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A field trial was conducted for 6 yr (1998 through 2003) at Scottsbluff, NE, to measure weed shifts following multiple applications of two rates of glyphosate or alternating glyphosate with nonglyphosate treatments in continuous corn or in a crop rotation of corn, sugarbeet, and spring wheat with all three crops resistant to glyphosate. After 6 yr, plant densities of common lambsquarters, redroot pigweed, hairy nightshade, and common purslane increased in the crop-rotation treatment compared with continuous corn. There were four weed control subplot treatments consisting of two in-crop applications of glyphosate at 0.4 or 0.8 kg ae/ha each spring, alternating two applications of glyphosate at 0.8 kg/ha one year with a nonglyphosate treatment the next year, or a nonglyphosate treatment each year. The composition of the weed population averaged across all four treatments shifted from kochia and wild proso millet to predominately common lambsquarters. After 3 yr of using glyphosate at 0.4 kg/ha twice each year, common lambsquarters density increased compared with that in the 0.8 kg/ha rate of glyphosate or alternating glyphosate treatments. By the sixth year, the density of common lambsquarters in the glyphosate at 0.4 kg/ha treatment had increased to the extent that corn grain yield was reduced 43% compared with corn grain yield in the 0.8 kg/ha glyphosate treatment. Using glyphosate at either rate for 6 yr decreased the densities of kochia, wild proso millet, and longspine sandbur, did not alter densities of redroot pigweed and green foxtail, and increased the density of hairy nightshade. In the low-rate treatment of glyphosate, the number of common lambsquarters seeds in the seed bank were 134 seeds/kg soil in 1998, declined to 15 seeds/kg by 2002, but began to increase in 2003 as the densities of plants not controlled by glyphosate increased.

Thirteen hard red winter wheat cultivars were evaluated for their ability to suppress summer annual weeds in grain production systems near North Platte, NE, from 1993 through 1997. ‘Turkey’, a 125-yr-old landrace selection, suppressed both broadleaf and grass weeds more than other cultivars. Some relatively new cultivars, such as ‘Arapahoe’, ‘Jules’, ‘Pronghorn’, and ‘Vista’ suppressed summer annual grasses almost as well as Turkey. Total weed density was negatively correlated with number of winter wheat stems/m2, mature winter wheat height, and lodging. Weed density after wheat harvest was positively correlated with delay in winter wheat seeding date and was negatively correlated with precipitation 0 to 30 d after winter wheat seeding, during tillering, tillering to boot stage, and heading to maturity stage. Mean air temperature 0 to 30 d after wheat seeding was positively correlated with weed density. In the spring, weed density was positively correlated with temperatures during the tillering stage, tillering to boot stage, and heading to maturity stage. Stinkgrass and witchgrass densities were positively correlated with severity of wheat leaf rust. The highest grain-producing cultivars included three medium height cultivars ‘Alliance’, Arapahoe, and ‘Niobrara’. Alliance wheat produced 53% more grain than Turkey, and the other two produced 43% more grain.

Marie Jasieniuk,1 Bruce D. Maxwell,2 Randy L. Anderson,3 John 0. Evans,4 Drew J. Lyon,5 Stephen D. Miller,6 Don W Morishita,7 Aex G. Ogg, Jr.,8 Steven Seefeldt,9 Phillip W.XT Stahlman,10 Francis E. Northam,'0 Philip Westra, 1 Zewdu Kebede, Gail A. Wicks'2 I Corresponding author. Department of Land Resources and Environmental Sciences, Montana State University, Bozeman, MT 59717; mariej@montana.edu; 2 Department of Land Resources and Environmental Sciences, Montana State University, Bozeman, MT 59717; 3 Central Plains Research Center, USDA-ARS, Akron, CO 80720; 4 Department of Plant, Soils, and Biometeorology, Utah State University, Logan, UT 84322; 5 Panhandle Research and Extension Center, University of Nebraska, Scotsbluff, NE 69361; 6 Department of Plant, Soil, and Insect Sciences, University of Wyoming, Laramie, WY 82071; 7 Twin Falls Research and Extension Center, University of Idaho, Twin Falls, ID 83303; 8 National A. cylindrica Research Program, P.O. Box 53, Ten Sleep, WY 82442; 9 USDA-ARS, Washington State University, Pullman, WA 99164; 10 Agricultural Research Center, Kansas State University, Hays, KS 67601; l1 Department of Bioagricultural Science and Pest Management, Colorado State University, Fort Collins, CO 80523; 12 West Central Research and Extension Center, University of Nebraska, North Platte, NE 69101 Crop yield loss-weed density relationships critically influence calculation of economic thresholds and the resulting management recommendations made by a bioeconomic model. To examine site-to-site and year-to-year variation in winter Triticum aestivum L. (winter wheat)-Aegilops cylindrica Host. (jointed goatgrass) interference relationships, the rectangular hyperbolic yield loss function was fit to data sets from multiyear field experiments conducted at Colorado, Idaho, Kansas, Montana, Nebraska, Utah, Washington, and Wyoming. The model was fit to three measures of A. cylindrica density: fall seedling, spring seedling, and reproductive tiller densities. Two parameters: i, the slope of the yield loss curve as A. cylindrica density approaches zero, and a, the maximum percentage yield loss as A. cylindrica density becomes very large, were estimated for each data set using nonlinear regression. Fit of the model to the data was better using spring seedling densities than fall seedling densities, but it was similar for spring seedling and reproductive tiller densities based on the residual mean square (RMS) values. Yield loss finctions were less variable among years within a site than among sites for all measures of weed density. For the one site where year-to-year variation was observed (Archer, WY), parameter a varied significantly among years, but parameter i did not. Yield loss functions differed significantly among sites for 7 of 10 comparisons. Site-to-site statistical differences were generally due to variation in estimates of parameter i. Site-to-site and year-to-year variation in winter T aestivum-A. cylindrica yield loss parameter estimates indicated that management recommendations made by a bioeconomic model cannot be based on a single yield loss function with the same parameter values for the winter T aestivumproducing region. The predictive ability of a bioeconomic model is likely to be improved when yield loss functions incorporating time of emergence and crop density are built into the model's structure.

