Studies were conducted in 1998 and 1999 in Ohio to determine the effect of postemergence (POST) application timing of glyphosate on weed control and grain yield in glyphosate-tolerant corn, and how this was influenced by corn planting date and the use of soil-applied herbicides. Glyphosate was applied based on giant foxtail height. Two applications of glyphosate provided better weed control than a single application, especially when applied to weeds 10 cm or less in early-planted corn. Yield was reduced occasionally with a single application on 5- or 10-cm weeds, because of weed re-infestation. Failure to control weeds before they reached a height of 15 to 30 cm also resulted in occasional yield loss. Application of atrazine or acetochlor plus atrazine prior to glyphosate did not consistently increase weed control or yield. Results suggested that glyphosate should be applied before weeds reach 15 cm in height to avoid corn grain yield loss.
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.
Studies were conducted to determine how a serine to threonine mutation at position 264 on the Qb binding niche of the D1 protein [urea-resistant/triazine resistant (UR/TR biotype)] in common purslane (Portulaca oleracea) impacted carotenoid and chlorophyll pigment pools and measurements of photochemical and non-photochemical quenching (NPQ) following applications of various inhibitors of carotenoid biosynthesis (CBI) and the Photosystem II (PSII) inhibitor diuron when applied alone, or in mixtures, as compared to wildtype (WT) purslane. Non-photochemical quenching decreased 138 to 531% in comparison to the untreated checks following any herbicide application. Most CBI herbicides and diuron did not change chl a and chl b in the UR/TR biotype, while these same herbicide treatments tended to sharply decrease chlorophyll pigments in the WT population. Zeaxanthin levels were sharply elevated when CBI herbicides were applied alone to both purslane biotypes. β-carotene reduced in both biotypes following herbicide applications in comparison to the untreated check. Neoxanthin, antheraxanthin, and lutein were generally increased or remained similar to the untreated controls in the herbicide treated UR/TR biotype, while levels of these carotenoids tended to decrease in the herbicide treated WT population. Diuron alone increased neoxanthin, antheraxanthin, lutein, and zeaxanthin by 4 to 200% in the UR/TR biotype, but decreased these same carotenoids 25 to 62% in the WT population. The applications of CBI and PSII herbicides demonstrate that redox signaling in response to this mutation in the D1 protein may impact the retention of plant pigment concentrations in the light harvesting complexes of PSII, which would be vital for stress tolerance in this biotype.
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