Chris M. Boerboom

Critical time of weed removal in glyphosate-resistant Glycine max

Abstract Field experiments were conducted in 1996 and 1997 to determine the effect of the rate and time of glyphosate application on weed emergence, survival, biomass, and Glycine max yield in reduced-tillage (RT) and no-tillage (NT) glyphosate-resistant G. max planted in rows spaced 18 (narrow-row) and 76 cm (wide-row). Glyphosate was applied at 0.42, 0.63, and 0.84 kg ae ha−1 at V2, V4, R1, and R4 growth stages. On separate plots, 0.84 kg ha−1 glyphosate was applied at each growth stage with hand weeding. A weed-free check was maintained with preemergence imazethapyr plus metolachlor supplemented with hand weeding, and a nontreated check was included. Weed population density before glyphosate application ranged from 239 to 606 plants m−2 in RT and 33 to 500 plants m−2 in NT systems. Setaria faberi and Chenopodium album were the predominant species. Weed control efficacy and crop yield were influenced more by application time than by glyphosate rate. Glyphosate applied at V2, V4, and R1 gave season-long control of weeds in 18-cm rows. In 76-cm rows, glyphosate applied at V2, V4, and R1 gave almost complete control of weeds, but broadleaf weeds emerged after application at V2. The critical time of weed removal, the time beyond which weed competition reduced G. max yield by 3% or more compared to the weed-free check, was at R1 and V4 in 18-cm RT G. max in 1996 and 1997, respectively, and at V2 in 76-cm RT G. max in both years. The predicted critical time of weed removal in 18- and 76-cm NT G. max was R1 and V4, respectively, in 1996 and R1 in 1997. This research showed that there was variation in the onset of the critical time of weed removal between tillage systems, as well as within tillage systems across years. The results indicate a single glyphosate application can prevent yield loss in narrow-row, glyphosate-resistant G. max under favorable conditions, but application timing becomes more critical in wide rows because the critical period of weed removal occurs earlier. Late-emerging weeds may warrant a second glyphosate application in wide-row G. max. Nomenclature: Glyphosate; imazethapyr; metolachlor; Chenopodium album L. CHEAL, common lambsquarters; Setaria faberi Herrm. SETFA, giant foxtail; Glycine max (L.) Merr., ‘Asgrow experimental 19505’, soybean.

Efficacy and Tolerance to HPPD-Inhibiting Herbicides in Sweet Corn

Abstract The use of POST herbicides has been limited in sweet corn because of the narrow spectrum of weed control or potential crop injury. Field experiments were conducted to evaluate the 4-hydroxyphenyl pyruvate dioxygenase (HPPD)-inhibiting herbicides mesotrione, tembotrione, and topramezone applied POST in sweet corn at three locations. Efficacy of mesotrione, tembotrione, and topramezone applied alone or mixed with atrazine was compared to other labeled POST herbicides following PRE S-metolachlor. Giant foxtail control was greater with tembotrione or topramezone than mesotrione alone or mixed with atrazine. Common lambsquarters, velvetleaf, and common ragweed were controlled 98% or greater with the HPPD-inhibiting herbicides when mixed with atrazine. Tolerance of six sweet corn hybrids was determined in the field when treated with 1× and 2× rates of these herbicides mixed with atrazine. Tolerance of six sweet corn hybrids to these herbicides was determined in the greenhouse when treated with 0.5, 1, 2, 4, 8, and 16 times the labeled rate. Differential hybrid tolerance to each herbicide was observed in both the field and greenhouse evaluations. Tembotrione killed ‘Merit’ in both evaluations. Excluding Merit, hybrids generally had good tolerance to tembotrione and topramezone in the field, but had differential tolerance to mesotrione. With the exception of Merit, hybrids generally had greater tolerance to tembotrione than topramezone and less tolerance to mesotrione in the greenhouse. These HPPD-inhibiting herbicides provide POST weed control, but the potential for sweet corn injury varies among the herbicides and hybrids and warrants further characterization. Nomenclature: Atrazine; mesotrione; S-metolachlor; tembotrione; topramezone;common lambsquarters, Chenopodium album L. CHEAL; common ragweed, Ambrosia artemisiifolia L. AMBEL; giant foxtail, Setaria faberi Herrm. SETFA; velvetleaf, Abutilon theophrasti Medicus ABUTH; corn, Zea mays L

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.

