In the first of 2 field studies, weed biomass and soybean seed yield were used to evaluate 16 soybean genotypes for competitive ability against 12 weed species at Rosemount, MN, in 1992 and 1993. The yield and ranking of soybean genotypes often varied with the weed species. Grass weed species reduced yields the most, and small-seeded broadleaf weeds reduced yields the least across years. ‘Parker’ was highly competitive, as it suppressed weed biomass and produced high soybean yield. ‘Kato,’ ‘Kasota,’ ‘Dawson,’ and ‘Glenwood’ minimized weed biomass and maintained soybean yield while in competition with grass weeds but yielded poorly relative to other soybean genotypes in weed-free conditions. ‘Lambert’ produced high soybean yield in weed-free conditions, but yield dropped markedly when in competition with grass weeds. ‘Grande,’ ‘Heifeng 25,’ and ‘Norman’ soybeans were poor competitive genotypes in weedy situations and low yielding in weed-free conditions. A 2nd field study conducted at Rosemount and St. Paul, MN, during 1993 evaluated 16 soybean genotypes under 4 levels and durations of weed pressure for weed competitiveness. Parker, ‘Sturdy,’ and M89-794 were most competitive in suppressing weed biomass and producing high yields. Lambert yielded fairly well but allowed high weed biomass. M89-1743, M89-1006, ‘Archer,’ and ‘Ozzie’ yielded poorly and did not sup press weed biomass production. No relationship was found between weed competitiveness and soybean canopy area, height, and volume measured 30–45 d after planting (DAP).

There is significant interest from processors in producing organic sweet corn and snap bean. However, large-scale production is necessary for this to be a practical and economical venture for processors. This study focused on the feasibility of managing weeds in organic sweet corn and snap bean, utilizing methods that are practical in large hectarage. Tactics such as rotary hoe, interrow cultivation, and a stale seedbed were evaluated alone or in combination. Hand-weeded and herbicide-based treatments were included for each crop for comparison. Percentage weed control, weed biomass, and crop yield were quantified, and net profit was calculated for each treatment. Organic weed management was feasible in snap bean, with yields similar among several of the organic treatments and the herbicide treatment in all 3 yr of the study. Interrow cultivation was the most effective means of organic weed control in snap bean. Organic weed management was possible in snap bean because it is a short-season crop and an effective competitor with weeds in the crop row. Organic weed management was more difficult in sweet corn because of the longer crop season and poor competition with weeds in the crop row. In sweet corn, the organic treatment involving three interrow cultivations was the only one consistently similar in yield to the herbicide treatments. Higher net profits were attained for most of the organic treatments in both crops because of the organic premium. Market saturation and organic premium adjustments are factors for grower consideration in this potential industry, particularly for sweet corn production.

Integrated weed management practices, such as crop rotation and increased seeding rates, potentially improve weed management. Yet, few studies compare competitive interactions of weeds with different crops. This research quantified the impact of Persian darnel on spring wheat, canola, and sunflower yield across different seeding rates. Increasing crop density increased yield when Persian darnel affected crop yield early in physiological development. Crop yield loss was estimated to reach 83, 70, and 57% for spring wheat, canola, and sunflower, respectively, at high Persian darnel densities. Persian darnel reduced spring wheat yield by limiting the number of tillers per plant and seed per tiller; reduced canola yield by limiting the number of branches per plant, pods per branch, and seed per pod; and reduced sunflower yield by limiting the number of seed per plant. Persian darnel affected crop growth early in physiological development, indicating that interspecific interference occurred early in the growing season. Cultural and resource management aimed at reducing Persian darnel impact on resource availability and crop yield components will reduce Persian darnel impact on crop yield.

Managing weed infestations in a spatially precise manner requires accurate and cost-effective weed identification techniques. The goal of our research was to quantify the accuracy of continuous weed presence–absence maps and assess how management based on those maps may affect producer net returns. Each continuous sampled map covered the entire field and contained vector polygons labeled as either wild oat presence or wild oat absence. The accuracy of the continuous wild oat maps at each sampling time was determined from georeferenced quadrats of wild oat densities. The accuracy of the continuous wild oat seedling maps ranged from 48.3 to 87.1% among the six site-years. The accuracy of the wild oat seedling maps improved by at least 8% when a 10-m buffer was included around areas mapped as wild oat presence. The accuracy of continuous wild oat panicle maps from the combine at harvest ranged from 65.8 to 90.9% among the six site-years. The variation in accuracy for the wild oat seedling maps among sites was greater than the accuracy of the panicle maps. Net returns ($ ha−1) for four site-years were calculated and compared for four possible weed management approaches on each field. A site-specific herbicide application to areas mapped as wild oat presence always generated higher net returns than a herbicide application over the entire field for four sites. A site-specific herbicide application to areas mapped as wild oat presence plus a surrounding 10-m buffer area only resulted in a higher net return in one of the 12 site-years compared with a site-specific herbicide application without the 10-m buffer. This site had the lowest (48.3%) wild oat seedling map accuracy, and uncontrolled wild oat had a high-yield effect. This research indicates that using a continuous weed sampling method based on presence or absence for site-specific herbicide application can be profitable over a herbicide application to the entire field, even with the associated technology cost and seedling map errors.

