Stephen D. Miller

Glyphosate-Induced Weed Shifts in Glyphosate-Resistant Corn or a Rotation of Glyphosate-Resistant Corn, Sugarbeet, and Spring Wheat

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. Nomenclature: Glyphosate; common lambsquarters, Chenopodium album L. CHEAL; common purslane, Portulaca oleracea L. POROL; green foxtail, Setaria viridis (L.) P. Beauv. SETVI; hairy nightshade, Solanum physalifolium Rusby SOLSA; kochia, Kochia scoparia (L.) Schrad. KCHSC; longspine sandbur, Cenchrus longispinus (Hack.) Fern. CCHPA; redroot pigweed, Amaranthus retroflexus L. AMARE; wild proso millet, Panicum miliaceum L. PANMI; corn, Zea mays L; spring wheat, Triticum aestivum L; sugarbeet, Beta vulgaris L.

Secale cereale interference and economic thresholds in winter Triticum aestivum

Abstract Secale cereale is a serious weed problem in winter Triticum aestivum–producing regions. The interference relationships and economic thresholds of S. cereale in winter T. aestivum in Colorado, Kansas, Nebraska, and Wyoming were determined over 4 yr. Winter T. aestivum density was held constant at recommended planting densities for each site. Target S. cereale densities were 0, 5, 10, 25, 50, or 100 plants m−2. Secale cereale–winter T. aestivum interference relationships across locations and years were determined using a negative hyperbolic yield loss function. Two parameters—I, which represents the percent yield loss as S. cereale density approaches zero, and A, the maximum percent yield loss as S. cereale density increases—were estimated for each data set using nonlinear regression. Parameter I was more stable among years within locations than among locations within years, whereas maximum percentage yield loss was more stable across locations and years. Environmental conditions appeared to have a role in the stability of these relationships. Parameter estimates for I and A were incorporated into a second model to determine economic thresholds. On average, threshold values were between 4 and 5 S. cereale plants m−2; however, the large variation in these threshold values signifies considerable risk in making economic weed management decisions based upon these values. Nomenclature: Secale cereale L. SECCE, rye; Triticum aestivum L., wheat.

Imazamox for Winter Annual Grass Control in Imidazolinone-Tolerant Winter Wheat

Field experiments were conducted at five locations in Kansas, Nebraska, and Wyoming to determine the effects of imazamox rate and application timing on winter annual grass control and crop response in imidazolinone-tolerant winter wheat. Imazamox at 35, 44, or 53 g ai/ha applied early-fall postemergence (EFP), late-fall postemergence, early-spring postemergence (ESP), or late-spring postemergence (LSP) controlled jointed goatgrass at least 95% in all experiments. Feral rye control with imazamox was 95 to 99%, regardless of rate or application timing at Hays, KS, in 2001. Feral rye control at Sidney, NE, and Torrington, WY, was highest (78 to 85%) with imazamox at 44 or 53 g/ha. At Sidney and Torrington, feral rye control was greatest when imazamox was applied EFP. Imazamox stunted wheat <10% in two experiments at Torrington, but EFP or LSP herbicide treatments in the Sidney experiment and ESP or LSP treatments in two Hays experiments caused moderate (12 to 34%) wheat injury. Wheat injury increased as imazamox rate increased. Wheat receiving imazamox LSP yielded less grain than wheat treated at other application timings in each Hays experiment and at Sidney in 2001. No yield differences occurred in one Torrington experiment. However, yields generally decreased as imazamox application timing was delayed in the other Torrington experiment. Generally, imazamox applied in the fall provided the greatest weed control, caused the least wheat injury, and maximized wheat yield. Nomenclature: Imazamox; feral rye, Secale cereale L. #3 SECCE; jointed goatgrass, Aegilops cylindrica Host # AEGCY; wheat, Triticum aestivum L. ‘CO980875’. Additional index words: Central Great Plains, herbicide-tolerant wheat, IMI-wheat. Abbreviations: EFP, early-fall postemergence; ESP, early-spring postemergence; KS-A, Hays, KS, experiment A; KS-B, Hays, KS, experiment B; LFP, late-fall postemergence; LSP, late-spring postemergence; MSO, methylated seed oil; NE, Sidney, NE; UAN, urea ammonium nitrate; WY-A, Torrington, WY, experiment A; WY-B, Torrington, WY, experiment B.

