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Velvetleaf (Abutilon theophrasti Medik.) is a troublesome broadleaf weed that competes with crops for resources such as soil moisture. Water stress can affect the ability of weed species to grow and produce seeds. The objective of this study was to determine the effect of degree of water stress on the growth and fecundity of A. threophrasti using soil moisture sensors under greenhouse conditions. Abutilon threophrasti seeds collected from a corn (Zea mays L.)–soybean [Glycine max (L.) Merr.] field were grown in silty clay loam soil, and plants were maintained at 100%, 75%, 50%, and 25% soil field capacity (FC) corresponding to no, light, moderate, and high water-stress conditions, respectively. Water was added daily to pots based on soil moisture levels detected by a Meter Group 5TM sensor to maintain the desired water-stress level required by treatment. Plants maintained at 100% FC had the maximum number of leaves (28 leaves plant−1), followed by 21 and 15 leaves plant−1 at 75% and 50% FC, respectively. Abutilon threophrasti at 100% and 75% FC achieved maximum plant height (108 to 123 cm) compared with 83 cm at 50% FC. Abutilon threophrasti maintained at 75% FC had the greatest growth index (79,907 cm3) followed by 72,197 cm3 at 100% FC and 64,256 cm3 at 50% FC. Seed production was similar at 100%, 75%, and 50% FC (288 to 453 seeds plant−1) compared with 2 seeds plant−1 at 25% FC. This is because the majority of plants maintained at 25% FC did not survive more than 77 d after transplanting. Seed germination was 96% to 100% at 100%, 75%, and 50% FC compared with 20% germination at 25% FC. Abutilon threophrasti can survive ≥50% FC continuous water-stress conditions, although with reduced leaf number, plant height, and growth index compared with 75% and 100% FC.

Palmer amaranth is the most problematic and troublesome weed in agronomic cropping systems in the United States. Acetolactate synthase (ALS) inhibitor and glyphosate-resistant (GR) Palmer amaranth has been confirmed in Nebraska and it is widespread in several counties. Soybean resistant to isoxaflutole/glufosinate/glyphosate has been developed that provides additional herbicide site of action for control of herbicide-resistant weeds. The objectives of this study were to evaluate herbicide programs for control of ALS inhibitor/GR Palmer amaranth and their effect on Palmer amaranth density and biomass, as well as soybean injury and yield in isoxaflutole/glufosinate/glyphosate–resistant soybean. Field experiments were conducted in a grower’s field infested with ALS inhibitor and GR Palmer amaranth near Carleton, Nebraska, in 2018 and 2019. Isoxaflutole applied alone or mixed with sulfentrazone/pyroxasulfone, flumioxazin/pyroxasulfone, or imazethapyr/saflufenacil/pyroxasulfone provided similar control (86%–99%) of Palmer amaranth 21 d after PRE (DAPRE). At 14 d after early-POST (DAEPOST), isoxaflutole applied PRE and PRE followed by (fb) POST controlled Palmer amaranth by 10% to 63% compared to 75% to 96% control with glufosinate applied EPOST in both years. A PRE herbicide fb glufosinate controlled Palmer amaranth 80% to 99% 21 d after late-POST (DALPOST) in 2018, and reduced density 89% to 100% in 2018 and 58% to 100% in 2019 at 14 DAEPOST. No soybean injury was observed from any of the herbicide programs tested in this study. Soybean yield in 2019 was relatively higher due to higher precipitation compared with 2018 with generally no differences between herbicide programs. This research indicates that herbicide programs are available for effective control of ALS inhibitor/GR Palmer amaranth in isoxaflutole/glufosinate/glyphosate-resistant soybean.