Field experiments were conducted to compare performance of glyphosate with three different boom arrangements in a winter wheat-fallow rotation near North Platte, NE, in 1994 and 1995. One boom was optically controlled, and the other boom was for broadcast herbicide applications. Spraying with both booms at the same time was called “dual boom.” The sprayers were tested during May, June, and July on two weed density levels established by applying glyphosate at 0.42 kg ae/ha with and without atrazine at 0.84 kg ai/ha in October following wheat harvest. The dual-boom and the broadcast herbicide applications were more efficient in controlling weeds than the optically controlled system. The dual boom reduced weed density 4.5-fold compared with the optically controlled sprayer used alone. Horseweed < 8 cm tall was more difficult to control with the optically controlled sprayer than redroot pigweed and kochia because of its cylindrical-shaped growth patterns. Barnyardgrass and green foxtail seedlings with an erect growth pattern were also difficult for the sensors to detect. Poorer control with the optically controlled sprayer was associated with failure to identify small weeds, chlorotic plants, inconsistency among sensors, and too wide a field of view (FOV), as sensors were spaced farther apart than presently recommended. The number of sensors on a boom needs to be increased to improve the performance of the optically controlled sprayer.

Downy brome is one of the most troublesome winter annual weeds in winter wheat-fallow rotations in the central Great Plains. A 3-yr seed burial study was initiated to determine how long downy brome seed remained germinable when placed on the soil surface or 2.5 cm deep at four different times in four environments. Only 1 to 7% of the downy brome seed survived after 1 yr on the soil surface in chemical fallow and stubble mulch when deposited in August, but survival varied in September, October, and November. In 1970, a year with low fall and winter precipitation, 36 to 46% of the seed placed on the soil surface of chemical fallow in September, October, and November survived, compared with 1 to 8% for stubble mulch tillage. Early spring tillage covered more seed with soil, and downy brome seed survival decreased. When fall and winter precipitation was normal, stubble mulch and chemical fallow had 1 to 20% germinable seed remaining. Induced dormancy existed in some years. More downy brome seed survived when placed on the soil surface of crested wheatgrass sod (14 to 50%) than on smooth brome sod (0 to 36%). No differences existed among environments when downy brome seed was buried 2.5 cm deep. Only 0.4% of downy brome seed buried 2.5 cm survived after 1 yr when averaged across all environments.

Field studies were conducted from 1986 through 1989 to determine the feasibility of using clomazone in a winter wheat-corn-fallow rotation. Clomazone at 1.1 kg ai/ha did not control emerged downy brome in autumn nor did it control redroot pigweed and tumble pigweed that emerged after a spring application of paraquat at 0.6 kg ai/ha. However, clomazone plus atrazine at 0.6 plus 2.2 kg ai/ha provided weed control in winter wheat stubble from October to corn harvest 12 mo later. Corn yields from this treatment were equal to or greater than the conventional practice of applying atrazine plus glyphosate at 2.2 plus 0.4 kg ae/ha in the fall followed by a spring application of metolachlor plus dicamba at 2.8 ai plus 0.3 kg ae/ha. When kochia densities were high, clomazone plus atrazine controlled triazine-resistant kochia better than metolachlor plus dicamba. Atrazine at 2.2 kg/ha applied in autumn followed by clomazone at 0.8 kg/ha in April generally caused more corn injury than when the same rate of clomazone was applied in autumn with the atrazine. The percent of bleached corn plants varied with planter, year, time of application, and rate of clomazone applied. The optimum time to apply clomazone at 0.6 kg/ha was in autumn with atrazine. This treatment caused some corn bleaching but produced the highest corn yield over 3 yr. In a dry autumn and spring, less clomazone injury occurred with a planter equipped with row cleaners than those without. The injury pattern associated with planters was inconsistent when precipitation was average and no injury occurred in a wet autumn and spring.

Ridge-till is an integrated weed management system that involves the physical movement of soil containing weed seeds away from the row with ridge-clearing equipment on the planter. Corn, grain sorghum, and soybean are the major crops planted using the ridge-till system. Weeds can be controlled with cultivation, competitive row crops, and herbicides. Weeds have adapted to the system but have been controlled through alternative management. Through modernization of equipment and herbicides, ridge-till has become an economic crop production practice. Integrating cultivation and herbicides controls a broader spectrum of weeds than cultivation or herbicides alone.

