The response of oat, foxtail millet, proso millet, and sunflower to atrazine and clomazone applied the previous fall was investigated two years in field studies near Akron, CO and Sidney, NE. Foxtail millet biomass, and proso millet and sunflower grain yields were not reduced when these crops were seeded into soil that had been treated the previous fall with atrazine and/or clomazone at rates of 0.6 or 1.1 kg ai/ha. Forage yield of oat was reduced 11 to 18% by some treatments, but this effect was not consistent over years or sites. Treatments containing 1.1 kg/ha of atrazine provided 1 to 5 wk of residual weed control in foxtail millet, proso millet, and sunflower. The study indicated that producers have flexibility in crop selection when using atrazine and clomazone in reduced- and no-till production systems.

Jointed goatgrass and downy brome continue to plague winter wheat producers in the western United States. Because there are no effective herbicides for in-crop control of these weeds, producers are seeking cultural practices that stimulate seed germination and deplete the soil seed bank. We determined the effect of limited tillage and crop canopy on seedling emergence of these grasses. One tillage operation with a sweep plow increased jointed goatgrass seedling emergence 74% in the first year but did not affect emergence in later years. Downy brome emergence was not affected by tillage. Jointed goatgrass seedlings emerged over 5 yr, whereas downy brome did not emerge after 3 yr. Seedling emergence of both species was two times greater in corn and barley than in proso millet. Producers will accrue more benefit for seedbank management with cultural strategies such as alternative rotations and competitive wheat canopies than with limited tillage using a sweep plow.

Producers in the central Great Plains are seeking alternative strategies to manage weeds because of herbicide resistance, narrow profit margins, and a lack of registered herbicides in some crops. Thus, we evaluated the impact of cultural systems in winter wheat and tillage on weed dynamics in corn, sunflower, and proso millet planted the year following wheat harvest. Weed seedling densities were 25 to 30% lower in a no-till system compared with minimum-till with a sweep plow. With no-till, cultural systems that produced more crop residue reduced weed densities an additional 15 to 30%, compared with the conventional system. Cultural system effect was eliminated in dry years and by tillage. Weed biomass was 10-fold less in proso millet than in corn. Crop residue management, critical for successful cropping in semiarid regions, also will help weed management by reducing weed density in summer annual crops, especially in no-till systems.

Field studies were conducted from 1990 through 1997 to evaluate the long-term effect of 2- and 3-yr rotations on the control of downy brome, jointed goatgrass, and feral rye in winter wheat. At the completion of the study, jointed goatgrass and feral rye densities averaged 8 plants/m2 and < 0.1 plant/m2 for the 2- and 3-yr rotations, respectively. Downy brome densities averaged < 0.5 plant/m2 for both the 2- and 3-yr rotations, with no treatment differences observed. Winter annual grasses were not eradicated after two cycles of the 3-yr rotations, but weed densities were reduced 10-fold compared to densities after one cycle and more than 100-fold compared with the 2-yr rotations. Wheat grain contamination with dockage and foreign material followed a similar trend. The 3-yr rotations were economically competitive with 2-yr rotations and provided superior control of the winter annual grass weeds.

Producers need alternative methods to manage jointed goatgrass because there are no herbicides that selectively control jointed goatgrass in winter wheat. The effect of crop residue incorporated in soil on reducing seedling growth of jointed goatgrass was examined in the greenhouse. Residues of corn, proso millet, safflower, grain sorghum, and winter wheat reduced fresh weight of jointed goatgrass by 70 to 85%. Applying N fertilizer at 33 or 66 kg ha−1 diminished this effect, indicating that residue stimulated microbial immobilization of N. Ethiozin applied preemergence at 0.2 μg g−1 soil reduced jointed goatgrass seedling growth by 50%, but combining crop residue with ethiozin did not synergistically improve control of jointed goatgrass. These results suggest that producers may favor winter wheat growth over jointed goatgrass by incorporating crop residue in soil with tillage before planting and by banding N with winter wheat seed at planting.

Proso millet is a short-season summer annual grass that is well adapted to the central Great Plains. Proso millet is commonly planted as a summer crop when winter wheat stands are lost due to adverse conditions. Sulfonylurea herbicides labeled for use in winter wheat prohibit planting proso millet for intervals up to 10 mo following application. A series of greenhouse and field studies determined proso millet tolerance to CGA-152005, metsulfuron, and triasulfuron soil residue. In the greenhouse, proso millet was not affected by soil-applied CGA-152005 at doses up to 160 g ai/ha, while metsulfuron and triasulfuron doses of 4 and 15 g ai/ha, respectively, inhibited proso millet biomass accumulation. In the field, metsulfuron and triasulfuron caused early season stunting and chlorosis at doses two to four times those recommended; however, grain yields were not affected. Organic matter and clay content were highly correlated with proso millet growth response to the herbicides under greenhouse conditions, but in the field, soil pH may have influenced herbicide bioavailability.