Tillage regimes can influence weed population dynamics and, consequently, the choice of appropriate weed management practices. Studies were conducted in 2016 and 2017 in a long-term (36-yr) grain sorghum [Sorghum bicolor (L.) Moench ssp. bicolor] experiment at Texas A&M University, College Station, to determine the impact of long-term no-till (NT) and conventional till (CT) systems on weed species dynamics. Higher densities of johnsongrass [Sorghum halepense (L.) Pers.], prostrate spurge [Chamaesyce humistrata (Engelm. ex A. Gray) Small], waterhemp [Amaranthus tuberculatus (Moq.) Sauer], and henbit (Lamium amplexicaule L.) were recorded in the NT system compared with the CT system. Further, the NT system showed greater weed diversity (Shannon-Wiener index, H = 0.8) and species richness (S = 6.2), compared with the CT system (H = 0.6, S = 4.2). Seedling emergence of some dominant weed species was also delayed in the NT system. In the CT system, 50% emergence of S. halepense (8.5 C base temperature) and waterhemp (10 C base temperature) occurred at 59 and 63 growing degree days (GDD), respectively, whereas 68 and 75 GDD, respectively, were required in the NT system. Further, a greater proportion (61%) of the viable seedbank was present at the top 5 cm of the soil in the NT system compared with the CT system (46%). Overall, findings from this 36-yr-long tillage experiment have revealed that the NT system had greater weed densities (especially of the perennial weed S. halepense) and a high proportion of weed seeds (particularly small-seeded annuals) on the topsoil layer, corroborating some earlier reports that were based on short-term investigations. Findings indicate that growers transitioning to NT systems should be mindful of potential shifts in weed species dominance and develop appropriate management tactics.

A waterhemp [Amaranthus tuberculatus(Moq.) J. D. Sauer] biotype (designated as “NER”) collected from a soybean [Glycine max(L.) Merr.] production field in eastern Nebraska survived the POST application of fomesafen at the labeled rate. The objectives of this study were to (1) quantify the level of resistance to protoporphyrinogen oxidase (PPO) inhibitors (acifluorfen, fomesafen, and lactofen) applied POST, (2) determine the mechanism of PPO-inhibitor resistance in the NER biotype, (3) determine whether NER possessed multiple resistance to acetolactate synthase (ALS)-, 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS)-, and photosystem II (PSII)-inhibiting herbicides, and (4) control NER with POST soybean herbicides. A whole-plant dose–response bioassay revealed that the NER biotype was 4- to 6-fold resistant to PPO-inhibiting herbicides depending on the known susceptible biotype (S1 or S2) used for comparison. A Kompetitive Allele Specific PCR (KASP™) assay was developed and performed for rapid and robust detection of the ΔG210 mutation (deletion of a codon) in thePPX2Lgene. All samples of the NER biotype tested positive for the ΔG210 mutation. Dose–response bioassays confirmed that the NER biotype was resistant to three additional herbicide sites of action. Chlorimuron and imazethapyr, both ALS inhibitors, applied at 32X the labeled rate resulted in Amaranthus tuberculatusis the first confirmed weed species in Nebraska to evolve resistance to four distinct herbicide sites of action, leaving no POST herbicide choice for effective control in glyphosate-resistant and conventional (non-transgenic) soybean.

Despite the best weed control efforts, weed escapes are often present in large production fields prior to harvest, contributing to seed rain and species persistence. Late-season surveys were conducted in cotton (Gossypium hirsutum L.) fields in Texas in 2016 and 2017 to identify common weed species present as escapes and estimate seed rain potential of Palmer amaranth (Amaranthus palmeri S. Watson) and waterhemp [A. tuberculatus (Moq.) J.D. Sauer], two troublesome weed species with high fecundity. A total of 400 cotton fields across four major cotton-producing regions in Texas [High Plains (HP), Gulf Coast (GC), Central Texas, and Blacklands] were surveyed. Results have revealed that A. palmeri, Texas millet [Urochloa texana (Buckley) R. Webster], A. tuberculatus, ragweed parthenium (Parthenium hysterophorus L.), and barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.] were the top five weed escapes present in cotton fields. Amaranthus palmeri was the most prevalent weed in the HP and Lower GC regions, whereas A. tuberculatus escapes were predominantly observed in the Upper GC and Blacklands regions. On average, 9.4% of an individual field was infested with A. palmeri escapes in the Lower GC region; however, it ranged between 5.1 and 8.1% in the HP region. Average A. palmeri density ranged from 405 (Central Texas) to 3,543 plants ha−1 (Lower GC). The greatest seed rain potential by A. palmeri escapes was observed in the upper HP region (13.9 million seeds ha−1), whereas the seed rain potential of A. tuberculatus escapes was the greatest in the Blacklands (12.9 million seeds ha−1) and the upper GC regions (9.8 million seeds ha−1). Results indicated that seed rain from late-season A. palmeri and A. tuberculatus escapes are significant in Texas cotton, and effective management of these escapes is imperative for minimizing seedbank inputs and impacting species persistence.