Carfentrazone Improves Broadleaf Weed Control in Proso and Foxtail Millets

Proso and foxtail millets are regionally important dryland crops for the semiarid portions of the Central Great Plains. However, few herbicides are registered for use in either crop. The efficacy of carfentrazone was studied in proso millet from 2003 through 2005 at the University of Nebraska High Plains Agricultural Laboratory located near Sidney, NE, and in foxtail millet in 2004 and 2005 at the University of Wyoming Sustainable Agriculture Research and Extension Center near Lingle, WY. Carfentrazone was applied POST at 9.0, 13.5, and 18.0 g ai/ha with combinations of 2,4-D amine, prosulfuron, and dicamba. Although leaves of treated plants exhibited localized necrosis, leaves emerging after treatment were healthy. Grain and forage yields were not affected by the application of carfentrazone. Dicamba and 2,4-D amine provided visual control of 30% or less for buffalobur. Adding carfentrazone to one or both of these herbicides improved buffalobur control to 85% or greater. Carfentrazone applied at 18.0 g/ha improved Russian thistle, kochia, and volunteer sunflower control in 2003, when plants were drought-stressed, but did not help with these and other weeds during wetter years. Carfentrazone provides proso millet producers with a way to selectively control buffalobur, a noxious weed in several western states. In foxtail millet, carfentrazone provides POST broadleaf weed control with little risk for serious crop injury. Crop injury has been a concern with 2,4-D, which is currently the only other herbicide registered for use in foxtail millet. Nomenclature: Carfentrazone, 2,4-D, dicamba, prosulfuron, buffalobur, Solanum rostratum Dun. SOLCU, kochia, Kochia scoparia (L.) Schrad. KCHSC, Russian thistle, Salsola iberica Sennen & Pau SASKR, foxtail millet, Setaria italica (L.) P. Beauv, proso millet, Panicum miliaceum L, sunflower, Helianthus annuus L

Evaluation of models predicting winter wheat yield as a function of winter wheat and jointed goatgrass densities

Abstract Three models that empirically predict crop yield from crop and weed density were evaluated for their fit to 30 data sets from multistate, multiyear winter wheat–jointed goatgrass interference experiments. The purpose of the evaluation was to identify which model would generally perform best for the prediction of yield (damage function) in a bioeconomic model and which model would best fulfill criteria for hypothesis testing with limited amounts of data. Seven criteria were used to assess the fit of the models to the data. Overall, Model 2, provided the best statistical description of the data. Model 2, regressions were most often statistically significant, as indicated by approximate F tests, explained the largest proportion of total variation about the mean, gave the smallest residual sum of squares, and returned residuals with random distribution more often than Models 1, and 3,. Model 2, performed less well based on the remaining criteria. Model 3, outperformed Models 1, and 2, in the number of parameters estimated that were statistically significant. Model 1, outperformed Models 2, and 3, in the proportion of regressions that converged on a solution and more readily exhibited an asymptotic relationship between winter wheat yield and both winter wheat and jointed goatgrass density under the constraint of limited data. In contrast, Model 2, exhibited a relatively linear relationship between yield and crop density and little effect of increasing jointed goatgrass density on yield, thus overpredicting yield at high weed densities when data were scarce. Model 2, had statistical properties that made it superior for hypothesis testing; however, Model 1s properties were determined superior for the damage function in the winter wheat–jointed goatgrass bioeconomic model because it was less likely to cause bias in yield predictions based on data sets of minimum size. Nomenclature:Jointed goatgrass, Aegilops cylindrica Host. AEGCY; winter wheat, Triticum aestivum L.

