Effects of glyphosate on weed management and reproductive biology of glyphosate-resistant corn

by 1979- Thomas, Walter Edward

THOMAS, WALTER EDWARD. Effects of glyphosate on weed management and reproductive biology of glyphosate-resistant corn. (Under the direction of Dr. John W. Wilcut). Field studies were conducted from 2000 to 2002 to evaluate crop tolerance, weed control, grain yield, and net returns in glyphosate-resistant corn with various herbicide systems. Weed management systems evaluated various preemergence (PRE) and postemergence (POST) herbicide options. Glyphosate formulations did not influence crop injury, weed control, or grain yield in either study. Effective annual grass control was not achieved with any PRE herbicide system. However, PRE herbicide treatment plus any glyphosate treatment controlled goosegrass, large crabgrass, and Texas panicum > 86%. Regardless of PRE herbicide options, any POST glyphosate treatment controlled common ragweed and common lambsquarters at least 88 and 89%, respectively. Ipomoea spp. were controlled at least 88% with a PRE herbicide fb glyphosate EPOST alone, glyphosate sequentially, or in EPOST mixture with either halosulfuron or mesotrione. The highest yielding herbicide systems used glyphosate applied once or twice. Net returns were highest for glyphosate weed management systems. Experiments conducted in the North Carolina State University Phytotron greenhouse and three field locations in 2002 determined the effect of 1.12 kg ai/ha of glyphosate at various application timings on pollen viability and seed set in two types (GA21 and NK603) of glyphosate-resistant corn. Regardless of hybrid, pollen viability was reduced in phytotron and field studies with glyphosate treatments applied POST at the V6 stage or later. Scanning and transmission electron microscopy indicated distinct morphological alterations and large vacuoles and lower starch accumulation associated with later glyphosate treatments that caused pollen viability reductions. Although glyphosate negatively influenced pollen viability, pollination and seed set were not influenced. Enzyme-linked immunoabsorbent assay examined CP4-EPSPS expression in DKC 64-10 (NK603) at anthesis and showed the greatest amount in pollen with progressively less in brace roots, ear leaf, anthers, roots, ovaries, silks, stem, flag leaf, and husk, respectively. Greenhouse experiments examined the effect of 112 and 280 g ai/ha of glyphosate applied at various sicklepod growth stages on reproductive development. No treatment differences were found for average pod length, seed germination, seed viability, and above ground biomass. The nontreated plants had 18 flowers/plant counted over 8 weeks. Glyphosate applied at 4 leaf (L), 8 L, 12 L, and sequentially at 4 L and 8 L, averaged over glyphosate rates, reduced flower production by 29, 5, 65, and 54%, respectively, compared to the nontreated. Similarly, glyphosate at 112 and 280 g/ha, averaged over treatment timings, reduced flower production by 19 and 58%, respectively, compared to the nontreated. Pollen viability measurements could not be analyzed due to limited flower production imposed by our treatment structure. The number of pods and seeds and total seed weight were reduced by 78, 80, and 81%, respectively, with 280 g/ha of glyphosate compared to the nontreated plants. Field studies conducted in 2001 and 2002 investigated the response of nontransgenic cotton to simulated glyphosate drift in a weed-free environment. Glyphosate treatments were applied EPOST at the 4-leaf growth stage of cotton at 0, 8.7, 17.5, 35, 70, 140, 280, 560, and 1,120 g ai/ha. Rates as low as 140 g/ha caused lint yield reductions depending on year and location. When averaged over all locations, lint yield reductions of 4, 49, 72, and 87% compared with nontreated cotton were observed with glyphosate rates of 140, 280, 560, and 1,120 g/ha, respectively. Visual injury and shikimic acid accumulation were evident at glyphosate rates > 70 g/ha. Collectively, visual injury and shikimic acid accumulation at 7 DAT may be used as a diagnostic indicator for potential yield reductions from simulated glyphosate drift.