Control of natural infestations of common lambsquarters and giant foxtail in 1993, 1994, and 1995 and of velvetleaf in 1994 and 1995 by postemergence application of glyphosate to glyphosate-resistant soybean planted in narrow (20 cm) and wide (76 cm) rows was evaluated. Planting glyphosate-resistant soybean in narrow rows and applying reduced rates of glyphosate when common lambsquarters, giant foxtail, and velvetleaf were at their actively growing stage 3 to 18 cm, 5 to 28 cm, and 3 to 20 cm tall, respectively, resulted in > 90% control. The effect of time of herbicide application was greater than the rate of herbicide application, especially within the wide-row soybean plantings. Applying imazethapyr in combination with glyphosate did not improve weed control or soybean yield compared with glyphosate alone.

Field trials were conducted at two sites in both 1997 and 1998 to evaluate soybean response and weed control with glufosinate alone or combined with quizalofop, lactofen, imazethapyr, flumiclorac, or bentazon plus acifluorfen in narrow-row, glufosinate-resistant soybean. Soybean injury ranged from 0 to 21% at 2 wk after treatment (WAT) and from 0 to 5% by 4 WAT. Glufosinate alone at 0.29 and 0.4 kg ai/ha controlled velvetleaf, common waterhemp, common ragweed, morningglory species, and giant foxtail greater than 85% in all studies. Mixtures containing glufosinate and other herbicides controlled these species greater than 81% but did not improve control over glufosinate alone. Estimates of weed biomass closely reflected visual control evaluations. However, giant foxtail biomass was higher for mixtures of glufosinate plus lactofen, flumiclorac, or bentazon and acifluorfen, indicating possible antagonism of glufosinate activity. At both locations, soybean yields were similar among most treatments, but that of the glufosinate plus lactofen treatment was lower when compared with other treatments. Additional trials evaluated soybean response and weed control with a preemergence herbicide followed by glufosinate postemergence (POST), glufosinate applied once or twice POST, and mixtures of glufosinate plus imazethapyr or flumiclorac POST in wide-row soybean. Glufosinate applied twice controlled common waterhemp, morningglory species, prickly sida, common cocklebur, and giant foxtail up to 39% greater than did glufosinate applied once. The addition of imazethapyr, but not flumiclorac, to glufosinate improved weed control when compared with glufosinate alone.

Greenhouse experiments were conducted to determine whether multiple applications of glyphosate and time of glyphosate application with regard to the crop's growth stage had a significant effect on the growth and development of glyphosate-resistant canola. Glyphosate was applied as single applications at the two-, four-, or six-leaf stage of canola; as sequential double applications at the two- and four-, two- and six-, and four- and six-leaf stages of canola; and as a triple application at all three stages. Of the plant growth parameters measured, single applications of glyphosate resulted in significant reductions to stem weight and shoot weight compared with nontreated plants, and multiple applications of glyphosate caused significant reductions to leaf area, leaf weight, stem weight, and shoot weight. Single applications of glyphosate were less injurious to glyphosate-resistant canola compared with multiple applications, and canola growth parameter reductions were greatest after earlier glyphosate applications.

Herbicide-resistant corn hybrids offer additional options for POST weed control in corn, and growers may benefit from information on the consistency of these weed-control strategies. Studies were conducted in Indiana, Illinois, Michigan, and Ohio, in 2000 and 2001, to evaluate weed control among herbicide strategies for imidazolinone-resistant, glufosinate-resistant, glyphosate-resistant, and conventional corn. Isogenic hybrids were utilized to minimize variation in growth and yield potential among hybrids. The glyphosate-resistant corn postemergence (glyphosate-POST) treatment provided more consistent control of giant foxtail than the PRE, conventional corn postemergence (conventional-POST), glufosinate-resistant corn postemergence (glufosinate-POST), and imidazolinone-resistant corn postemergence (imi-POST) treatments. All four POST treatments were more consistent and provided greater control than the PRE treatment of the large-seeded broadleaf weeds velvetleaf, giant ragweed, common cocklebur, and morningglory species. Conventional-POST and imi-POST were more consistent than glufosinate-POST and glyphosate-POST treatments in controlling giant ragweed. There were no statistical differences in the variability of PRE or POST treatments for control of common lambsquarters, common ragweed, and redroot pigweed. Corn yield varied among locations and years. The glyphosate-POST treatment did not reduce yield relative to the weed-free treatment, the imi-POST and glufosinate-POST treatments each reduced yield in one of eight locations, and the conventional-POST treatment reduced yield in three of eight locations.