Weed species–area relationships as influenced by tillage

Abstract The relationship between species richness and sample area has been characterized in many natural communities but has rarely been examined in crop–weed communities. We determined the species–area relationship in short-term (≤4 yr) and long-term (>15 yr) moldboard-plowed (MP), chisel-plowed (CP), and no-tillage (NT) fields cropped to corn and in short-term MP, CP, and NT fields cropped to soybean. A total of 10 corn fields and 10 soybean fields were sampled for species richness in 14 nested sample areas that ranged from 0.0625 to 512 m2. The influence of sample area on frequency of species occurrence was also determined. Species richness was greater in long-term NT fields than in tilled or short-term NT fields. The species–area relationship in tilled and short-term NT fields was best described by an exponential function. In contrast, a power function was the best fit for the species–area relationship in long-term NT fields. The functional minimum area required to represent 75% of the total weed species in tilled and short-term NT fields was 32 m2. A functional minimum area could not be determined in long-term NT fields because species richness continued to increase over the range of sample areas. Regression functions predicted that sample areas of 1 m2 would contain less than 50% of the observed maximum species richness in these fields. Sample areas of 36 m2 in tilled and short-term NT fields and 185 m2 in long-term NT fields were predicted to measure 75% of observed maximum species richness in these fields. Pigweed species and common lambsquarters occurred at high frequencies and were detected in most sample areas. This information could be used to better define sample area requirements and improve sampling procedures for species richness of weed communities. Nomenclature: Common lambsquarters, Chenopodium album L. CHEAL; corn, Zea mays L.; pigweed species, Amaranthus spp.; soybean, Glycine max (L.) Merr.

Residual Weeds of Processing Sweet Corn in the North Central Region

Abstract Knowledge of weed community structure in vegetable crops of the north central region (NCR) is poor. To characterize weed species composition present at harvest (hereafter called residual weeds) in processing sweet corn, 175 fields were surveyed in Illinois, Minnesota, and Wisconsin from 2005 to 2007. Weed density was enumerated by species in thirty 1-m2 quadrats placed randomly along a 300- to 500-m loop through the field, and additional species observed outside quadrats were also recorded. Based on weed community composition, population density, and mean plant size, overall weed interference level was rated. A total of 56 residual weed species were observed and no single species dominated the community of NCR processing sweet corn. Several of the most abundant species, such as common lambsquarters and velvetleaf, have been problems for many years, while other species, like wild-proso millet, have become problematic in only the last 20 yr. Compared to a survey of weeds in sweet corn more than 40 yr ago, greater use of herbicides is associated with reductions in weed density by approximately an order of magnitude; however, 57% of fields appeared to suffer yield loss due to weeds. Sweet corn harvest in the NCR ranges from July into early October. Earlier harvests were characterized by some of the highest weed densities, while late-emerging weeds such as eastern black nightshade occurred in fields harvested after August. Fall panicum, giant foxtail, wild-proso millet, common lambsquarters, and velvetleaf were the most abundant species across the NCR, yet each state had some unique dominant weeds. Nomenclature: Common lambsquarters, Chenopodium album L. CHEAL; eastern black nightshade, Solanum ptychantum Dunal. SOLPT; fall panicum, Panicum dichotomiflorum Michx. PANDI; giant foxtail, Setaria faberi Herrm. SETFA; velvetleaf, Abutilon theophrasti Medic. ABUTH; wild-proso millet, Panicum miliaceum L. PANMI; sweet corn, Zea mays L