The objectives of this study were to model the influence of herbicides, wilt disease, and mechanical treatments on velvetleaf population dynamics, annualized net return (ANR), and economic optimum threshold (EOT) in a 20-yr rotation involving alternate years of corn and soybean. Mechanical treatments were interrow cultivation in corn and rotary hoeing in soybean. Herbicides at a quarter (¼×) rate or lower did not reduce velvetleaf seed banks without mechanical treatments in the absence of wilt. Herbicides at full (1×) and half (½×) rates decreased velvetleaf seed banks 95% within 6 and 20 yr, respectively, when there was no wilt. Herbicides at ½× rates with mechanical treatments reduced the seed bank 95% in only 10 yr, but mechanical treatments did not increase the rate of seed bank decline with 1× rates. Wilt infection had to occur annually to reduce velvetleaf seed banks as effectively as herbicides at 1× rates alone. ANR was maximized with herbicides at reduced rates, even though they were not as effective at reducing seed banks as were 1× rates. The herbicide rate required to maximize ANR increased as the initial velvetleaf seed bank density increased. Mechanical treatments and wilt decreased the herbicide rate required to maximize ANR. In fact, wilt infection increased the ANR of herbicides at reduced rates. The EOT was 0.55 and 0.4 seedlings m−2 when velvetleaf was managed with herbicides at 1× and ½× rates, respectively. Mechanical treatment had no effect on EOT, but wilt increased the EOT. Herbicides at reduced rates should only be used to manage velvetleaf in fields with a low seed bank density when integrated with mechanical treatments or when the field has a history of wilt. Herbicides should be used at 1× rates when fields have a large velvetleaf seed bank and when integrated management practices are not used.

Glyphosate-resistant crop species have increased in number over the past decade as growers eagerly adopt this simple and effective weed management technology. Glyphosate-resistant wheat cultivars are being developed and may soon be available to growers. The objective of this paper is to discuss the pest management implications of glyphosate-resistant wheat in the western United States, a region stretching from the Great Plains to the Pacific Ocean that produces more than 80% of the nation's wheat crop. The benefits of glyphosate-resistant wheat include: (1) improved weed control, particularly of difficult-to-control weeds, such as winter annual grasses belonging to the Aegilops, Avena, Bromus, Lolium, Poa, Secale, and Setaria genera; (2) an ability to control weeds resistant to currently available wheat herbicides; (3) an extended application window for control of late-emerging weeds; and (4) improved crop safety. Although these benefits are not to be minimized, they need to be considered in the light of the concerns surrounding this new technology in wheat. These concerns are about (1) the lack of an equally effective and affordable herbicide to control glyphosate-resistant volunteer wheat, which may increase wheat diseases such as wheat streak mosaic and Rhizoctonia root rot; (2) the possibility that overreliance on glyphosate will lead to species shifts, with unknown consequences for weed management in wheat; and (3) the use of multiple glyphosate-resistant crops in rotation with glyphosate-resistant wheat, which could rapidly increase glyphosate-resistant weeds, thereby limiting the future utility of glyphosate. If, or when, glyphosate-resistant wheat becomes commercially available, it will require careful management to sustain its usefulness. We have proposed several areas of research that we feel are critical to help develop sound management guidelines for deployment and use of this new weed management technology in wheat. These include (1) developing effective “green bridge” management strategies, i.e., using cultural and chemical approaches to control plants that sustain insect vector populations between wheat crop periods; (2) predicting potential weed species shifts resulting from the use of glyphosate-resistant wheat; and (3) developing management systems that include herbicide-resistant wheat on a rotational basis and rotating the use of glyphosate with other weed management strategies in the fallow period to minimize the potential development of glyphosate-resistant weeds or weed communities.

