a negative relationship between random effects for the shape parameterlrc(natural log of the rate constant, indicating the speed at which emergence progressed) and thermal time (base 10 C) during the seed burial period October through March (r= −0.51, P = 0.03). One possible implication of this result is that cold winter temperatures are required to break dormancy in giant ragweed seeds. By taking advantage of advances in statistical computing approaches, development of robust regional models now is possible for explaining arable weed seedling emergence progress across wide regions.

Cultivated sorghum [ (L.) Moench subsp. ] can interbreed with a feral weedy relative shattercane [ nothosubsp. (Steud.) de Wet ex Davidse]. Traits introduced from cultivated sorghum could contribute to the invasiveness of a shattercane population. An experiment was conducted to determine the potential for pollen-mediated gene flow from grain sorghum to shattercane. Shattercane with juicy midrib () was planted in soybean [ (L.) Merr.] fields during 2 yr in concentric arcs at varying distances from a 0.39 ha sorghum pollen source with dry midrib (). The arcs were placed so that prevailing winds would carry sorghum pollen to the shattercane. Seven hundred twenty seeds from each of over 300 shattercane panicles in anthesis during sorghum pollen shed each year were collected. Progeny were evaluated by phenotype to determine rate of hybridization. Hybridization averaged 3.6% within the source in 2008 and 16.0% in 2009 and declined as distance increased. Hybridization as high as 2.6% for an individual panicle was measured at the farthest distance evaluated (200 m). Wind direction and speed were also measured and their product affected hybridization rate for all pollination periods. Results indicate that genes from cultivated sorghum will likely be introduced into shattercane populations at distances of at least 200 m and that rate of hybridization is dependent on weather factors such as wind. Source size is also important in determining hybridization rate but was not studied here.

Cover crops can provide many benefits in agroecosystems, including the opportunity for improved weed control. However, the weed suppressive potential of cover crops may depend on the species (or mixture of species) chosen, and the method of cover crop termination and residue management. The objective of this study was to determine the effects of cover crop mixture and mechanical termination method on weed biomass and density, and relative crop yield in an organic cropping system. A field experiment was conducted from 2009 to 2011 near Mead, NE, where spring-sown mixtures of two, four, six, and eight cover crop species were included in a sunflower–soybean–corn crop rotation. Cover crops were planted in late March, terminated in late May using a field disk or sweep plow undercutter, and main crops were planted within 1 wk of termination. Terminating cover crops with the undercutter consistently reduced early-season grass weed biomass, whereas termination with the field disk typically stimulated grass weed biomass relative to a no cover crop control (NC). The effects of cover crop mixture were not evident in 2009, but the combination of the undercutter and the eight-species mixture reduced early-season weed biomass by 48% relative to the NC treatment in 2010. Cover crops provided less weed control in 2011, where only the combination of the undercutter and the two-species mixture reduced weed biomass (by 31%) relative to the NC treatment. Termination with the undercutter resulted in relative yield increases of 16.6 and 22.7% in corn and soybean, respectively. In contrast, termination with the field disk resulted in a relative yield reduction of 13.6% in soybean. The dominant influence of termination method highlights the importance of appropriate cover crop residue management in maximizing potential agronomic benefits associated with cover crops.

Soybean cyst nematode (SCN) is the most yield-limiting pathogen of soybean in the United States. Henbit is a prevalent winter annual weed species in no-till fields and is reported to be an alternative host of SCN. A greenhouse study was conducted to evaluate how the development of SCN on henbit roots was affected by herbicide mode of action and time of herbicide application. Henbit plants were grown in watertight pots placed in a water bath bench that kept soil temperature constant (27 ± 1 C) during the study. Ten d after transplanting, pots were inoculated with approximately 1,000 SCN eggs. At 7, 14, or 21 d after inoculation (DAI), henbit plants were sprayed with recommended dose of either glyphosate (870 g ae ha−1) or 2,4-D (1,070 g ae ha−1). The experiment was arranged in a randomized complete block design with five replications per treatment, and two experimental runs separated in time. At 28 DAI, the total number of SCN cysts and eggs, and plant shoot and root dry weight per pot were determined. Henbit root and shoot biomass increased as the time of herbicide application was delayed. Glyphosate reduced root biomass more than 2,4-D, but no differences in shoot biomass were detected. The number of SCN cysts per henbit plant and eggs per cyst increased as the herbicide application was delayed from 7 to 21 DAI. Glyphosate reduced the number of cysts found on henbit roots more than 2,4-D, especially at earlier application times. On plants treated with glyphosate, SCN-females produced only half the number of eggs of SCN-females on henbit plants treated with 2,4-D, regardless of time of application. These results indicate that early control of henbit plants, especially with glyphosate, can reduce SCN reproduction potential in SCN infested fields.

