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Field studies were conducted at 35 sites throughout the north-central United States in 1998 and 1999 to determine the effect of postemergence glyphosate application timing on weed control and grain yield in glyphosate-resistant corn. Glyphosate was applied at various timings based on the height of the most dominant weed species. Weed control and corn grain yields were considerably more variable when glyphosate was applied only once. The most effective and consistent season-long annual grass and broadleaf weed control occurred when a single glyphosate application was delayed until weeds were 15 cm or taller. Two glyphosate applications provided more consistent weed control when weeds were 10 cm tall or less and higher corn grain yields when weeds were 5 cm tall or less, compared with a single application. Weed control averaged at least 94 and 97% across all sites in 1998 and 1999, respectively, with two glyphosate applications but was occasionally less than 70% because of late emergence of annual grass and Amaranthus spp. or reduced control of Ipomoea spp. With a single application of glyphosate, corn grain yield was most often reduced when the application was delayed until weeds were 23 cm or taller. Averaged across all sites in 1998 and 1999, corn grain yields from a single glyphosate application at the 5-, 10-, 15-, 23-, and 30-cm timings were 93, 94, 93, 91, and 79% of the weed-free control, respectively. There was a significant effect of herbicide treatment on corn grain yield in 23 of the 35 sites when weed reinfestation was prevented with a second glyphosate application. When weed reinfestation was prevented, corn grain yield at the 5-, 10-, and 15-cm application timings was 101, 97, and 93% of the weed-free control, respectively, averaged across all sites. Results of this study suggested that the optimum timing for initial glyphosate application to avoid corn grain yield loss was when weeds were less than 10 cm in height, no more than 23 d after corn planting, and when corn growth was not more advanced than the V4 stage.

Field experiments were conducted at four locations in Kansas in 1999 and 2000 to evaluate grain sorghum response to simulated drift rates of four herbicides. Imazethapyr, glufosinate, glyphosate, and sethoxydim were applied at 1/3, 1/10, 1/33, and 1/100 of the use rate when plants were 10 to 20 cm tall. Visible crop injury increased as rates of each herbicide increased. Glyphosate and imazethapyr caused the most injury and glufosinate the least. Data show that some plants that were significantly injured 2 wk after treatment (WAT) recovered 8 WAT. However, some plants that received the highest rate of imazethapyr or glyphosate died. Grain sorghum yields were reduced only when injury was severe. This research showed that the potential for sorghum injury from off-target herbicide drift is greater from imazethapyr and glyphosate than from sethoxydim or glufosinate.

The selection of herbicide-resistant weeds in grain sorghum production has prompted researchers to explore alternative herbicides to prevent, delay, and manage herbicide-resistant weed biotypes. Greenhouse and field experiments were conducted to evaluate the differential response of sorghum hybrids to POST application of mesotrione. In a greenhouse experiment, 85 sorghum hybrids were treated with 0, 52, 105, 210, and 315 g ai/ha mesotrione when plants were at the three- to four-leaf collar stage. Sorghum response ranged from susceptible to tolerant sorghum hybrids. ‘Pioneer 84G62’, ‘Pioneer 85G01’, and ‘Triumph TR 438’ were the three most susceptible, whereas ‘Dekalb DKS35-70’, ‘Frontier F222E’, and ‘Asgrow Seneca’ were the three most tolerant hybrids. One week after treatment (WAT), the mesotrione rate causing 50% visible injury ranged from 121 to 184 and 64 to 91 g/ha in the most tolerant and susceptible hybrids, respectively. Mesotrione dose–response studies were conducted under field conditions on four sorghum hybrids. One WAT, injury symptoms were greater (up to 23%) in Pioneer 85G01 than in Asgrow Seneca (< 14%). However, all plants appeared normal by the end of the growing season. In addition, sorghum yields were not reduced by mesotrione treatments as verified by correlation coefficient analysis.

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