Due to depressed corn and soybean prices over the last few years in the United States, growers in Nebraska are showing interest in no-tillage (hereafter referred to as no-till) conventional (non–genetically engineered [non-GE]) soybean production. Due to the increasing number of herbicide-resistant weeds in the United States, weed control in no-till non-GE soybean using POST herbicides is a challenge. The objectives of this study were to compare PRE-only, PRE followed by (fb) POST, and PRE fb POST with residual (POST-WR) herbicide programs for Palmer amaranth and velvetleaf control and soybean injury and yield, as well as to estimate the gross profit margins and benefit–cost ratio of herbicide programs. A field experiment was conducted in 2016 and 2017 at Clay Center, NE. The PRE herbicides tested in this study resulted in ≥95% Palmer amaranth and velvetleaf control at 28 d after PRE (DAPRE). Averaged across the programs, the PRE-only program controlled Palmer amaranth 66%, whereas 86% and 97% control was obtained with the PRE fb POST and PRE fb POST-WR programs, respectively, at 28 d after POST (DAPOST). At 28 DAPOST, the PRE fb POST herbicide programs controlled velvetleaf 94%, whereas the PRE-only program resulted in 85% control. Mixing soil-residual herbicides with foliar-active POST programs did not improve velvetleaf control. Averaged across herbicide programs, PRE fb POST programs increased soybean yield by 10% and 41% in 2016 and 2017, respectively, over the PRE-only programs. Moreover, PRE fb POST-WR programs produced 7% and 40% higher soybean yield in 2016 and 2017, respectively, compared with the PRE fb POST programs. The gross profit margin (US$1,184.3 ha−1) was highest under flumioxazin/pyroxasulfone (PRE) fb fluthiacet-methyl plusS-metolachlor/fomesafen (POST-WR) treatment; however, the benefit–cost ratio was highest (6.1) with the PRE-only program of flumioxazin/chlorimuron-ethyl.

Stakeholders were surveyed across Nebraska to identify the problem weeds and assess common weed management practices. A total of 425 responses were returned across four Nebraska extension districts (Northeast, Panhandle, Southeast, and West Central). Collectively, 61.2% of total farmed or scouted areas in Nebraska were under no-till production, and corn and soybean were the major crops (82.3% of total farmed or scouted area). Common waterhemp, horseweed, and kochia were the most problematic weeds statewide. Widespread occurrence of glyphosate-resistant (GR) weeds such as common waterhemp, horseweed, kochia, and Palmer amaranth were a serious problem in GR crop production. Additionally, 60% of growers in Nebraska reported the presence of at least one GR weed species on their farms. The most commonly used preplant burndown herbicides were 2,4-D and glyphosate, followed by saflufenacil and dicamba. In Nebraska, 74% and 59% of corn and soybean growers, respectively, were using PRE herbicides; however, more than 80% of growers were using POST herbicides for in-crop weed management. Atrazine alone or in premix or tank mix with mesotrione, S-metolachlor, or acetochlor were the most widely applied PRE herbicides in corn and grain sorghum, whereas the most commonly used PRE herbicides in soybean were the inhibitors of acetolactate synthase (ALS) and protoporphyrinogen oxidase (PPO). Glyphosate was the most frequent choice of the survey respondents as a POST herbicide in GR corn and soybean; 2,4-D was the most commonly used POST herbicide in grain sorghum and wheat. In Nebraska, only 5.2% of total crop area was planted with glufosinate-resistant crops. Most of the respondents (89%) were aware of the new multiple herbicide–resistant crops, and 80% of them listed physical drift and volatility of the auxinic herbicides as their primary concern. Fortyeight percent of survey respondents identified herbicide-resistant weed management as their primary research and extension priority.Nomenclature: 2,4-D; acetochlor; atrazine; glufosinate; glyphosate; mesotrione; S-metolachlor; common waterhemp, Amaranthus rudis J.D. Sauer; horseweed, Conyza canadensis (L.) Cronq.; kochia, Kochia scoparia (L.) Schrad.; Palmer amaranth, Amaranthus palmeri S. Wats.; corn, Zea mays L.; grain sorghum, Sorghum bicolor (L.) Moench ssp. bicolor; soybean, Glycine max (L.) Merr.; wheat, Triticum aestivum L.