A class of phenolic compounds, ortho-dihydroxyphenols (hereafter “o-DHP”), has been implicated with seed survival. Based on expectations of the growth-differentiation balance hypothesis, we predicted that seed o-DHP concentration exhibits a curvilinear response to increasing resource availability in the maternal environment, with maximum o-DHP occurring at moderate resource levels. To test this hypothesis, Abutilon theophrasti seeds were produced under field conditions at two locations. Each location included twelve maternal environments established through factorial combinations of soil compost (+/-), species assemblage (A. theophrasti with and without maize), and soil nitrogen fertilizer (0, 0.5× or 1× local recommendations for maize). Resource availability with respect to A. theophrasti growth was summarized by above-ground biomass at seed harvest (maternal biomass). Results indicated that seed o-DHP concentrations increased then decreased in response to increasing maternal biomass. This relationship was modeled with a unimodal function specific to location (Location 1, y = 1.18 + 0.03xe-0.02x, pseudo-R2 = 0.59, p = 0.003; Location 2, y = 1.40 + 0.006xe-0.005x; pseudo-R2 = 0.34, p = 0.05). Seed protein concentrations remained constant across maternal biomass levels. Because inherent vulnerability to predation and decay is considered a consequence of chemical protection relative to nutritional offering, our results suggest that A. theophrasti seed susceptibility to lethal attack is influenced by resource levels in the maternal environment. More broadly, our results suggest that the growth-differentiation balance hypothesis can be extended to maternal effects on seed phenolics.

Knowledge of environmental factors influencing demography of weed species will improve understanding of current and future weed invasions. The objective of this study was to quantify regional-scale variation in vital rates of giant ragweed and common sunflower. To accomplish this objective, a common field experiment was conducted across seven sites between 2006 and 2008 throughout the north central U.S. maize belt. Demographic parameters of both weed species were measured in intra- and interspecific competitive environments, and environmental data were collected within site-years. Site was the strongest predictor of belowground vital rates (summer and winter seed survival and seedling recruitment), indicating sensitivity to local abiotic conditions. However, biotic factors influenced aboveground vital rates (seedling survival and fecundity). Partial least squares regression (PLSR) indicated that demography of both species was most strongly influenced by thermal time and precipitation. The first PLSR components, both characterized by thermal time, explained 63.2% and 77.0% of variation in the demography of giant ragweed and common sunflower, respectively; the second PLSR components, both characterized by precipitation, explained 18.3% and 8.5% of variation, respectively. The influence of temperature and precipitation is important in understanding the population dynamics and potential distribution of these species in response to climate change.