Dryland rotations are changing in the semiarid Great Plains because of no-till systems. Producers now rotate summer annual crops such as corn with winter wheat and fallow, which can disrupt weed population growth because of diverse life cycles among crops. This study estimated changes in weed populations as affected by rotation design, with the goal of suggesting crop sequences that lower weed community density. We used an empirical life-cycle simulation based on demographics of jointed goatgrass and green foxtail to compare various rotations consisting of winter wheat, corn, proso millet, and fallow across a 12-yr period. The simulation indicated that designing rotations to include a 2-yr interval when seed production of either jointed goatgrass or green foxtail is prevented will drastically reduce weed populations. Arranging four different crops in sequences of two cool-season crops, followed by two warm-season crops was the most beneficial for weed management. Fallow, if used, serves in either life-cycle category. However, if the same crop is grown 2 yr in a row, such as winter wheat, the benefit of rotation design on weed density is reduced considerably. Impact of rotation design on weed density was enhanced by improving crop competitiveness with cultural practices. Rotations with balanced life-cycle intervals not only reduce weed density but enable producers to use alternative weed management strategies, improve effectiveness of herbicides used, and minimize herbicide resistance.

Producers in the semiarid Great Plains are exploring alternative crop rotations, with the goal of replacing winter wheat–fallow. In 1993, a study was established to compare performance of eight rotations comprised of various combinations with winter wheat (W), spring wheat (SW), dry pea (Pea), safflower (Saf), corn (C), sunflower (Sun), proso millet (M), or fallow (F). After 8 years, we characterized weed communities by recording seedling emergence in each rotation. Seventeen species were observed, with downy brome, kochia, horseweed, and stinkgrass comprising 87% of the community. Rotations with the least number of weed seedlings were W–F and SW–W–C–Sun; in comparison, weed density was six-fold higher in W–M. Density of downy brome and kochia was highest in W–M compared with other rotations, whereas stinkgrass and green foxtail were prominent in proso millet of the W–M and W–C–M rotations. Horseweed established readily in safflower and dry pea. In the semiarid Great Plains, designing rotations in a cycle of four that includes cool- and warm-season crops can be a key component of integrated weed management.

Herbicide efficacy, coverage, and retention were evaluated for spray applied through Drift Guard, Turbo TeeJet, AI TeeJet, and TurboDrop drift-reducing nozzles compared to a conventional flat-fan nozzle. Percentage spray coverage detected on water-sensitive cards was greater for conventional and Drift Guard nozzles than for Turbo TeeJet, AI TeeJet, and TurboDrop nozzles. Spray without adjuvants was retained better by redroot pigweed for treatments applied with conventional and Drift Guard nozzles than Turbo TeeJet, AI TeeJet, and TurboDrop nozzles. However, spray with adjuvants was retained similarly for all nozzle types when averaged over spray adjuvant and two weed species. The efficiency of spray retention was greater for spray applied in 47 than in 190 L/ha spray volume for all nozzles. Paraquat and glyphosate, representing contact and translocated herbicides, respectively, provided similar grass species control for all nozzle types, regardless of spray volume. Paraquat and glyphosate were also equally or more effective in 47 compared to 190 L/ha spray volume.

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 gai/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.

Proso millet is an important short-season summer cereal in western Nebraska, southeast Wyoming, and eastern Colorado. The objective of this study was to evaluate proso millet tolerance to saflufenacil applied early preplant (EPP) or PRE. Field studies were conducted in Lingle, WY and Sidney, NE in 2008 and 2009. A dose–response study was conducted in the greenhouse at the University of Wyoming in Laramie, WY to determine proso millet cultivar response to saflufenacil applied at six rates from 0 to 400 g ai ha−1. In the field, saflufenacil was applied EPP and PRE at 50 and 100 g ha−1. Proso millet stands were reduced by an average of 33 and 23% by PRE and EPP treatments compared with the nontreated check; however, proso seed yields were not affected by saflufenacil timing or rate. In the greenhouse, ‘Panhandle’ and ‘Dawn’ exhibited less tolerance to saflufenacil than ‘Sunrise’, the cultivar used in the field studies.

MON 37500 is a sulfonylurea herbicide that selectively controls Bromus spp. in winter wheat. Field studies were conducted near Sidney, NE, and Archer, WY, to determine the sensitivity of corn, foxtail millet, grain sorghum, proso millet, and sunflower to soil residues of MON 37500. MON 37500 was applied to winter wheat at 0, 35, 69, and 139 g/ha in the autumn of 1997. Rotational crops were no-till seeded into the standing residues of the previous year's crop from 1999 through 2001. Grain yields for corn, foxtail millet, and proso millet planted 18 to 20 mo after herbicide application were not affected by soil residues of MON 37500. In contrast, average grain yields of grain sorghum were reduced from 1,760 to 30 kg/ha at Archer and from 4,480 to 390 kg/ha at Sidney as MON 37500 rates increased from 0 to 139 g/ha. Thirty to 32 mo after herbicide application, average grain yields of grain sorghum were reduced from 2,360 to 620 kg/ha at Sidney and average aboveground biomass was reduced from 4,000 to 1,800 kg/ha at Archer as MON 37500 rates increased from 0 to 139 g/ha. Nineteen to 20 mo after herbicide application, average sunflower seed yields were reduced from 1,450 to 20 kg/ha at Archer and from 1,830 to 540 kg/ha at Sidney as MON 37500 rates increased from 0 to 139 g/ha. Visual injury was observed 31 to 32 mo after herbicide application, but drought in 2000 prevented collection of seed yield data. In the High Plains, foxtail millet, proso millet, and corn may be successfully grown 18 to 20 mo after the application of MON 37500 to winter wheat. Successful production of grain sorghum and sunflower may require a minimum recrop interval between treatment and planting of >36 mo.