A premix of atrazine, bicyclopyrone, mesotrione, and S-metolachlor (ABMS) has recently been registered for broad-spectrum weed control in corn (Zea mays L.) in the USA. The objectives of this study were to compare the efficacy of ABMS applied preemergence (PRE) with other commonly used PRE herbicides for weed control, corn injury, and yield in no-tillage (no-till) and reduced-tillage (reduced-till) corn production systems, and to determine the biologically effective doses of ABMS for controlling Palmer amaranth (Amaranthus palmeri S. Wats.) in field conditions. Field experiments were conducted in 2015 and 2016 at South Central Agricultural Laboratory, Clay Center, Nebraska, USA. ABMS applied at the labeled dose (2.89 kg ai ha−1) resulted in 92–99% Palmer amaranth control in both tillage systems. A similar level of Palmer amaranth control (86–99%) was observed with mesotrione plus rimsulfuron in the no-till system; however, the control was higher with ABMS than with other premixes in the reduced-till system at 42 days after treatment (DAT) and at harvest. Applications of ABMS at 2.89 kg ai ha−1 provided 99 and 81% control of velvetleaf (Abutilon theophrasti Medik.) and foxtail (Setaria spp.), respectively, in the no-till system at 28 DAT, whereas control was ≥93% in the reduced-till system. ABMS applied at 2.89 kg ai ha−1 resulted in 3% corn injury at 14 DAT regardless of the tillage system, whereas 15% corn injury was observed with acetochlor plus clopyralid plus flumetsulam, and dimethenamid-P plus saflufenacil in the reduced-till. ABMS or mesotrione plus rimsulfuron at labeled doses resulted in 16.0–16.3 t ha−1 corn yield, comparable to the weed-free control (16.4 t ha−1). The biologically effective doses of ABMS to provide 90% control (ED90) of Palmer amaranth at 42 DAT were 2.44 and 2.81 kg ai ha−1 in no-till and reduced-till systems, respectively. The efficacy of ABMS for broadleaf weed control and early-season grass weed control, and corn yield was the same or sometimes better than most of the PRE herbicides tested in this study; therefore, ABMS can be considered as an additional option for management of problem weeds, including Palmer amaranth in corn in the USA.

Although the United States has no areas considered strictly tropical, there are subtropical and warm humid regions in the south where agricultural production is high. Practices in these regions, and the results of research on their effects towards carbon sequestration in soils, are certainly transferable to tropical regions. Soil management (e.g., tillage and amendment) and crop management (e.g., cropping system and cover crops) practices in the United States (with emphasis on the southern region) as well as new technologies and advances are covered in this chapter. Regulatory pathways for increasing carbon stores in managed agricultural lands in the United States are unlikely; therefore, willingness on the part of farmers to adopt practices aligned with C sequestration goals must be engendered. Reduced tillage and cover crop inclusion are being adopted more commonly in the United States. Reuse of organic waste materials for the benefit of agricultural production is also increasing. Breeding for greater or more stable root mass, and new methods for monitoring the flux of C from the soil to the atmosphere both represent exciting frontiers of discovery in this area.