The objectives of this study were to use a computer simulation model to predict the influence of herbicides and mechanical treatments on giant foxtail population dynamics, annualized net return (ANR), and the giant foxtail economic optimum threshold (EOT) in a corn–soybean rotation over 20 yr. Mechanical treatments were interrow cultivation in corn and rotary hoe in soybean. Herbicides at full (1 ×) and half (½ ×) rates applied alone reduced giant foxtail seedbank 95% within 4 and 8 yr, respectively. Predicted seedbank dynamics had more variability when managed with herbicides at ½ × than at 1 × rates applied alone. Mechanical treatments integrated with herbicide at ½ × rates resulted in giant foxtail seedbank and variability similar to herbicides at 1 × rates applied alone. ANR was maximized when herbicides were applied between ⅜ × and 9/16 × rates applied alone. As initial giant foxtail density increased from 100 to 10,000 seeds m−2, the herbicide rate that maximized ANR increased. Economic optimum thresholds (EOTs) did not vary when herbicides were applied at different rates, but integrating mechanical treatment with herbicides increased the EOT from 0.1 to 0.7 seedlings m−2. Sensitivity analysis determined that giant foxtail seedbank demographics, seedling survival, and seed production per plant had the most influence on model predictions. Model sensitivity varied little between 1 × and ½ × rates. Integrating herbicides and mechanical treatment decreased the sensitivity of the model to perturbations in parameter estimates. Herbicides at reduced rates were more profitable over the long term than 1 × rates, but risk of herbicide failure increased as rate decreased. Integration of herbicides applied at reduced rates with mechanical treatments increased ANR and minimized the risk of herbicide failure compared to herbicides applied at 1 × rates alone.

Uncontrolled kochia plants that regrow after small-grain harvest can produce substantial numbers of seeds. An average of 4,100 seeds per plant were produced between harvest (late July to mid August) and the first killing frost (late September) at three locations in Montana. Field experiments were conducted to determine the optimal timing of postharvest herbicide applications to prevent kochia from producing viable seeds. Herbicide treatments were applied at three timings from late August to mid September. The most effective treatments were glyphosate (631 g/ha) and paraquat (701 g/ha) applied at the second application timing (late August to early September). These treatments reduced kochia seed production by 92% or greater at each site. Kochia regrowth by this time had sufficient leaf area for herbicide absorption, but few viable seed had been produced. Herbicide treatments at the first and third application timings were generally less effective and more variable in reducing kochia seed production. Sulfentrazone (157 g/ha) and 2,4-D (561 g/ha) were not as effective at reducing seed production as other herbicide treatments.

Field studies were conducted in 1995 and 1996 to determine the rate response of velvetleaf seedling survival, seed production, and shoot biomass to postemergence herbicides in corn and soybean. Dicamba and imazethapyr were applied to corn and soybean, respectively, at 1, ½, ¼, ⅛, 1/16, 1/32, and 0× labeled rates. Velvetleaf mature plant density was linearly related to seedling density, thus indicating that seedling survival was not density dependent, even after seedling densities exceeded 150 plants m−2. Seedling survival as influenced by herbicide was described by a dose–response curve in corn and soybean. In corn, seedling survival ranged from 0 to 48% across herbicide treatments and years. Seedling survival was greater at the ½× or lower herbicide rates than at the 1× rate. In soybean, maximum seedling survival was 61 and 14% in 1995 and 1996, respectively, and minimum seedling survival was less than 2% in each year. Seedling survival was less in 1996 than in 1995 because velvetleaf was infected with wilt in 1996. In soybean, seedling survival was 20 times greater when treated with herbicides at the ½× rate than when treated at the 1× rate in 1995, but seedling survival was similar when herbicides were applied at 1, ½, ¼, and ⅛× rates in 1996. Velvetleaf fecundity (seeds per plant) was dependent on mature plant density in 1995 but was density independent in 1996. Fecundity as influenced by herbicide was described by dose–response curves in corn each year and in soybean in 1995. In 1995, velvetleaf treated with herbicides at ½× and ¼× rates produced 20 to 30 times more seed per square meter than when treated with herbicides at the 1× rate. Differences in seed per square meter were exaggerated by high densities of velvetleaf. Seed per square meter did not differ between velvetleaf treated with herbicides at 1× or ½× rates in corn or soybean in 1996. Wilt infection of velvetleaf in 1996 was the likely cause of differences in herbicide performance between years. Herbicides at reduced rates were not effective at limiting seedling survival and seed production compared to 1× rates in the absence of wilt. As a result, long-term management of velvetleaf with herbicides at reduced rates likely will be difficult, especially in areas with high densities, unless integrated with other management practices.

