Alvin J. Bussan

Response of Setaria faberi demographic processes to herbicide rates

Abstract 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. Nomenclature: Nicosulfuron; sethoxydim; Setaria faberi, Herrm. SETFA, giant foxtail; Zea mays L., ‘Wyffels W549’ and ‘Dekalb 404SR’, corn; Glycine max (L.) Merr. ‘Dairyland DSR 250/STS’, soybean.

Implementing and conducting on-farm weed research with the use of GPS

Abstract 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. Nomenclature: Fenoxaprop; imazamethabenz; tralkoxydim; kochia, Kochia scoparia L. KCHSC; Russian thistle, Salsola iberica Sennen and Pau SASKR; spring wheat, Triticum aestivum L; wild oat, Avena fatua L. AVEFA.

Accuracy and cost effectiveness of GPS-assisted wild oat mapping in spring cereal crops

Abstract 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 (

Modeling the integrated management of velvetleaf in a corn–soybean rotation

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. Nomenclature: Wild oat, Avena fatua L. AVEFA.

Postharvest kochia (Kochia scoparia) management with herbicides in small grains

Abstract 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. Nomenclature: Corn, Zea mays L.; soybean, Glycine max (L.) Merr.; velvetleaf, Abutilon theophrasti Medikus, ABUTH; wilt, Verticillium dahliae Kleb.

Response of velvetleaf demographic processes to herbicide rate

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. Nomenclature: Glyphosate; paraquat; sulfentrazone; 2,4-D; kochia, Kochia scoparia (L.) Schrad. #3 KCHSC. Additional index words: Seed production. Abbreviations: AMS, ammonium sulfate.

Spring Wheat, Canola, and Sunflower Response to Persian Darnel ( Lolium persicum ) Interference

Abstract 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. Nomenclature: Dicamba; imazethapyr; corn, Zea mays L. ‘Wyffels W549’ and ‘Dekalb 404SR’; soybean, Glycine max (L.) Merr. ‘Dairyland DSR 250/STS’; velvetleaf, Abutilon theophrasti Medicus ABUTH; wilt, Verticillium dahliae Kleb.

Wild oat (Avena fatua) habitat and water use in cereal grain cropping systems

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. Nomenclature: Persian darnel, Lolium persicum Boiss. & Hoh. #3 LOLPS; canola, Brassica napus Koch.; spring wheat, Triticum aestivum L.; sunflower, Helianthus annuus L. Additional index words: Modeling, rooting depth, seeding rate, seedling establishment, weed interference, yield components, yield loss. Abbreviations: MSU, Montana State University; NGP, Northern Great Plains.

Modeling the integrated management of giant foxtail in corn–soybean

Abstract 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. Nomenclature: Wild oat, Avena fatua L. AVEFA; spring wheat, Triticum aestivum L.

Persian Darnel (Lolium persicum) Fecundity Response to Spring Wheat, Canola, and Sunflower Interference

Abstract 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. Nomenclature: Corn, Zea mays L.; giant foxtail, Setaria faberi Herrm. SETFA; soybean, Glycine max (L.) Merr.