Barnyardgrass is a problem in winter wheat fields in the central Great Plains following harvest. Glyphosate plus 2,4-D efficacy on barnyardgrass with or without atrazine was investigated under various watering events at three sites in west central Nebraska. Two experiments were initiated at each site. One experiment dealt with rainfall and the other rainfall plus irrigation. Main plots in stubble fields infested with barnyardgrass were sprayed with glyphosate plus 2,4-D at 0.4 plus 0.7, 0.5 plus 0.8, and 0.6 plus 1.0 kg ae ha−1alone and with 2.2 kg ai ha−1atrazine. Barnyardgrass was under drought stress in unwatered areas in 1991 at two sites, but the 1993 site was waterlogged. Glyphosate plus 2,4-D at 0.6 plus 1.0 kg ha1controlled 62, 99, and 100% of the barnyardgrass at the three sites 30 DAT. Nevertheless, control was reduced to 48, 88, and 35% when atrazine was combined with glyphosate plus 2,4-D. Poorest control came when plants were under stress from drought or waterlogging and atrazine was included in the spray solution. In subplots barnyardgrass plants in a 0.8 m2area were watered 6, 3, or 1 d before spraying and 1, 4, 7, or 11 d after spraying. By watering drought-stressed barnyardgrass 6 d before spraying, control generally was similar or better than with later watering dates. Watering 6 d before spraying with glyphosate plus 2,4-D plus atrazine at 0.6 plus 1.0 plus 2.2 kg ha−1reduced but did not eliminate antagonism from atrazine.

The effect of herbicide and urea-ammonium nitrate (UAN) combinations on winter wheat injury in absence of noncompetitive weeds and weed control during a winter wheat-fallow and a winter wheat-sorghum-fallow rotation were investigated. Winter wheat was planted at different dates to obtain different growth stages for spraying in the spring. Winter wheat produced greater grain yields when planted Sept. 15 or Sept. 25, 1987, 1988, and 1989 vs. Sept. 1 at North Platte, NE, while at Sidney, NE, grain yield was higher in wheat planted on Sept. 10 or Sept. 20, 1988, compared to Aug. 26. Spring-applied UAN increased grain yield on wheat planted Sept. 10 compared to no UAN in 1988–89 at Sidney, but not in 1987–88, while at North Platte, grain yields were not affected by UAN. At Sidney 2,4-D ester at 0.6 kg ae ha−1, 2,4-D amine plus dicamba at 0.3 plus 0.1 kg ae ha−1, metsulfuron at 0.007 kg ai ha−1plus 0.25% nonionic surfactant (NIS), and metsulfuron plus 2,4-D ester at 0.007 plus 0.3 kg ha−1plus NIS decreased grain yields compared to one handweeding. At North Platte in 1988–89, when UAN was applied with 2,4-D ester, 2,4-D amine plus dicamba, or metsulfuron plus 2,4-D plus NIS grain yields were reduced compared to the handweeded check on wheat planted Sept. 15. Occasionally, metsulfuron plus 2,4-D ester plus NIS treated wheat yielded less grain than metsulfuron plus NIS treated wheat. One or more herbicide treatments reduced wheat grain yields 4 of 15 application dates. Crop injury was related to growth stage and health of winter wheat when treatments were applied. Wheat under stress was more susceptible to herbicide damage than healthy wheat. Metsulfuron and metsulfuron plus 2,4-D controlled kochia, tumble thistle, and redroot pigweed better after wheat harvest than 2,4-D or 2,4-D plus dicamba at North Platte, but allowed summer annual grass weeds to grow. Yields of grain sorghum planted after a 10-mo fallow period were higher following winter wheat treated with three of four herbicides than the handweeded treatment.

Spring small grains were not as competitive with barnyardgrass and witchgrass as winter wheat. Winter wheat grain yields were greater than spring barley or spring wheat in 1986, 1987, and 1988 and oat in 1986 and 1988. Barnyardgrass, stinkgrass, and witchgrass control with glyphosate plus 2,4-D plus atrazine at 0.6 plus 0.8 plus 1.7 kg ha−1was usually less when the herbicides were applied to stubble of spring small grain versus winter wheat due to the advanced weed growth at treatment Barnyardgrass and witchgrass were more difficult to control than stinkgrass, redroot pigweed, tumble pigweed, kochia, and tumble thistle. No-till corn planted into winter wheat stubble had fewer barnyardgrass and witchgrass than corn planted into spring wheat stubble. The addition of metolachlor plus atrazine at 1.7 plus 0.6 kg ha−1eliminated differences among small grain cultivars in weed control in corn. Corn grain yields from winter wheat plots were greater than other small grains in 1989 because of better weed control and more crop residue.

Herbicidal control of triazine-resistant (TR) kochia was evaluated in no-till grain sorghum. Herbicides were applied early preplant (EPP) or POST. In EPP experiments, herbicides plus nonionic surfactant at 0.25% v/v were applied 7, 2, or 0 wk before planting (WBP). Fluorochloridone at 0.8 kg ai/ha applied 7 or 2 WBP, pyridate at 1.0 kg ai/ha applied 2 WBP, and paraquat at 0.4 kg ai/ha applied 0 WBP controlled 94 to 99% of TR kochia; a prepackaged mixture of glyphosate plus 2,4-D at 0.3 plus 0.6 kg ae/ha and paraquat at 0.4 kg/ha applied 7 or 2 WBP controlled 71 to 82% of TR kochia; and 2,4-D ester at 0.6 kg ae/ha applied 2 WBP controlled 75% of TR kochia. Linuron at 0.8 kg ai/ha and atrazine at 2.2 kg ai/ha were ineffective. In the POST herbicide experiments, where paraquat plus metolachlor at 0.6 plus 1.7 kg ai/ha were applied 17 d before planting, various combinations and rates of bentazon plus atrazine, bromoxynil, and dicamba with adjuvants provided good control of TR kochia that was less than 8 cm tall.