Published in Agron. J. 104:1425–1435 (2012) Posted online 1 Aug. 2012 doi:10.2134/agronj2012.0185 Copyright © 2012 by the American Society of Agronomy, 5585 Guilford Road, Madison, WI 53711. All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. C crops have been shown to provide many environmental and agronomic services within agroecosystems. These include reduced soil erosion, increased biological diversity (e.g., microbes, insects, and birds), increased nutrient cycling and biological N2 fixation, increased soil organic matter, improved weed control, and increased crop yield (Pimentel et al., 1992; Pimentel et al., 1995; Sainju and Singh, 1997; Williams et al., 1998; Altieri, 1999; Reddy et al., 2003; Teasdale et al., 2007). While cover crops have traditionally been used as a soil conservation tool (Pimentel et al., 1995), there is increasing interest in using cover crops to enhance agronomic crop performance. However, maximizing agronomic benefits associated with cover crops will depend on appropriate species choice and residue management (Ashford and Reeves, 2003; Wortman et al., 2012). Selecting a single species is often popular among farmers due to the ease of planting, uniform development, and predictable termination efficacy of the cover crop (Creamer et al., 1995; Mirsky et al., 2009). However, multi-species mixtures may increase productivity, stability, resilience, and resource-use efficiency of the cover crop community (Tilman, 1996; Tilman et al., 1997, 2001; Trenbath, 1999; Wortman et al., 2012). Despite the demonstrated benefits, on-farm adoption remains limited due to farmer concerns about the potential cost and management implications of cover crop use. One of the top concerns among farmers is the amount of soil water used by cover crops, potentially reducing available soil moisture for the cash crop. During seasons with average and above-average rainfall conditions, differences in available soil moisture among cover crop species and mixtures are often undetectable. However, when cover crop productivity is high and precipitation becomes limiting, species can differ greatly in their effects on soil moisture (Unger and Vigil, 1998; Daniel et al., 1999). While transpiration demands will undoubtedly vary among species, the method of cover crop termination and residue management may have a greater impact on available soil moisture during main crop growth. Daniel et al. (1999) found that volumetric soil moisture (%) was increased by as much as 2.4% to a depth of 61 cm when cover crops were terminated with herbicides in a no-till system compared to conventional termination with a field disk. Soil water savings associated with no-till practices have been well documented (Blevins et al., 1983; De Vita et al., 2007), but the additional benefits of cover crop residue in a conservation tillage system are not as clear. Liebl et al. (1992) found that transpiration reduced available soil moisture during dry periods, but following no-till termination cover crop residue conserved soil moisture relative to a no-till system without cover crops. Given that the driest portion of the growing season in the western Corn Belt typically occurs after cover crop growth (i.e., June–August), potential soil moisture savings offered by the residue (post-termination) throughout the growing season may negate moisture deficits observed during cover crop growth. AbstrAct

Weed-suppressive soils contain naturally occurring microorganisms that suppress a weed by inhibiting its growth, development, and reproductive potential. Increased knowledge of microbe–weed interactions in such soils could lead to the identification of management practices that create or enhance soil suppressiveness to weeds. Velvetleaf death and growth suppression was observed in a research field (fieldA) that was planted with high populations of velvetleaf, which may have developed via microbial mediated plant–soil feedback. Greenhouse studies were conducted with soil collected fromfieldA(soilA) to determine if it was biologically suppressive to velvetleaf. In one study, mortality of velvetleaf grown for 8 wk insoilAwas greatest (86%) and biomass was smallest (0.3 g plant−1) in comparison to soils collected from surrounding fields with similar structure and nutrient content, indicating that suppressiveness ofsoilAwas not likely caused by physical or chemical factors. WhensoilAwas autoclaved in another study, mortality of velvetleaf plants in the heat-treated soil was reduced to 4% compared to 55% for the untreated soil, thus suggesting that suppressiveness ofsoilAis biological in nature. A third set of experiments showed that suppressiveness to velvetleaf could be transferred to an autoclaved soil by amending the autoclaved soil with untreatedsoilA; this provided additional evidence for a biological basis for the effects ofsoilA.The suppressive condition in these greenhouse experiments was associated with high soil populations of fusaria.Fusarium lateritiumwas the most frequently isolated fungus from roots of diseased velvetleaf plants collected fromfieldA, and also was the most virulent when inoculated onto velvetleaf seedlings. Results of this research indicate that velvetleaf suppression can occur naturally in the field and thatF. lateritiumis an important cause of velvetleaf mortality infieldA.

Knowledge of the soil nitrogen (N) supply and the N mineralization potential of the soil combined with an understanding of weed-crop competition in response to soil nutrient levels may be used to optimize N fertilizer rates to increase the competitive advantage of crop species. A greenhouse study (2006) and field studies (2007 to 2008) in Illinois and Nebraska were conducted to quantify the growth and interference of maize and velvetleaf in response to varying synthetic N fertilizer rates in soils with high and low N mineralization potential. Natural soils were classified as having ‘‘low mineralization potential’’ (LMP), while soils amended with composted manure were classified as having ‘‘high mineralization potential’’ (HMP). Maize and velvetleaf were grown in monoculture or in mixture in both LMP and HMP soils and fertilized with zero, medium, or full locally recommended N rate. In the greenhouse, velvetleaf interference in maize with respect to plant biomass increased as N rate increased in the HMP soil, whereas increasing N rate in the LMP soil reduced velvetleaf interference. In contrast, velvetleaf interference in maize decreased as N rate increased regardless of soil class in the field experiment. With respect to grain yield, velvetleaf interference in maize was unaffected by N rate or soil class. In both greenhouse and field experiments, velvetleaf biomass was greater in the HMP soil class, whereas maize interference in velvetleaf was generally greater in the LMP soil class. While soil N levels influenced weed-crop interference in the greenhouse, the results of the field study demonstrate the difficulty of controlling soil nutrient dynamics in the field and support a maize fertilization strategy independent of weed N use considerations. Nomenclature: Velvetleaf, Abutilon theophrasti Medic. ABUTH; maize, Zea mays L.