The introduction of Clearfield (CL) rice cultivars resistant to imidazolinone herbicides, acetolactate synthase (ALS) inhibitors, has raised concerns of gene flow to weedy rice genotypes collectively called “red rice” that infest rice-growing areas in the southern United States. This experiment was conducted to study hybridization between CL rice and red rice using simple sequence repeats (SSR) markers, identify mutations in the ALS gene of imazethapyr-resistant red rice, and to detect the introgression of the ALS-resistant gene from CL rice into red rice. Natural outcrossing experiments between CL rice and strawhull (SH) red rice were set up in Stuttgart, AR, in 2002 and 2003. Putative red rice hybrids were detected among volunteer plants in the following year. Hybridization was confirmed using SSR markers, and introgression of the resistant ALS gene from CL rice to red rice was detected by ALS gene sequencing. The ALS gene sequences of U.S. rice cultivars ‘Bengal’ and ‘Cypress’, SH red rice, CL rice (CL161), and imazethapyr-resistant red rice/CL rice hybrids were compared. Nucleotide sequences of the ALS gene from the rice cultivars were identical. Three point mutations were present in the SH red rice ALS gene coding region relative to Bengal/Cypress. One of these resulted in the substitution of Asp630 for Glu630. The ALS gene sequences of confirmed hybrids were identical to that of the herbicide-resistant pollen source, CL161. We identified four ALS gene mutations in the herbicide-resistant red rice hybrids relative to the susceptible rice cultivars. One point mutation, resulting in a substitution of Ser653 with Asn, was linked to ALS resistance in callus tissue derived from a Kinmaze rice line from Japan. The other three mutations (Ser186—Pro, Lys416—Glu, and Leu662—Pro) are novel. This experiment confirmed that gene flow from imidazolinone-resistant rice resulted in herbicide-resistant red rice plants.

Wheat cultivars resistant to imazamox will facilitate selective chemical control of many winter annual grass weeds, including jointed goatgrass, downy brome, and feral rye. These three weed species respond differently to imazamox postemergence treatments with feral rye, demonstrating more tolerance than jointed goatgrass or downy brome; therefore, growth chamber studies were conducted to evaluate imazamox absorption in all three weed species and translocation and metabolism in jointed goatgrass and feral rye. Adding nonionic surfactant (NIS) or methylated seed oil increased absorption in jointed goatgrass and feral rye but not in downy brome, compared to imazamox applied alone. Imazamox applied with NIS and urea ammonium nitrate resulted in the highest absorption in each species: 97, 91, and 92% of applied 14C for jointed goatgrass, downy brome, and feral rye, respectively, 48 h after treatment (HAT). Imazamox translocation from the treated leaf was similar for jointed goatgrass and feral rye across seven harvest intervals between 0 and 96 HAT. Shoot tissues of jointed goatgrass and feral rye accumulated 17 and 14% of applied 14C, respectively, by 96 HAT. Differential translocation of imazamox into root tissue was observed within 12 HAT; by 96 HAT, 20% of applied 14C translocated to jointed goatgrass roots compared to 27% for feral rye. Imazamox was readily metabolized in both weed species. At 96 HAT, 73 and 98% of the applied 14C was metabolized in the treated leaves of jointed goatgrass and feral rye, respectively. Metabolism was consistently higher in feral rye than in jointed goatgrass in all plant parts 96 HAT. On a whole-plant basis, metabolism was 25% greater in feral rye than in jointed goatgrass. The differential response of jointed goatgrass and feral rye to foliar applications of imazamox appears to be related to differences in translocation and metabolism but not in absorption.