Winter wheat cultivars that are competitive with weeds help control weeds in crop rotations. Ten winter wheat cultivars were evaluated for interference with summer annual grasses in the wheat and the subsequent grain sorghum crop in a winter wheat-ecofallow sorghum-fallow rotation in which there are two 10–mo fallow periods and two crops in 3 yr during 1983 to 1987. The medium–tall (100 to 109 cm tall) and medium–statured (90 to 99 cm tall) winter wheat cultivars (‘Buckskin’, ‘Siouxland’, ‘Lancota’, ‘Centurk 78’, and ‘Brule’) were more competitive than medium-short (80 to 89 cm tall) and short (68 to 79 cm tall) cultivars (‘Eagle’, ‘Homestead’, ‘Colt’, ‘Vona’, and ‘TAM 101’). Atrazine plus paraquat was applied to all cultivars 30 d after wheat harvest. When grain sorghum was planted in areas previously seeded with medium–tall and medium-statured winter wheat, summer annual grass weed biomass in sorghum was 61% less than in grain sorghum seeded into areas previously planted with medium-short and short wheat cultivars. Use of pendimethalin plus 2,4–D in winter wheat and glyphosate plus alachlor in grain sorghum eliminated differences in summer annual grass weed density and weed biomass among wheat cultivars. Sorghum grain yields were improved 7% when herbicides were used in the winter wheat and sorghum but value of the increase was less than cost of herbicides. Substituting less costly herbicides for herbicides used in this study still would not have been enough to pay for cost of herbicides for five cultivars. Grain sorghum grown on weed–free stubble of medium–tall and medium–statured winter wheat produced more grain than grain sorghum grown after medium–short and short-statured winter wheat by 5%. Volunteer wheat density during the fallow period following grain sorghum was lower in areas originally seeded to Centurk 78 and Siouxland wheat while volunteer wheat density was higher in areas planted to Homestead and Vona.

The presence of wheat straw mulch in no-tillage systems can increase corn yields in the central Great Plains, but information is needed on the optimal mulch level and the toxicity of metolachlor on corn growth. Research was conducted to determine the effect of winter wheat straw mulch levels of 0, 1.7, 3.4, 5.1, and 6.8 t ha–1and metolachlor rates of 0, 0.5, 1.0, and 1.5X-rates on corn growth and yield in a winter wheat-ecofallow-corn-fallow rotation at three locations across Nebraska. Response of corn to different mulch levels and metolachlor rates varied with climate and location. In general, early corn growth was retarded by increasing amounts of mulch due to reduced soil temperatures, but after tasseling corn grew taller under increasing mulch levels because of increased soil moisture. Soil water content, kernel moisture at harvest, stover dry matter, total dry matter, ears per plant, and kernel weight increased with increasing mulch level. Corn grain yield reached a maximum at a mulch level of 5.1 t ha–1. Kernel weight, kernel number, and grain yield increased with higher mulch levels and 0.5 and 1.0X metolachlor rates as weed competition was reduced, but decreased at the 1.5X-rate of metolachlor due to corn injury. Lack of sufficient growing degree-days to compensate for lower soil temperatures or corn injury reduced corn yields at the higher mulch levels and 1.5X-rate of metolachlor in west-central and western Nebraska. From a practical standpoint, a mulch level of 3.4 to 5.1 t ha–1and a metolachlor rate near the X-rate should increase corn yield (14 to 15%) over unmulched corn in the central Great Plains.

Experiments were conducted to determine the influence of nitrogen (N) fertility and winter wheat cultivars on weed infestations in a winter wheat-ecofallow sorghum-fallow rotation near North Platte, NE. Centurk 78 and Lancota winter wheat suppressed density and growth of barnyardgrass and green foxtail significantly more than Eagle winter wheat before and after wheat harvest. Increasing N rates applied to winter wheat decreased annual grass weed population and weed yields. However, 67 and 101 kg N ha−1reduced winter wheat grain yields compared to 34 kg N ha−1. Plots treated at 2.8 plus 0.3 kg ai ha−1of atrazine plus paraquat 31 d after wheat harvest had more barnyardgrass before grain sorghum planting in 1983 than plots treated 17 d after wheat harvest but the reverse was true for green foxtail after grain sorghum emergence in 1984. Increasing N rates from 34 kg ha−1to 67 and 101 kg ha−1in the previous wheat crop decreased weed density before and after grain sorghum planting. There was no advantage in weed control in the grain sorghum from applying N to winter wheat in the fall vs. spring.

We conducted research to determine if soybeans can be grown successfully in a no-till environment, in the semi-arid areas of the central Great Plains near North Platte, NE. Soybeans planted no-till into winter wheat stubble that was sprayed with glyphosate yielded more than when planted into soil that was rototilled in a winter wheat-soybean-fallow rotation. However, grain yield averaged only 420 kg ha-1during 1975, 1976, and 1977. No-till soybean grown in a winter wheat-grain sorghum-soybean rotation during 1982 through 1985 yielded an average of 1370 kg ha-1. Low yields were associated with lack of precipitation during the fallow period after winter wheat harvest or grain sorghum harvest and during the soybean pod elongation and filling period. Several herbicides gave excellent weed control in soybeans when applied either after wheat harvest, early preplant, or at planting time. None of the herbicides persisted long enough to reduce grain yields of winter wheat planted into the soybean residue. With present production costs these nonirrigated rotations are not economical in the semi-arid region of the central Great Plains of the United States.