Weeds cause crop loss by reducing the quantity of available resources to which the crop responds. Therefore, predicting the impact of weeds on crops can only be achieved through a quantitative understanding of resource depletion by both crop and weed species and their unique response to resource supply. Soil water availability is the most important factor limiting crop yield worldwide (Begg and Turner 1976; Boyer 1982). Crop production in the Great Plains region of the United States is characterized by highly variable rainfall and, consequently, variable soil water availability (Ruiz-Barradas and Nigam 2005). A comparative understanding of crop and weed transpiration in response to water availability may provide valuable insight into the mechanisms of crop yield loss in water-limited environments and provide the background for predicting crop–weed competition for soil water. When soil water supply is not sufficient to meet plant demand, transpiration is reduced (Kropff 1993; Sadras and Milroy 1996). Kropff (1993) suggested that under waterlimiting conditions, plant growth is reduced in proportion to the reduction in whole-plant transpiration rate. Therefore, actual plant growth can be predicted as the product of potential growth under non–water-limiting conditions and the ratio of actual to potential transpiration (Ta/Tp). The response of the Ta/Tp of a species to the progressive drying of soil varies little across a wide range of conditions when expressed as a function of transpirable soil water content (FTSW; Sinclair 2005; Sinclair et al. 1998). Velvetleaf is a detrimental weed in corn, soybeans, cotton, and sorghum in the United States (Bridges 1992, Spencer 1984). Patterson and Flint (1983) found little variation in transpiration efficiency (dry-matter production/transpiration) among five C3 weeds (including velvetleaf ) and soybean. However, smooth pigweed (Amaranthus hybridus L.), a C4 species, had approximately twice the transpiration efficiency of the C3 species. Plants that exhibit C3 and C4 photosynthetic pathways differ in how they fix CO2 during photosynthesis, which subsequently affects stomatal conductance and transpiration (Monson et. al 1984). In general, the transpiration efficiency of C4 plants is twice that of C3 plants (Begg and Turner 1976). We know of no reports of the comparative water use of velvetleaf (C3) and corn, a C4 species. Thus, the objective of this research was to determine the relative transpiration of corn and velvetleaf as the plants were subjected to drought stress by limiting soil water availability.

While weed management is consistently a top priority among farmers, there is also growing concern for the conservation of biodiversity. Maintaining diverse weed communities below bioeconomic thresholds may provide ecosystem services for the crop and the surrounding ecosystem. This study was conducted to determine if weed diversity, density and biomass differ within and among organic and conventional crop rotations. In 2007 and 2008, we sampled weed communities in four long-term crop rotations near Mead, Nebraska using seedbank analyses (elutriation and greenhouse emergence) and above-ground biomass sampling. Two conventional crop rotations consisted of a corn (Zea mays) or sorghum (Sorghum bicolor)–soybean (Glycine max)–sorghum or corn–soybean sequence and a diversified corn or sorghum–sorghum or corn–soybean–wheat (Triticum aestivum) sequence. Two organic rotations consisted of an animal manure-based soybean–corn or sorghum–soybean–wheat sequence and a green manure-based alfalfa (Medicago sativa)–alfalfa–corn or sorghum–wheat sequence. Species diversity of the weed seedbank and the above-ground weed community, as determined by the Shannon diversity index, were greatest in the organic green manure rotation. Averaged across all sampling methods and years, the weed diversity index of the organic green manure rotation was 1.07, followed by the organic animal manure (0.78), diversified conventional (0.76) and conventional (0.66) rotations. The broadleaf weed seedbank density in the tillage layer of the organic animal manure rotation was 1.4×, 3.1× and 5.1× greater than the organic green manure, diversified conventional and conventional rotations, respectively. The grass weed seedbank density in the tillage layer of the organic green manure rotation was 2.0×, 6.1× and 6.4× greater than the organic animal manure, diversified conventional and conventional rotations, respectively. The above-ground weed biomass was generally greatest in the organic rotations. The broadleaf weed biomass in sorghum and wheat did not differ between organic and conventional rotations (CRs), but grass weed biomass was greater in organic compared to CRs for all crops. The above-ground weed biomass did not differ within CRs, and within organic rotations the grass weed biomass was generally greatest in the organic green manure rotation. The weed seedbank and above-ground weed communities that have accumulated in these rotations throughout the experiment suggest a need for greater management in long-term organic rotations that primarily include annual crops. However, results suggest that including a perennial forage crop in organic rotations may reduce broadleaf weed seedbank populations and increase weed diversity.

