Field studies were conducted in 2004 and 2005 to determine the effect of soybean row width and population on weeds, canopy closure, crop yield, and economic return in glyphosate-resistant soybean. Soybean leaf area index (LAI) was greater in 19- and 38-cm, compared with 76-cm rows from 8 to 12 wk after planting in the low, moderate, and high soybean populations. Canopy closure was delayed by 2 wk in the moderate population in 76-cm rows compared with the high population in 19-cm rows. Fewer weeds emerged in 19-cm, compared with 76-cm rows following glyphosate application, and increasing the soybean population within a row width did not influence late-season weed emergence. Weed biomass in the weedy control was greater in the very low soybean population compared with the high soybean population within each row width; however, weed biomass in the weedy control was similar in the high and moderate soybean populations. Soybean yield in the weed-free and 10-cm glyphosate treatment did not differ, and yield was greater in 19-cm rows planted at moderate or high, compared with low populations. There was no difference in weed-free soybean yield at low, moderate, and high populations within 38- and 76-cm rows. Gross margins were usually greater in 19- and 38-cm, compared with 76-cm rows. The gross margin for soybean planted in 19-cm rows was usually greater at moderate or high soybean populations compared with lower populations. In 76-cm rows, the gross margin was greatest at the low and moderate soybean populations. When rainfall or other factors limited soybean yield, increasing the soybean population from approximately 300,000 plants/ha to 445,000 plants/ha in 19-, 38-, and 76-cm rows did not result in quicker canopy closure, reduced weed emergence, or greater soybean yield and gross margins.

Repeated use of protox-inhibiting herbicides has resulted in a common waterhemp (Amaranthus rudisSauer) biotype that survived lactofen applied up to 10 times the labeled rate. Field and greenhouse research evaluated control options for this biotype of common waterhemp. In the field, PRE applications of flumioxazin at 72 g ai ha−1, sulfentrazone at 240 g ai ha−1, and isoxaflutole at 70 g ai ha−1controlled common waterhemp >90% up to 6 weeks after treatment. POST applications of fomesafen at 330 g ai ha−1, lactofen at 220 g ai ha−1, and acifluorfen at 420 g ai ha−1resulted in <60% visual control of common waterhemp, but differences were detected among herbicides. In the greenhouse, glyphosate was the only herbicide that controlled protox resistant waterhemp. The majority of herbicide activity from POST flumioxazin, fomesafen, acifluorfen, and lactofen was from foliar placement, but control was less than 40% regardless of placement. Control of common waterhemp seeded at weekly intervals after herbicide treatment with flumioxazin, fomesafen, sulfentrazone, atrazine, and isoxaflutole exceeded 85% at 0 weeks after herbicide application (WAHA), while control with isoxaflutole was greater than 60% 6 WAHA. PRE and POST options for protox-resistant common waterhemp are available to manage herbicide resistance.

Corn (Zea mays L.) growers continue to show interest in identifying hybrids that offer high yields in narrow rows (<30 inches). Research evaluated the response of 10 phenotypically different hybrids planted at 30,000 and 36,000 seeds/acre in 15‐ and 30‐inch rows from 2011 to 2013 under dry growing conditions in Missouri. Precipitation was up to 35% less (2012) than the 10‐year average in the region during this research. Grain yields were 2.7% greater in 15‐inch rows than in 30‐inch rows when averaged over seeding rate and hybrid, with four of the 10 hybrids having greater yields in narrower rows. Drought affected overall yields, though an early‐maturing hybrid (P0461HR) and a drought‐tolerant hybrid (P1151AM) yielded more than the others. The higher‐yielding hybrids showed no yield difference based on row spacing. In 15‐inch rows, high ear placement and short plants of an early‐maturing hybrid (P0461HR) with average drought tolerance and a late‐maturing hybrid (33T57) with above‐average drought tolerance yielded more than a hybrid (P0621HR) with average drought tolerance and low ear placement, but yields were similar in 30‐inch rows. However, with both row spacings, late‐maturing, drought‐tolerant hybrids yielded more than hybrids with below‐average drought tolerance. Corn seeded at 30,000 seeds/acre (27,900 plants/acre at harvest) yielded 25% more in an extreme drought year (2012) than corn seeded at 36,000 seeds/acre (32,600 plants/acre at harvest), but no yield difference appeared in 2011 and 2013. Our study indicates that appropriate plant densities and hybrid selection were needed to successfully integrate narrow‐row corn into drought‐prone claypan soils.

Control of common lambsquarters with glyphosate in Michigan soybean fields has been inconsistent. Stem-boring insects and evidence of insect tunneling were found inside the stems of common lambsquarters plants not controlled with glyphosate. In 2004 and 2005, field surveys and studies were conducted to identify and evaluate the prevalence of stem-boring insects in common lambsquarters in Michigan and Indiana soybean fields to determine whether tunneling by insects occurred before or following POST glyphosate applications and to evaluate the effect of glyphosate rate, application timing, and insect tunneling on the control of common lambsquarters with glyphosate. Two insect species, the beet petiole borer (Cosmobaris americana) from the Curculionidae family and an unidentified leafminer fly larvae from the Agromyzidae family were found inside common lambsquarters stems. Leafminer larvae were present in Michigan soybean fields in mid- to late-June, when most POST glyphosate applications are made in Michigan and Indiana; however, beet petiole borer larvae were not found in common lambsquarters stems until mid-July and would only be present in common lambsquarters plants if glyphosate applications occurred at that time. Results from three field experiments in East Lansing, MI, demonstrated the variability in common lambsquarters control. Control ranged from 79 to 98%, 75 to 99%, and 49 to 97% from glyphosate applied at 0.84 kgae/ha to 10-, 25-, and 46-cm common lambsquarters, respectively. In general, applying glyphosate to common lambsquarters plants 10 cm or less, or increasing the glyphosate rate beyond 0.84 kgae/ha, improved common lambsquarters control. Insect tunneling by leafminer and beet petiole borer larvae did not contribute to reduced common lambsquarters control with glyphosate applied to 10- and 25-cm common lambsquarters.

Field surveys were conducted to evaluate the prevalence of stalk-boring insects in giant ragweed in Indiana and Michigan soybean fields. Greenhouse studies were also conducted to determine whether stalk-boring insects had a negative impact on control of giant ragweed with glyphosate. In the June 2005 field surveys, 18 to 30% of all giant ragweed plants sampled contained stalk-boring insects or insect tunnels. Languriidae, Noctuidae, Pyralidae, and Tortricidae families were found most often at the time glyphosate was being applied to soybean fields to control giant ragweed. Cerambycidae and Curculionidae families were typically found later in the season after herbicide applications were completed. In the August field surveys in Indiana, 28 to 62% of the giant ragweed plants that showed evidence of stalk-boring insects were not controlled by POST herbicide applications suggesting that control was compromised by the presence of stalk-boring insects. In greenhouse studies, glyphosate efficacy on 15-cm-tall giant ragweed was enhanced by the presence of stalk-boring insects; however, glyphosate efficacy on 45-cm plants was reduced by the presence of stalk-boring insects. Overall, this research suggests that there is a possibility that stalk-boring insects could reduce glyphosate efficacy on giant ragweed.