In field studies conducted in 1971 through 1977 weed yields were higher in EPTC (S-ethyl dipropylthiocarbamate) and alachlor [2-chloro-2′,6′-diethyl-N-(methoxymethyl)acetanilide]-treated areas as compared to handweeded plots. Weeds present in EPTC-and alachlortreated areas did not result in a significant field bean (Phaseolus vulgaris L. ‘Great Northern 59’) yield reduction as compared to hand-weeded plots. For every 1000 kg of dry weeds present in field beans there was a corresponding decrease in field bean yields of 208 kg/ha. Weed yields produced in plots treated with alachlor, dinitramine (N4,N4-diethyl-α,α,α-trifluoro-3,5-dinitrotoluene-2,4-diamine), EPTC, metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide], trifluralin (α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine), and profluralin [N-(cyclopropylmethyl)-α,α,α-trifluoro-2,6-dinitro-N-propyl-p-toluidine] were significantly less when herbicides were incorporated with the rotary tiller as compared to incorporation with the spike-tooth harrow. Incorporation method was not as critical with butralin [4-(1,1-dimethylethyl)-N-(1-methylpropyl)-2,6-dinitrobenzenamine], with spike-tooth harrow incorporation of herbicides providing similar weed control to that obtained with the rotary tiller. Weed densities and yields were higher in plots seeded with field bean determinate variety ‘Great Northern 1140’ than in plots seeded with indeterminate variety ‘Tara’, when either EPTC or alachlor were applied preplant and incorporated. Neither field bean variety competed well with weeds when no herbicide was used.

Research on the timing of herbicide application on no-till sorghum [Sorghum bicolor(L.) Moench.] planted into undisturbed winter wheat (Triticum aestivumL.) stubble was conducted at North Platte, NE, during 1980–1982. Applying some herbicides 41 and 25 days prior to planting sorghum maintained weed control, reduced sorghum injury, and increased sorghum yields when compared to application at planting. It was necessary to apply cyanazine {2-[[4-chloro-6-(ethylamino)-1,3,5-triazin-2-yl]amino]-2-methylpropanenitrile} at 2.7 kg ai/ha 41 days prior to planting to avoid sorghum injury. Metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] + 2,4-D [(2,4-dichlorophenoxy)acetic acid] at 2.2 + 0.3 kg/ha reduced grass yields 97, 98, and 99%, while reduction with alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide] + 2,4-D at 2.8 + 0.3 kg/ha was 93, 41, and 63%, respectively, when herbicides were applied 0, 25, and 41 days prior to planting.

Visible wheat (Triticum aestivumL.) injury, delay in bloom date, and yield reduction were observed following herbicide treatments in growing wheat. Protein content and volume weight of winter wheat were not significantly affected by any herbicide treatment. Some treatments reduced annual grass and broadleaf population by 90% or more 6 weeks after spraying and maintained weed control 8 weeks in untilled winter wheat stubble. Effective herbicide treatments that caused the least winter wheat injury in 1977 and 1978 included metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazin-5(4H)-one] + alachlor [2-chloro-2′,6′-diethyl-N-(methoxymethyl)acetanilide] at 0.3 + 2.8 kg/ha, metribuzin + oryzalin (3,5-dinitro-N4,N4-dipropylsufanilamide) at 0.3 + 1.8 kg/ha, metribuzin + pendimethalin [N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine] at 0.3 + 2.2 kg/ha, metribuzin + SN-533 [N-ethyl-N-propyl-3-(propylsulfonyl)-1H-1,2,4-triazole-1-carboxamide] at 0.3 + 0.8 kg/ha, and R-40244 [1-m-trifluoromethylphenyl)-3-chloro-4-chloromethyl-2-pyrrolidone] at 1.1 kg/ha. Herbicide treatments that showed the most potential to control annual grass and broadleaf weeds selectively in tillered winter wheat and stubble in 1978 include metribuzin + metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] at 0.3 + 2 kg/ha, metribuzin + oryzalin at 0.3 + 1.4 kg/ha, and metribuzin + pendimethalin at 0.3 + 2 kg/ha.

Research was conducted to determine the effect of winter wheat (Triticum aestivumL.) straw mulch level on weed control in a winter wheat-ecofallow corn (Zea maysL.)-fallow rotation at North Platte and Sidney, NE, in 1981 and 1982. Wheat straw mulch was established at 0, 1.7, 3.4, 5.1, and 6.8 Mg/ha in stubble fields. After application of 1.5 times the recommended rate at corn planting, metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] concentration remained higher in unmulched soil than in mulched soil for more than 4 months, due to interception of metolachlor by the mulch. Even though the amount of metolachlor in the soil was reduced by mulch, weed control was not reduced and increased with increasing mulch level. Thus, increasing metolachlor rate was not necessary to maintain adequate weed control in no-till winter wheat stubble since mulch itself provided some measure of weed control.