Glyphosate Susceptibility in Common Lambsquarters (Chenopodium Album) Is Influenced by Parental Exposure

Abstract Field studies were carried out at two sites in 2005 using common lambsquarters seed collected from long-term research plots near Scottsbluff, NE; Fort Collins, CO; and Torrington, WY, to determine the effect of herbicide selection pressure on glyphosate susceptibility. Parental herbicide exposure influenced the level of glyphosate susceptibility exhibited by a subsequent generation. Common lambsquarters selected from historical plots receiving continuous and exclusive use of glyphosate exhibited lower mortality in response to 420 g ae ha−1 glyphosate compared with selections from nonglyphosate treatment histories. Selections from rotating glyphosate treatment histories demonstrated an intermediate tolerance response. Differences in response were also influenced by environmental conditions. Nomenclature: Glyphosate; common lambsquarters, Chenopodium album L., CHEAL

Wheat Plant Density Influences Jointed Goatgrass (Aegilops cylindrica) Competitiveness1

Jointed goatgrass is a problem weed in winter wheat production areas of the Great Plains. Winter wheat seeding rates are easily adjusted by the growers and influence competition by some weeds. Field experiments were initiated in Kansas, Nebraska, and Wyoming using winter wheat cultivars selected from leading adapted cultivars from each region to determine the effect of wheat plant density in the fall on jointed goatgrass competitiveness. Three winter wheat seeding rates (50, 67, and 84 kg seeds/ha) were used at Hays, KS, and Sidney, NE, and four seeding rates (33, 50, 67, and 101 kg seeds/ha) were used at Torrington and Archer, WY. An analysis of covariance model was fit with winter wheat fall plant density as the covariate. In 1996, winter wheat grain contamination (dockage) was reduced at the rate of about 6% for every 10 additional wheat plants/m2 above the threshold density of 70 plants/m2 at Archer, WY, and at the rate of about 0.5% for every 10 additional wheat plants/m2 above the threshold density of 110 plants/m2 at Hays, KS. At Hays the reduction occurred only with the semidwarf cultivar ‘Vista’. Increased wheat density reduced jointed goatgrass reproductive tillers in four out of six location–year combinations and biomass in two out of four location–year combinations. Despite the lack of a consistent reduction in jointed goatgrass competitiveness as the result of increased wheat density, increased seeding rates may be a good, low-cost, long-term investment as part of an integrated jointed goatgrass control program in winter wheat. Nomenclature: Jointed goatgrass, Aegilops cylindrica Host. #3 AEGCY; winter wheat, Triticum aestivum L. Additional index words: Interference; plant competition.

Imazamox rates, timings, and adjuvants affect imidazolinone-tolerant winter wheat cultivars

Irrigated field experiments were conducted near Torrington, WY, during the 2001 to 2002 (year 1) and 2002 to 2003 (year 2) winter wheat growing seasons to evaluate cultivar response to different imazamox rates, adjuvants, and application timings. Five cultivars were treated postemergence in the early fall (EF), late fall (LF), or early spring (ES) with imazamox at 54 or 108 g ai/ha, including either nonionic surfactant (NIS) at 0.25% or methylated seed oil (MSO) at 1% (v/v) as adjuvants. A 28% urea ammonium nitrate solution at 1% (v/v) was included with all treatments. Spring injury was more severe in year 1 than year 2. Severe spring injury on ‘AP502 CL’, ‘Above’, ‘IMI-Fidel’, ‘IMI-Jagger’, and ‘IMI-Madsen’ was linked to fall application of 108 g/ha imazamox with MSO. Imazamox applied at 108 g/ha plus MSO applied in the fall consistently injured all cultivars more than the same rate with NIS and 54 g/ha imazamox regardless of adjuvant and timing, although severity of injury in the experiments differed between EF and LF timings in years 1 and 2, respectively. Correlation analysis supports injury reduced reproductive tillers per meter of row and wheat yields and increased the number of seeds per spike in year 1. The reduction of reproductive tillers per meter of row in year 1 was likely the result of severe injury caused by 108 g/ha imazamox applied in the EF coupled with little snow cover to protect against cold winter temperatures. Wheat yield in year 1 was reduced by 108 g/ha imazamox applied in the early fall; however, imazamox applied at 54 g/ha with either adjuvant in EF, LF, or ES were safe. Yield parameters and wheat yields in year 2 were not affected by imazamox rate, adjuvant, timing, or interactions of these factors. Nomenclature: Imazamox, 2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-5-(methoxymethyl)-3-pyridinecarboxylic acid; winter wheat, Triticum aestivum L. ‘Above’, ‘AP502 CL’, ‘IMI-Fidel’, ‘IMI-Jagger’, ‘IMI-Madsen’. Additional index words: Clearfield winter wheat. Abbreviations: AHAS, acetohydroxyacid synthase; ALS, acetolactate synthase; EF, early fall; ES, early spring; IMI, imidazolinone; LF, late fall; MSO, methylated seed oil; NIS, nonionic surfactant; RTMRs, reproductive tillers per meter of row; TKW, thousand kernel weight.