Phenology of common lambsquarters growth parameters

Abstract Research was conducted to characterize the phenology of common lambsquarters growth parameters as influenced by climatic variation among years. Treatments included soybean or corn grown alone, common lambsquarters with soybean or corn, and common lambsquarters grown alone. Common lambsquarters leaf area and plant height phenology differed among years and was variable within treatments. Conversely, crop leaf area and plant height phenology did not differ among years and was less variable within a treatment than common lambsquarters. Weed relative leaf area and relative volume differed among years because of differences in crop and common lambsquarters leaf area and plant height phenology. Differences in common lambsquarters relative leaf area and relative volume among years may explain differences in previously reported crop yield responses to weed infestations between sites and years. Although common lambsquarters relative leaf area and relative volume differed among years, variability as indicated by regression coefficients of determination was also high within year and treatment. Crop leaf area and plant height phenology were well described by regression equations, with r2 values greater than 0.68. Therefore, low coefficients of determination for relative leaf area and relative volume models were attributed to variability in common lambsquarters within a treatment. Nomenclature: Common lambsquarters, Chenopodium album L. CHEAL; field corn, ‘Dekalb DK493SR’, Zea mays L.; soybean, ‘Asgrow XP19505RR’ and ‘AG2101RR’, Glycine max L. Merr.

Significance of atrazine in sweet corn weed management systems.

Abstract Weed management systems used by sweet corn growers, including the role of atrazine, are poorly characterized. Management records of 175 fields throughout the major sweet corn production areas of the Midwest were surveyed from 2005 to 2007. Seventy-four percent of sweet corn fields in the Midwest were grown in rotation with soybean or corn. Interrow cultivation was used on 48% of fields, and atrazine use was higher in those fields without interrow cultivation. A majority of fields (54%) received both PRE and POST herbicide applications. Mesotrione was applied below the registered use rate in two-thirds of the fields in which it was used POST. Atrazine rates in sweet corn were highest when the preceding crops were other vegetables, compared to preceding crops of soybean or corn. Selective herbicides are used extensively in U.S. sweet corn production, accounting for 94% of total weed management expenditures which average

Predicting soybean yield loss in giant foxtail (Setaria faberi) and common lambsquarters (Chenopodium album) communities

123/ha. Growers treated 66% of fields with one or more applications of atrazine at an average total use rate of 1.35 kg ai/ha. The estimated annual net cost to replace atrazine in U.S. sweet corn production with the broad spectrum broadleaf herbicide, mesotrione, is

Growth interactions in communities of common lambsquarters (Chenopodium album), giant foxtail (Setaria faberi), and corn

9.2 million.

Estimating giant foxtail cohort productivity in soybean based on weed density, leaf area, or volume

Abstract Widespread use of crop yield loss models based on weed density has been limited on account of spatial and temporal variability. Furthermore, research characterizing crop yield loss associated with two or more weed species is lacking for many cropping systems. Therefore, research was conducted to characterize giant foxtail and common lambsquarters leaf area, height, and shoot volume in soybean, to quantify the relative competitive ability of giant foxtail and common lambsquarters in a mixed–weed species environment, and to assess weed density, weed relative leaf area, and weed relative volume as predictors of soybean yield loss. Based on weed density, coefficient estimates of percent soybean yield loss as giant foxtail or common lambsquarters densities approached zero differed between years. In contrast, coefficient estimates of maximum soybean yield loss were similar between years. Based on weed relative leaf area, estimates of giant foxtail or common lambsquarters damage coefficients differed between years. Similarly, estimates of maximum soybean yield loss associated with common lambsquarters leaf area differed between years, whereas estimates of maximum soybean yield loss associated with giant foxtail leaf area did not change over time within a growing season or between years. Based on weed relative volume, estimates of giant foxtail or common lambsquarters damage coefficients differed between years. Similarly, estimates of maximum soybean yield loss associated with common lambsquarters volume differed between years, whereas estimates of maximum soybean yield loss associated with giant foxtail volume did not change over time within a growing season or between years. Based on weed density, weed relative leaf area, or weed relative volume, giant foxtail was more competitive than common lambsquarters in terms of soybean yield loss. Temporal variability of weed density, weed relative leaf area, and weed relative volume indicates that additional parameters may be required to accurately predict weed–crop interactions in a multiple–weed species community. Nomenclature: Giant foxtail, Setaria faberi Herrm. SETFA; common lambsquarters, Chenopodium album L. CHEAL; soybean, Glycine max (L.) Merr. ‘Asgrow AG2101’.