Atrazine [2chloro-4(ethyla.mino)6(isopropylamino)-s-triazine] retention by standing and flat wheat (Triticum aestivum L.) stubble was determined over time. Immediately following an application of 1.7 kg/ha atrazine, approximately 60% of the herbicide had been intercepted by the stubble and 40% was found in the underlying soil. After 3 weeks and 50 mm of rainfall, atrazine on standing and flat stubble had decreased by 90 and 63%, respectively, while atrazine in the soil increased nearly twofold. No atrazine was found in stubble 9 weeks after application, and only 17% of that originally applied remained in the upper 4 cm of the soil. Successive alternate-day applications of 12.5, 25, or 50 mm water on 1 4C-atrazine retention by stubble was determined. Atrazine loss from stubble was greatest during the first water application, but there was no linear relationship between water application rate and retention of the herbicide. Atrazine loss from stubble decreased significantly following the second water application in all treatments. The amount of water applied did not influence atrazine loss following the third washing. Subsequent loss after the fourth, fifth, sixth, and seventh washings was 'Received for publication January 24, 1983 and in revised form July 5, 1983. Published as Paper No. 7077, Journal Series, Nebraska Agric. Exp. Stn. Research reported was part of senior author's Ph.D. Thesis. 2Grad. Student and Asst. Prof., Dep. Agron., Univ. of Nebraska, Lincoln, NE 68583 and Prof., Dep. Agron., Univ. of Nebraska, North Platte Station, North Platte, NE 69101. not significant at any water application volume. After the seventh washing, atrazine residues were greatest on stubble which had received the least amount of water. Additional index words. Minimum till, crop residue, stubble mulch.

A survey of 146 fields was conducted to investigate herbicide performance in winter wheat-producing areas of southwestern Nebraska during August and September of 1986. Only 55% of the fields received an excellent rating for weed control and stubble quality; one third rated as unacceptable. Weed control after wheat harvest was improved by planting ‘Bounty 310’, ‘Siouxland’, ‘Vona’, and ‘Centura’ winter wheat cultivars rather than ‘Mustang’, ‘Hawk’, ‘Pioneer 2656’, and ‘Wings'. Fertilizing winter wheat in the fall, planting wheat at the optimum date, high wheat stem density, using a winter wheat-corn-fallow rotation, not spraying herbicides after wheat harvest on days that it rained or air temperature exceeded 35 C, and spraying weeds when they were small also improved weed control in wheat stubble. Nine months after wheat harvest, fields treated with atrazine before July 16 had more volunteer wheat than fields treated later.

Handweeding sugarbeets (Beta vulgaris L. 'Mono Hy D2') for 8 weeks after planting prevented sugarbeet yield losses. Weeds invading sugarbeet plots at the two - leaf stage (4 weeks after planting) reduced sugarbeet yields 26%. Cycloate (S - ethyl N- ethylthiocyclohexanecarbamate) caused more visual sugarbeet injury and stand reduction than did ethofumesate ((?) - 2- ethoxy - 2,3 - dihydro - 3,3 - dimethyl - 5- benzofuranyl methanesulfonate). Broadleaf and grass weeds differed in their response to cycloate and ethofumesate, but overall, total weed yields were lower in plots treated with cycloate. Ethofumesate injured sugarbeets more when applied before planting and incorporated into the soil than when applied preemergence. Herbicides applied when sugar - beets had four to six leaves generally injured sugarbeets less, but controlled weeds less effectively, than when sugar - beets had two to four leaves. None of these herbicide treat - ments were as effective as handweeding in providing season - long weed control. Every 1120 kg/ha of oven dry weeds present in sugarbeet fields corresponded to a decrease in sugarbeet root yields of 10 000 to 11 500 kg/ha. Additional index words. Herbicide incorporation; preplant, preemergence, and postemergence application; interference; competition.

This research was conducted near North Platte, NE, over an 18-yr period to determine the feasibility of using herbicides to replace tillage as the weed control method in a winter wheat (Triticum aestivumL.)-sorghum [Sorghum bicolor(L.) Moench.]-fallow rotation. Five tillage treatments [two tillage and three reduced or no-till treatments] were used on the same plots during the duration of this experiment on a Holdrege silt loam (Typic Argiustolls). Herbicides effectively replaced tillage for weed control. The no-till plots treated with atrazine [6-chloro-N-ethyl-N′-(1-methylethyl)-1,3,5-triazine-2,4-diamine] after wheat harvest had higher sorghum and winter wheat yields, higher crop residue remaining on the soil surface, and lower weed yields than tilled plots. The most difficult weeds to control were volunteer wheat and barnyardgrass [Echinochloa crus-galli(L.) Beauv. # ECHCG]. Soil surface pH decreased over time because of increased use of nitrogen. The pH in nontilled plots was significantly lower than in tilled plots due to lack of soil mixing. Exchangeable calcium was the predominant cation leached from the top 5 cm but showed accumulation between the 5- to 12.5-cm depth. Organic matter content showed little change over time.

Twenty-five winter wheat (Triticum aestivumL.) cultivars were sprayed with herbicides when in the tillering stage in April at North Platte, NE. In 1978, ‘Lindon’ and ‘Vona’ stands were reduced and grain yields of ‘Larned′, Lindon, ‘Roughrider′, 'Sage′, 'Scout 66′, 'Sentinel′, ‘Turkey′, and Vona cultivars were reduced by metribuzin [4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazin-5(4H)-one] plus pendimethalin [N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine] at 0.3 plus 2.8 kg ai/ha. There were no differences among cultivars in response to herbicides in 1979, 1981, and 1982. During 1980, wheat was more tolerant to a mixture of pendimethalin and 2,4-D [(2,4-dichlorophenoxy)acetic acid] than metribuzin plus pendimethalin. In 1981 and 1982, combinations of 2,4-D with pendimethalin or metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] did not reduce wheat yields. The mixture of pendimethalin plus 2,4-D performed satisfactorily in 4 yr of use.