The evolution of glyphosate and acetolactate synthase (ALS) inhibitor-resistant common waterhemp in the Midwestern United States has reduced the number of effective POST herbicide options for management of this problem weed in glyphosate-resistant soybean. Moreover, common waterhemp emerges throughout the crop growing season, justifying the need to evaluate herbicide programs that provide season-long control. The objectives of this study were to compare POST-only and PRE followed by (fb) POST herbicide programs for control of glyphosate-resistant common waterhemp in glyphosate-resistant soybean. Field experiments were conducted in 2013 and 2014 in Dodge County, NE, in a field infested with glyphosate-resistant common waterhemp. Programs containing PRE herbicides resulted in ≥83% control of common waterhemp and densities of ≤35 plantsm–2at 21 d after PRE (DAPRE). Post-only herbicide programs resulted in –2at 14 d after early-POST (DAEPOST) treatment. PRE fb POST herbicide programs, including saflufenacil plus imazethapyr plus dimethenamid-P, sulfentrazone plus cloransulam, orS-metolachlor plus metribuzin, fb fomesafen plus glyphosate;S-metolachlor plus fomesafen fb acifluorfen plus glyphosate resulted in >90% control of glyphosate-resistant common waterhemp throughout the growing season, reduced density to ≤7plantsm–2, ≥92% biomass reduction, and soybean yield >2,200kg ha–1. Averaged across herbicide programs, common waterhemp control was 84%, and density was 15 plants m–2with PRE fb POST herbicide programs compared with 42% control, and density of 101 plants m–2with POST-only herbicide programs at harvest. Results of this study indicated that PRE fb POST herbicide programs with effective modes of action exist for season-long control of glyphosate-resistant common waterhemp in glyphosate-resistant soybean.

Glyphosate-resistant (GR) common waterhemp has become a significant problem weed in Nebraska and several Midwestern states. Several populations of GR common waterhemp are also resistant to acetolactate synthase (ALS)-inhibiting herbicides, making them difficult to control with POST herbicides in GR soybean. Glufosinate-resistant (GFR) soybean is an alternate system for controlling GR common waterhemp, justifying the need for evaluating glufosinate-based herbicide programs. The objectives of this study were to compare POST-only herbicide programs (including one-pass and two-pass POST programs) with PRE followed by (fb) POST herbicide programs for control of GR common waterhemp in GFR soybean and their effect on common waterhemp density, biomass, and soybean yield. Field experiments were conducted in 2013 and 2014 near Fremont, NE in a grower’s field infested with GR common waterhemp. Glufosinate applied early- and late-POST provided 76% control of GR common waterhemp at 14 d after late-POST (DALPOST) compared with 93% control with a PRE fb POST program when averaged across treatments. The PRE application of chlorimuron plus thifensulfuron plus flumioxazin,S-metolachlor plus fomesafen or metribuzin, saflufenacil plus dimethenamid-P fb glufosinate provided ≥95% control of common waterhemp throughout the growing season, reduced common waterhemp density to ≤2.0 plants m─2, caused ≥94% biomass reduction, and led to 1,984 to 2,210 kg ha─1soybean yield. Averaged across treatments, the PRE fb POST program provided 82% common waterhemp control at soybean harvest, reduced density to 23 plants m─2at 14 DALPOST, and caused 86% biomass reduction and 1,803 kg ha─1soybean yield compared with 77% control, 99 plants m─2, 53% biomass reduction, and 1,190 kg ha─1yield with POST-only program. It is concluded that PRE fb POST programs with multiple effective modes of action are available for control of GR common waterhemp in GFR soybean.

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.

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.

The global policy agenda of agriculture is based around producing sufficient food, feed, and fiber to meet the needs of the world’s growing population (Anonymous 2009). Additionally, agricultural cropping systems have been challenged to create oil-based fuels and plant-based bio-products, reduce use of pesticides, and develop crops tolerant to biotic and abiotic stresses. Therefore, crop cultivars with higher yields and resistance to important pests are required to meet food requirements in a sustainable manner. Technological advances for developing bio-products, improved crop cultivars, and pest and/or herbicide-tolerant crops are predicated on the use of plant biotechnology.

A premix of atrazine, bicyclopyrone, mesotrione, and S-metolachlor has recently been commercialized for pre-emergence (PRE) and early post-emergence (POST) control of broadleaved and annual grass w...