Traditionally, herbicide efficacy has been evaluated by visual ratings, but these data provide little insight to the biological response of weeds to herbicides. Field studies were conducted in 1995 and 1996 to determine the rate response of Setaria faberi seedling survival, seed production, and biomass to postemergence herbicides in Zea mays and Glycine max. Nicosulfuron and sethoxydim were applied to Z. mays and G. max, respectively, at 1×, 12×, 14×, 18×, 116×, 132×, and 0× the label rate. Mature plant density of S. faberi was linearly related to seedling density, indicating that seedling survival was not density dependent. Based on a nonlinear dose–response analysis, maximum S. faberi survival was 55% in Z. mays across years and 60 and 45% in G. max in 1995 and 1996, respectively. Minimum survival was 0% except for Z. mays in 1996 when it was 13%. The minimum survival was greater in Z. mays in 1996 due to greater survival of late cohorts than in 1995. Setaria faberi seedling survival was greater in 12× than 1× herbicide treatments in Z. mays and G. max each year. Setaria faberi seed production was related to mature plant density with a negative exponential function. Seed production per plant was similar between 1× and 12× rates in Z. mays and among 1×, 12×, and 14× rates in G. max each year. However, seed production per square meter was greater in 12× than 1× treatments due to greater seedling survival. Regardless, seed production per square meter was 95% less in the 12× herbicide treatment compared to seed production by untreated plants in Z. mays and G. max.

The adoption of precision technologies that spatially register measurements using global positioning systems (GPS) greatly facilitates conducting large-scale on-farm research by farmers. On-farm experiments that utilize producer equipment include variations in agronomic practices that occur in situations where we want to predict the effect of inputs on yield. The domain of inference for such on-farm studies therefore more closely matches that desired by researchers. To investigate the feasibility of on-farm research using GPS, a study was conducted to evaluate the potential benefit of site-specific weed management. The study utilized producer-maintained field-scale equipment on four Montana farms in dryland spring wheat production. Paired site-specific and whole-field herbicide treatment areas were established in 0.9 to 1.9-ha blocks using consultant weed maps and a geographic information system (GIS). Yield was unaffected by herbicide treatment strategy (site-specific or broadcast). Minimal detectable yield differences were evaluated for the experimental design (0.2 T ha−1). Net returns increased when the percentage of field infested by wild oat decreased. Visual ratings of wild oat density taken at harvest indicated no difference in wild oat control between treatments in two of four site-years. This research suggests that producer-owned equipment can be used to compare treatments, but the accuracy and subsequent power of such comparisons are likely to be low.

The advent of site-specific weed management has generated research aimed at predicting weed spatial distributions from existing weed maps or correlations with soil properties and edaphic factors. Forecasting the spatial distribution of annual weeds requires knowledge of fecundity, dispersal, management, and suitable habitat distribution. We hypothesized that wild oat habitat was limited by field-scale heterogeneity in plant-available water. We eliminated seed number and dispersal limitations by seeding wild oat in areas with and without historical wild oat patches in three similarly managed spring wheat fields that differed in soil properties and wild oat infestations and were situated within a 160-km radius of Great Falls, MT. Wild oat habitat was quantified by wild oat leaf area growth rate, mature shoot biomass, seeds produced per plant, biomass water use efficiency, and competitive ratio with spring wheat. Soil texture and plot elevation correlated with existing wild oat patch areas in individual fields, but no site properties consistently correlated with wild oat patch areas in all three fields. Soil water use (SWU) and almost all habitat-defining variables for wild oat were similar between historic patch and nonpatch areas. Wild oat grew and produced seed regardless of existing patch boundaries and field-scale heterogeneity in SWU. This research suggested that (1) wild oat habitat may be unlimited in cereal grain cropping systems of the Northern Great Plains and (2) soil properties are a poor predictor of weed distribution for a generalist such as wild oat.

Persian darnel control options are limited and unmanaged populations can cause substantial crop yield loss. Integrating crop diversification and higher crop seeding rates into a cropping system might improve Persian darnel management. Field experiments were conducted to determine the effect of different crops and increased crop seeding rates on Persian darnel fecundity. Persian darnel produced up to 2,800 seeds per plant and 53,000 seeds/m2 when grown without competition. Increasing crop density reduced Persian darnel tillers per plant, seed weight, and fecundity. Increasing crop density reduced Persian darnel fecundity 0.4 to 0.2% per spring wheat plant, 0.6 to 0.1% per canola plant, and 16 to 8% per sunflower plant. Persian darnel fecundity was impacted the greatest by reduced weed seedling establishment, which was caused by crop competition and seeding sunflower late in the spring preceded by a nonselective herbicide application. Results indicated delaying the seeding of spring crops or including a late-seeded warm season crop, like sunflower or safflower, in the cropping system is an effective weed management tool for reducing Persian darnel fecundity.