Weeds cause crop loss indirectly by reducing the quantity of resources available for growth. Quantifying the effects of weed interference on nitrogen (N) supply, crop growth, and N nutrition may assist in making both N and weed management decisions. Experiments were conducted to quantify the effect of N addition and weed interference on soil nitrate-N (NO3-N) over time and the dependence of corn growth on NO3-N availability, determine the corn N nutrition index (NNI) at anthesis, and evaluate if relative chlorophyll content can be utilized as a reliable predictor of NNI. Urea was applied at 0, 60, and 120 kg N/ha to establish N treatments. Season-long weedy, weed-free, and five weed interference treatments were established by delaying weed control from time of crop planting to the V3, V6, V9, V15, or R1 stages of corn development. Soil NO3-N ranged from 20 kg N/ha without N addition to 98 kg N/ha with 120 kg N/ha added early in the season, but crop and weed growth reduced soil NO3-N to 10 kg N/ha by corn anthesis. Weed presence reduced soil NO3-N by up to 50%. Average available NO3-N explained 29 to 40% of the variation in corn shoot mass at maturity. Weed interference reduced corn biomass and NNI by 24 to 69%. Lack of N also reduced corn NNI by 13 to 46%, but reduced corn biomass by only 11 to 23%. Nondestructive measures of relative chlorophyll content predicted corn NNI with 65 to 85% accuracy. Although weed competition for factors other than N may be the major contributor to corn biomass reduction, the chlorophyll meter was a useful diagnostic tool for assessing the overall negative effects of weeds on corn productivity. Further research could develop management practices to guide supplemental N applications in response to weed competition.

Root morphology will affect interplant competition for soil nutrients. Research was conducted to assess fine root fraction, mean root radius, specific root length, root length density, and nitrogen (N) uptake per unit fine root biomass of corn and velvetleaf over time and in response to nitrogen supply. Pots containing either corn or velvetleaf were embedded in the ground and received one of three N treatments. Plants were destructively sampled on 10 dates during each of two years and root subsamples analyzed using root scanning software. While corn root morphology was more responsive to N supply than velvetleaf, velvetleaf N uptake per unit fine root length was greater than that of corn at similar biomass. Results suggest that, in lieu of modifying root morphology to increase uptake efficiency when N is deficient, velvetleaf may in vest more root biomass to produce a deeper tap root to reach nutrients deeper in the profile.

Injury to weeds from sublethal doses of POST herbicides may reduce the effect of weed interference on crop yield. Information on how herbicide dose influences weed mortality, growth, and seed production is needed to assess the potential benefit of applying reduced herbicide doses. Field experiments were conducted at Mead, NE, in 2001 and 2002 to quantify velvetleaf mortality, growth, and corn–velvetleaf interference in response to varying doses of three POST herbicides. Untreated velvetleaf at six densities (0, 1, 3, 6, 12, and 20 plants m−1corn row) was grown in mixture with corn to establish a baseline corn–velvetleaf interference relationship. Treated velvetleaf at a density of 20 plants m−1row received one of five doses of dicamba, halosulfuron, or flumiclorac. Untreated velvetleaf height, biomass, and seed capsule production were greater in 2002 than 2001 and declined with increasing velvetleaf density in both years. Corn yield was not affected by untreated velvetleaf in 2001, but yield loss increased with increasing velvetleaf density in 2002. Mortality of herbicide-treated velvetleaf was 56% greater in 2001 than 2002 and increased with increasing herbicide dose. Maximum height of treated velvetleaf was similar for all treatments in 2001 but declined with increasing herbicide dose in 2002. Biomass and seed production of treated velvetleaf varied among herbicides in 2002 and decreased with increasing dose. Corn yield was not influenced by velvetleaf in 2001, but yield loss in response to herbicide-treated velvetleaf declined with increasing herbicide dose in 2002. Results show that the assumption that weeds surviving herbicide application are as competitive as untreated weeds is incorrect. Reduction in growth and resource consumption by herbicide-damaged weeds reduced the negative effects of weeds on corn.