CGA-43089 [α-(cyanomethoximino)-benzacetonitrile] seed treatment was evaluated at three Nebraska locations during 1979 and 1980 for grain sorghum [Sorghum bicolor(L.) Moench] protection from injury by metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide], four other chloroacetanilide herbicides, and three metolachlor +s-triazine combinations. Acetochlor [2-chloro-N-(ethoxymethyl)-6′-ethyl-o-acetotoluidide] caused the most frequent and severest injury to unprotected and protected sorghum. Soilincorporated metolachlor produced more consistent injury than preemergence application to unprotected sorghum, but placement did not affect CGA-43089 protection. Sorghum-stand reduction was the type of injury most frequently encountered, but crop yield was least affected. CGA-43089 always protected the grain sorghum from yield reduction with these herbicides.

Downy brome (Bromus tectorumL. # BROTE) may be a troublesome weed in winter small grains, perennial legumes, perennial grasses grown for seed, and orchards. In Nebraska, winter wheat (Triticum aestivumL.) yields have been depressed 30% by downy brome populations of 11 to 22 plants/m2(8). In Oregon, downy brome densities of 108 to 160 and 538 plants/m2reduced yields by 40 and 92%, respectively (22, 23). In Idaho, wheat yields were depressed 20 to 40% with 55 to 110 plants/m2(18). Downy brome has also been shown to lower yields and quality of hay from infested fields of alfalfa (Medicago sativaL.) (29). In perennial grass seed fields, downy brome may cause a serious seed quality problem (16). Some grass seed lots have contained as much as 50 to 75% downy brome seed.

An experiment involving six winter wheat (Triticum aestivumL.) cultivars, an early-April herbicide application on wheat and on four dates after wheat harvest, and the growth of a subsequently planted corn (Zea maysL.) crop was conducted at North Platte, NE. ‘Centurk 78’ suppressed barnyardgrass [Echinochloa crus-galli(L.) Beauv. # ECHCG] more than ‘Bennett’ and ‘Eagle’ in the growing wheat and after wheat harvest in July, but there were no differences in weed yield among cultivars in corn planted 11 months later. Herbicides applied to the tillering wheat in early April improved weed control in wheat and the subsequent corn crop. Also, herbicides were applied 5, 25, 45, and 300 days after wheat harvest. Weed growth increased and soil water decreased as spraying dates were delayed. Herbicides applied 5 days after harvest did not maintain adequate weed control in the corn planted 11 months after wheat harvest and low corn yield resulted. Plots receiving herbicides 300 days after wheat harvest had the least soil water in the fall after wheat harvest but the best weed control in corn and highest corn yields because of better weed control in corn.

The persistence of five herbicides in six soils across Nebraska can be ranked from greatest to least as follows: 5-bromo-3-isopropyl-6-methyluracil (isocil) at 5 and 25 1b/A, 2-chloro-4,6-bis-(isopropylamino)-s-triazine (propazine) at 3 and 9 1b/A, 2-chloro-4-ethylamino-6-isopropylamino-s-triazine (atrazine) at 3 and 9 1b/A, trichlorobenzyl chloride (hereinafter referred to as TCBC) at 7 and 49 1b/A, and 3-(3,4-dichlorophenyl)-1-methoxyl-1-methylurea (linuron) at 3 and 9 1b/A. Soil texture differences (sandy loam, very fine sandy loam, silt loam, and silty clay loam) had a greater influence on herbicide residue carryover than did climatic differences across Nebraska during 1962 to 1968. Soil carryover of herbicide residues was greater in coarse rather than fine-textured soils and in the drier regions of western than in eastern Nebraska. Leaching of herbicides into the soil profile was an avenue of herbicide dissipation.

Downy brome (Bromus tectorumL.) seedling emergence was greatest from soil depths of 1 inch or less, but occasionally seedlings emerged from depths of 4 inches. Downy brome seed covered by soil germinated more rapidly than those seed on the soil surface. More downy brome seedlings emerged, and from greater depths, from coarse-textured soils than fine-textured soils when moisture was not limiting. Soil type did not influence longevity of downy brome seed buried in the soil. Most (98%) 8-month-old downy brome seed buried 8 inches in the soil germinated but did not emerge in 1 year; and none remained viable in the soil after 5 years. The moldboard plow was more effective in reducing downy brome populations than a sweep plow or one-way disk in a continuous winter wheat (Triticum aestivumL.) cropping system.

The use of 1,8-naphthalic anhydride, at 0.5% by corn (Zea maysL. ‘501D’) seed weight, eliminated most of the corn yield loss caused byS-ethyl dipropylthiocarbamate (EPTC) at 3.4, 6.7, or 10.0 kg/ha. The seed protectant partially reduced the moderate corn yield loss fromS-ethyl diisobutylthiocarbamate (butylate) at 9.0 kg/ha. This seed protectant provides a new approach to increased weed control while maintaining selectivity in the production of corn.

