Effect of Postemergence Glyphosate Application Timing on Weed Control and Grain Yield in Glyphosate-Resistant Corn: Results of a 2-Yr Multistate Study¹

Abstract

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.

Biologically effective dose of glyphosate as influenced by weed size in corn

“…ha −1 was not sufficient to provide 90% or greater control of barnyardgrass at 30 cm height. Results are similar to Gower et al (2003) that found greater than 90% control with glyphosate when applied at 840 g a.e. ha −1 to annual grasses that were up to 30 cm in height.…”

“…2). Similarly, Gower et al (2003) reported glyphosate applied when weeds were 5, 10, 15, 23, and 30 cm in height resulted in corn yield of 101%, 97%, 93%, 91%, and 79% compared to the weed-free control, respectively. Other studies have also shown that when weeds are not controlled during early stages of corn development, significant yield losses can occur (Anderson 1996;Jordan et al 1997;Hartzler et al 2006).…”

“…ha −1 was not sufficient to reduce common ragweed dry matter 90% or greater when applied when common ragweed was 30 cm in height at the time of glyphosate application. In other studies, Gower et al (2003) found that glyphosate applied at 840 g a.e. ha −1 to annual broadleaf weeds including common ragweed provided less than 90% control when applied to weeds at 5 cm in height and greater than 90% control when applied to weeds that were at 10, 15, 23, or 30 cm in height.…”

“…ha −1 was sufficient to provide 90% or greater control of green foxtail at all application timings. Similarly, Gower et al (2003) found that glyphosate applied at 840 g a.e. ha −1…”

Weed management options in transgenic stacked and non transgenic maize (Zea mays) hybrids-A review

“…Broadleaf weed control was more than 93 per cent with a single glyphosate application when broadleaf weeds were 15 cm or more in height. Although grass weeds present at the time of application of early emergence were controlled, regardless of size, tall broadleaf weeds could be more difficult to control (Gower et al, 2003). Puricelli and Tuesca (2005) found that sequential application of glyphosate provided excellent control of early emergence annual broad leaved weeds such as Portulaca oleracea and grass annuals such as Digeria sanguinalis.…”