Crop planting date and canopy density influence interactions between weeds and sweet corn (Zea mays L.); however, little is known about sweet corn growth response to weed interference. Field studies were conducted in 2004 and 2005 near Urbana, IL, to quantify the influence of planting date and weed interference on growth of sweet corn height, leaf area, aboveground biomass, and phenological development. Crop growth response to weed interference (presence or absence) was determined for sweet corn planted early May (EARLY) and late June (LATE). Dominant weed species included barnyard-grass [Echinochloa crus-galli (L.) Beauv.], common lambsquarters (Chenopodium album L.), common purslane (Portulaca oleracea L.), green foxtail [Setaria viridus (L.) Beauv.], redroot pigweed (Amaranthus retroflexus L.), and velvetleaf (Abutilon theophrasti Medicus) at densities ranging from 95 to 256 plants m -2 . Weed interference reduced sweet corn's absolute height growth rate, maximum leaf area index (LAI), absolute LAI growth rate, with some of the largest effects on crop growth observed in the EARLY planting date. Silk emergence was delayed by weeds for EARLY planted sweet corn, but not LATE. Moreover, the LATE planting date resulted in 9% taller crop plants with 36% lower maximum LAI. Relative to an EARLY planting date, lower yield losses due to weeds for LATE sweet corn correspond to greater resiliency of crop growth and silk emergence to weed interference.

Weeds compete with crops for light, soil water, and nutrients. Nitrogen (N) is the primary limiting soil nutrient. Forecasting the effects of N on growth, development, and interplant competition requires accurate prediction of N uptake and distribution within plants. Field studies were conducted in 1999 and 2000 to determine the effects of variable N addition on monoculture corn and velvetleaf N uptake, the relationship between plant N concentration ([N]) and total biomass, the fraction of N partitioned to leaves, and predicted N uptake and leaf N content. Cumulative N uptake of both species was generally greater in 2000 than in 1999 and tended to increase with increasing N addition. Corn and velvetleaf [N] declined with increasing biomass in both years in a predictable manner. The fraction of N partitioned to corn and velvetleaf leaves varied with thermal time from emergence but was not influenced by year, N addition, or weed density. With the use of the [N]–biomass relationship to forecast N demand, cumulative corn N uptake was accurately predicted for three of four treatments in 1999 but was underpredicted in 2000. Velvetleaf N uptake was accurately predicted in all treatments in both years. Leaf N content (NL, g N m−2leaf) was predicted by the fraction of N partitioned to leaves, predicted N uptake, and observed leaf area index for each species. Average deviations between predicted and observed corn NLwere < 88 and 12% of the observed values in 1999 and 2000, respectively. Velvetleaf NLwas less well predicted, with average deviations ranging from 39 to 248% of the observed values. Results of this research indicate that N uptake in corn and velvetleaf was driven primarily by biomass accumulation. Overall, the approaches outlined in this paper provide reasonable predictions of corn and velvetleaf N uptake and distribution in aboveground tissues.

Ecophysiological simulation models provide a quantitative method to predict the effects of management practices, plant characteristics, and environmental factors on crop and weed growth and competition. The INTERCOM interplant competition model was parameterized, calibrated by monoculture data for three cowpea (Vigna unguiculata) genotypes that differed in growth habit, common sunflower ( Helianthus annuus) and common purslane (Portulaca oleracea), and used to simulate competition of cowpea cover crops with sunflower or purslane. The simulation results were compared with observations from field competition experiments in 2003 and 2004. INTERCOM more accurately simulated actual field data for the com petition of cowpea genotypes and sunflower than companion field experiments for the competition of cowpea and purs lane. The validated simulation model of cowpea and sunflower at two densities was used to study the effects of cowpea growth habit on final biomass production of cowpea and sunflower. The model suggested that erect growth habit was more competitive than semi-erect and prostrate growth habit, when cowpea genotypes were grown with sunflower. Cowpea leaf area distribution was important to higher cowpea biomass production, while cowpea height growth was important to reduce sunflower biomass. Our simulation approach is suggested as a method for crop breeders to gauge the likely success of selection for competitive crops before undertaking expensive long-term breeding experiments.