Common Sunflower (Helianthus annuus) and Green Foxtail (Setaria viridis) Interference in Dry Bean1

Field experiments were conducted in 1994 and 1995 under sprinkler irrigation at the University of Wyoming Research and Extension Center at Torrington to evaluate the effects of season-long interference and the effects of duration of interference of several common sunflower and green foxtail densities, alone or in combination, on pinto bean yield. Green foxtail densities did not significantly affect pinto bean yield in 1994 and reduced yield only at the highest density in 1995. In contrast, sunflower densities reduced pinto bean yield, except at the lowest density in 1994. Pinto bean yield was reduced as the combined density of green foxtail and sunflower increased. Compared with yield losses from each weed species alone, yield reductions from mixed species were additive in 1994 and at low weed densities in 1995 and less than additive at higher weed densities in 1995. The minimum number of weeds per m of row that will economically reduce pinto bean yield was estimated to be 1.6 to 2.9 for green foxtail and 0.12 to 0.2 for sunflower. Pinto bean yield reduction increased as the duration of green foxtail and sunflower interference increased, whether grown alone or in combination. The maximum duration that green foxtail, sunflower, and green foxtail plus sunflower can interfere with pinto bean before causing economical losses was estimated to be 4.5, 3.2, and 2.5 wk, respectively. Nomenclature: Common sunflower, Helianthus annuus L. #3 HELAN; green foxtail, Setaria viridis (L.) Beauv. # SETVI; pinto bean, Phaseolus vulgaris L. ‘Bill Z’. Additional index words: Competition, time of removal.

Canada Thistle (Cirsium arvense) Control in Established Alfalfa (Medicago sativa) Grown for Seed Production1

Canada thistle is one of the most troublesome and difficult weed species to control in established alfalfa grown for seed production. Current tools available for control are limited because of cultural management strategies associated with seed production. Alfalfa seed losses due to Canada thistle interference include both reduced yields from competition and increased seed loss during seed cleaning operations. Additional tools are needed to alleviate these losses. Field experiments were conducted in 1998, 1999, and 2000 at two locations in Park County, WY, to evaluate Canada thistle control and alfalfa tolerance to several postemergence herbicides. Bentazon, imazamox, imazethapyr, and MCPB were applied, alone or in combination, at different Canada thistle growth stages. Methylated seed oil (MSO) was added at 1.5% v/v to the treatments containing imazamox or imazethapyr. MCPB applied alone when Canada thistle was 7.5- or 15-cm tall caused severe alfalfa injury (28 to 40%) and resulted in less Canada thistle control (23 to 27%). Imazamox or imazethapyr applied alone when Canada thistle was 15-cm tall did not cause any significant alfalfa injury but resulted in unsatisfactory Canada thistle control (29 to 35%). Bentazon was the only treatment containing a single herbicide that provided more than 50% Canada thistle control. The treatments providing the best balance between Canada thistle control (>80%) and alfalfa injury (<13%) were a single application of bentazon combined with either imazamox or imazethapyr. These two treatments also yielded the highest, more than 800 kg/ha. Split applications of bentazon combined with imazamox or imazethapyr were similar to single applications. Nomenclature: Bentazon; imazamox; imazethapyr; MCPB; Canada thistle, Cirsium arvense (L.) Scop. #3 CIRAR; Alfalfa, Medicago sativa L. Additional index words: Application timing, herbicide mixtures, methylated seed oil, MSO, postemergence, weed management, CIRAR. Abbreviations: IMI, imidazolinone-resistant; MSO, methylated seed oil.