Under certain conditions, application of glyphosate to glyphosate-resistant (GR) cotton can lead to fruit shedding and yield reductions. Field studies were conducted at the Texas Agricultural Experiment Station using GR cotton, cv. ‘DeltaPine 5690RR’, to determine if application method and timing affect cotton fruit retention. Glyphosate at 1.12 kg ai ha−1 was precisely postdirected (PD), postdirected with 25% foliage coverage (PDFC), or applied over the top (OT) at the 8- or 18-leaf stage after an initial topical application of 1.12 kg ha−1 glyphosate at the four-leaf stage. In one of the years of this study, 8 PD, 18 PDFC, and 18 OT reduced yield. In 1999 and 2000, 8 PDFC and 8 OT applications of glyphosate caused yield loss, mainly due to lower mean boll weight. Glyphosate applied topically at the eight-leaf stage also affected the Position 1 boll retention throughout the plant in both years. Glyphosate contact with leaves and stems should be avoided when applying glyphosate after the four-leaf stage to prevent possible yield loss.

This study investigated factors that influence occurrence and persistence of plant DNA in the soil environment in three crop rotations. In each rotation, soil was sampled in May before planting, in July and August while crops were growing, and in October after harvest. Total DNA was recovered from soil samples taken at two different depths in the soil profile and quantified. Three target plant genes (corn CP4 epsps, corn 10-kD Zein, and soybean CP4 epsps) also were quantified in these DNA extracts using species-specific quantitative real-time PCR assays. In general, total plant DNA content in the soil environment was greatest when the crop was growing in the field and decreased rapidly after harvest. Nevertheless, low levels of target plant DNA were often still detectable the following spring. Age of rotation did not influence target DNA quantities found in the soil environment. Data were collected for a combination of 10 location-years, which allowed for estimation of the variance components for six factors including time of sampling, year, location, crop, sampling depth, and herbicide to total and target DNA content in the soil samples. Mean target recombinant DNA content in soil was influenced most strongly by time of sampling and year (85 and 6%, respectively), whereas total soil DNA content was less dynamic and was most strongly influenced by location and year (49 and 25%, respectively). Over the duration of this study, no accumulation of transgenic plant DNA in the soil environment was observed.

Sclerotinia stem rot is an important soybean disease. An increase in phytoalexin production with herbicide treatments may reduce the incidence of this disease in soybean. Research was conducted to determine soybean response, Sclerotinia sclerotiorum lesion development, and phytoalexin production in glyphosate-resistant and -susceptible soybean cultivars treated with protoporphyrinogen oxidase–inhibiting herbicides. Necrosis of soybean leaves 7 d after postemergence application of oxyfluorfen at 17.5 g ai/ha, carfentrazone at 1.8 g ai/ha, sulfentrazone at 9.0 g ai/ha, fomesafen at 280 g ai/ha, acifluorfen at 425 g ai/ha, flumiclorac at 30 g ai/ha, CGA-248757 at 4 g ai/ha, and oxadiazon at 280 g ai/ha was equal to or less than lactofen at 70 g ai/ha. In a detached leaf bioassay, S. sclerotiorum lesion diameter was reduced by oxyfluorfen, carfentrazone, sulfentrazone, lactofen, fomesafen, flumiclorac, and oxadiazon compared with the untreated control. Furthermore, lesion diameter on untreated leaves of soybean treated with oxyfluorfen, carfentrazone, sulfentrazone, lactofen, fomesafen, acifluorfen, flumiclorac, CGA-248757, and oxadiazon was reduced compared with the untreated control. Lactofen and sulfentrazone increased leaf phytoalexin production similarly, but neither herbicide affected stem phytoalexin production compared with the untreated control. Glyphosate-resistant and near-isogenic–susceptible cultivars responded similarly when inoculated with S. sclerotiorum in the detached leaf bioassay. Glyphosate-resistant S20-B9 and P93B01 produced more phytoalexins than glyphosate-susceptible S 19-90 and P9281. Herbicide treatments may increase phytoalexin production in leaves of treated plants, but levels in the stem do not explain protection from Sclerotinia stem rot.

A 4-year study was initiated in 1997 to provide canola producers with information on the consequences of various rotational intervals with the use of new disease and herbicide-resistant canola varieties. The study was conducted at three locations in Alberta, Canada (Ellerslie, Strathmore, and Warburg). At each location, four canola rotations were established: continuous canola, and canola seeded in 1 of 2, 3, or 4 years. Canola varieties included the conventional varieties ‘AC Excel’ and ‘Quantum’, the glyphosate-resistant variety ‘Quest’, imidazolinone-resistant ‘45A71’, and a glufosinate-resistant hybrid, ‘Invigor 2153’. In the fourth year of the study, when canola was grown in all treatments, weed densities, weed species diversity, and evenness were determined preseeding and before and after in-crop herbicide application. Canola yield was greatest in the northern ecoregions of the Boreal Transition (Warburg) and Aspen Parkland (Ellerslie), and lowest in the Moist Mixed Grassland ecoregion (Strathmore). Weed populations increased and population diversity decreased and became less even where rotations were less diverse; in continuous canola and in the 1-in-4–year rotation. As expected, weed densities increased in poorly competitive, conventional canola varieties compared to herbicide-resistant varieties sprayed with their broader-spectrum herbicides. Where weed densities were high, variety/herbicide system became a critical factor affecting crop yield. Under these conditions, the herbicide-resistant varieties ‘Quest’ and ‘Invigor 2153’ typically outyielded the conventional varieties of ‘AC Excel’ and ‘Quantum’. Canola yield was highest when grown in a 1-in-3– or a 1-in-4–year rotation, although the 1-in-3–year rotation generally had lower weed densities, and allowed high-value canola to be grown more frequently in rotation.

Herbicide-resistant (HR) rice varieties offer U.S. rice producers a powerful tool for control of red rice infestations. However, improved weed control can shorten crop rotations and boost yields, resulting in expanded rice production and lower domestic market prices. Declining market returns diminish the benefits of HR rice adoption and substantially reduce net returns for nonadopters. More competitive prices increase U.S. rice exports, causing a slight decline in world rice prices. The dependence of the rice marketing loan program on world prices prevents loan deficiency payments from adequately offsetting producers' market revenue losses. U.S. consumers gain from lower rice prices.