Dissipation of 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine (atrazine) in sprinkler-irrigated fields was faster in a silty clay loam soil of eastern Nebraska than in loam soils of central or western Nebraska. Atrazine dissipated faster under fallow than under corn (Zea mays L.) culture in western Nebraska, but there was no difference in atrazine loss whether in corn or fallow in central or eastern Nebraska. In central and western Nebraska, atrazine persisted in the soil for longer than 1 year. In eastern Nebraska under sprinkler irrigation, most of the atrazine had dissipated before the following growing season. Sprinkler irrigation markedly reduced atrazine longevity in soils studied.

In field trials in 1967-8, sorghum cv. Nebraska 505 was sown with or without cultivation, sown on the surface or with drill units equipped to open furrows 2.5-5 cm deep and 2.24 kg atrazine/ha was applied pre-sowing on 15 Apr., 30 Apr. or 15 May or pre-emergence on 17 May, or plots were handweeded left unweeded or cultivated. Cultivation of the unweeded plots increased yields from 0.94 to 1.87 t grain/ha and for all treatments the average grain yield without cultivation was 2.25 t/ha, which was increased to 2.79 t/ha by cultivation. Cultivation increased the effectiveness of atrazine, but the highest average yield of 3.24 t/ha was given by hand weeding. Atrazine treatment did not control weeds adequately, or damaged the sorghum. In some cases, decreasing sorghum stands increased the number of ears/plant and the size of the ears. Sorghum plant populations were higher where furrow opening was used at sowing, than with surface sowing.ADDITIONAL ABSTRACT:In trials at Lincoln and North Platte, Nebraska during 1967-68 soil-incorporated, pre-sowing treatments with atrazine at 2.24 kg/ha on 15th April, 30 April and 15th May gave inadequate control of grass weeds and resulted in lower sorghum grain yields than a pre-em. application on 17th May. At North Platte in both years the sorghum stand was reduced following the treatment on 30th April but grain yields were not significantly affected as the remaining plants produced more and larger heads/plant. Sorghum stand and yield were improved when the crop was sown with a planter unit equipped with furrow openers which removed 2.5 to 5 cm of surface soil and weed yields were lower compared to plots where sorghum was sown without removing the surface soil. Atrazine + one cultivation improved weed control and increased grain yields but yields were still below those on the hand-weeded plots

Experiments were conducted at Crawford and Kimball, Nebraska, during 1962, 1963, 1964, and 1965 to determine the effect of 13 herbicides on plains pricklypear (Opuntia polyacantha Haw.). The best control was obtained with 2-(2,4,5-trichlorophenoxy)propionic acid (silvex), 4-amino-3,5,6-trichloropicolinic acid (picloram), 3,6-dichloro-o-anisic acid (dicamba), and (2,4,5-trichlorophenoxy) acetic acid (2,4,5-T). Silvex and picloram were superior to dicamba and 2,4,5-T in controlling plains pricklypear, but picloram and dicamba injured the native grasses. Rotary hoeing plains pricklypear cladodes just prior to spraying improved herbicide performance. Simulated railing was not a consistent control measure. Evaluation of plains pricklypear control was made 1 and 2 years after herbicide application.

The dissipation and detoxification of 3,6-dichloro-o-anisic acid (dicamba), 4-amino-3,5,6-trichloropicolinic acid (picloram), and 2,3,6-trichlorobenzoic acid (2,3,6-TBA) was followed for 6 years at three locations across Nebraska. Herbicide persistence was determined by growing field bioassays of field beans (Phaseolus vulgarisL.) and soybeans [Glycine max(L.) Merr.]. Picloram showed the greatest soil persistence, 2,3,6-TBA was intermediate, and dicamba showed the least persistence across Nebraska. The residual phytotoxicity of picloram in soil was not necessarily dependent upon rainfall but was greatest in the fine textured soil with higher organic matter content.

The competitiveness of 10 sorghum [Sorghum bicolor(L.) Moench] hybrids to weeds and their tolerances to commonly used sorghum herbicides were studied across Nebraska. The three best weed competitors in this study were sorghum hybrids ‘RS 671′, ‘RS 690′, and ‘AKS 663′; whereas the three poorest weed competitors were ‘AKS 614′, ‘NB 505′, and ‘RS 633′. Sorghum hybrids showing a significant yield reduction from a 2× rate of 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine (atrazine) were ‘SD 441′, ‘SD 451′, and ‘NB 505′. ‘SD 441 also suffered a significant yield reduction from a normal rate of 2-chloro-N-isopropylacetanilide (propachlor) applied preemergence plus a 4× rate of (2,4-dichlorophenoxy)acetic acid (2,4-D) applied postemergence. With the above exceptions, sorghum hybrids generally showed considerable tolerance to 2× rates of herbicides commonly used on sorghum.

Weed competition in this experiment had a more marked effect on sorghum [Sorghum bicolor (L.) Moench, var. RS 610] grain yields than did row spacings or sorghum populations. Sorghum grown in 20-inch, as compared to 40-inch, rows had higher grain and stover yields and lower weed yields and 1000-seed weight. Sorghum populations of 42,000 to 50,000 plants per acre produced higher grain and stover yields than populations of 26,000 to 28,000 plants per acre which had greater weed yields, heads per plant, seed weight per head, seed moisture at harvest, and 1000-seed weight. In hand weeded plots, sorghum yields were not affected by row spacing or sorghum population; but as weed competition increased, sorghum yields increased with narrower rows and higher plant populations. Sorghum yields were reduced 4, 12, and 18% by delaying weeding of plots 3, 4, and 5 weeks as compared with 2 weeks after planting. Weeds that did not emerge until 4 weeks after planting did not reduce sorghum yields.