An improved understanding of crop stress from multiple pests is needed for better implementation of integrated pest management (IPM) strategies. Field studies were conducted in 2003 and 2004 at two locations in eastern Nebraska to describe the effects of simulated early-season insect defoliation of soybean and duration of weed interference on soybean growth. Three levels of simulated defoliation (undefoliated, 30, and 60%) and seven durations of weed interference (weedy and weed free; weed removal at V2, V4, V6, R3, and R5) were evaluated in a split-plot design. Defoliation significantly reduced soybean leaf-area index (LAI), total dry matter (TDM), and crop height in season-long weedy treatments only. Biomass partitioning during vegetative and reproductive growth was affected by both defoliation and weed interference. Increase in soybean relative growth rate (RGR) and biomass production soon after defoliation occurred (e.g., V5 stage) indicated potential defense mechanism by which soybean is able to adjust its physiology in response to the loss of leaf area. Weed interference combined with defoliation caused the greatest yield losses up to 97%. Results from this study indicate the need for monitoring early-season insect density and weed growth to determine if simultaneous control of both pests may be needed.

Differences in weed population dynamics with respect to within-field heterogeneity are not well documented despite increasing interest in site-specific management of agro-ecosystems. The focus of this study was to determine if me chanical weed management (cultivation) and/or soil factors help to explain observed within-field distributions of feral common sunflower ( Helianthus annuus L.). The ridges and furrows created by the ridge–tillage system adds additional microsites to existing spatial heterogeneity for soil characteristics such as soil organic carbon (SOC) concentration. Experimental areas were selected on the basis of naturally high or low SOC concentration. Cultivation resulted in 100% mortality of H. annuus seedlings growing in the middle of furrows. Cultivation of pre-emergence herbicide treated and no-herbicide ridges resulted in small but statistically significant ( α = 0.05) reductions in seedling survival. No differences were detected in H. annuus canopy height, stem diameter, stem length, or vegetative biomass between high and low SOC environments. Neither total reproductive biomass (P = 0.49) nor the biomass of flowers near physiological maturity (an estimate of fecundity; P = 0.59) were affected by SOC environment. Late season H. annuus lodging was observed to reduce reproductive biomass. Juvenile plants that survived mechanical weed control efforts grew and produced reproductive biomass similarly across SOC environments. The lack of difference in vegetative and reproductive characteristics between high and low SOC environments suggests that SOC (or the edaphic conditions associated with greater or lesser SOC level) was not critical in contributing to the observed distribution of H. annuus from juvenile to flowering stages of growth within well-fertilized, irrigated agricultural habitats.

Decision support systems (DSSs) have been developed to assist producers and consultants with weed management decisions. WeedSOFT is a DSS currently used in several states in the north-central region of the United States. Accurate estimates of crop yield loss due to weed interference are required for cost-effective weed management recommendations. WeedSOFT uses competitive indices (CIs) to predict crop yield loss under multiple weed species, weed densities, and relative times of weed emergence. Performance of several WeedSOFT versions to predict soybean yield loss from weed competition was evaluated using CI values in WeedSOFT version 9.0 compared to new CI values calculated from weed dry matter, weed volume, and soybean yield loss in two soybean row spacings (19 and 76 cm) and two relative weed emergence times (at soybean emergence and first trifoliate leaf stage). Overall, new CI values improved predictions of soybean yield loss by as high as 63%. It was especially true with using new CI values based on yield loss compared to those based on weed dry matter or weed volume. However, there were inconsistencies in predictions for most weed species, suggesting that additional modifications are needed to further improve soybean yield loss predictions.

To develop more effective pest-management strategies, it is essential to understand how different pests interact with each other and the crop. Field studies were conducted in 2003 and 2004 at two Nebraska locations to determine the effects of early-season crop defoliation on the critical time for weed removal (CTWR) in narrow-row soybean. Three soybean defoliation levels were selected to simulate 0, 30, and 60% leaf tissue removal by the bean leaf beetle. Weeds were allowed to compete with the crop until V2, V4, V6, R3, and R5 growth stages. There were also season-long weedy and weed-free treatments. Results indicated that the CTWR in soybean occurred earlier as defoliation levels increased from 0 to 60%. The CTWR occurred at V3, V2, and V1 growth stage for 0, 30, and 60% defoliation levels, respectively. Overall, 60% defoliation resulted in earlier CTWR by at least 14 d. Yield losses from defoliation and weed interference were primarily associated with a reduction in number of pods per plant−1. ...