Comparison of weed control strategies in glyphosateresistant soybean [Glycine max (L.) Merr.] in Atlantic Canada

Abstract

Glyphosate-resistant soybean cultivars with low corn heat unit requirements have only recently become available in the Atlantic region of Canada. In a field experiment near Charlottetown, Prince Edward Island, in 1998 and 1999, we evaluated the effect of time of application of glyphosate and application of selected herbicides, flumetsulam, metolachlor, imazethapyr, metribuzin, and bentazon on weed control and tolerance and yield of three glyphosate-resistant soybean cultivars having different maturity ratings. Glyphosate was applied at 0.90 kg ai ha-1 at the first, second or third trifoliate leaf stage, and sequentially at the first and third or second and third trifoliate stages. The sequential treatments of glyphosate resulted in 89 to 100% weed control, and yields achieved with glyphosate applied at the second or third trifoliate stages did not usually differ from sequential treatments. The other selected herbicides did not injure the cultivars and within the group, all treatments resulted in comparable yields. Species that emerge in a single early-season flush were controlled at 6 wk after treatment with glyphosate applied as a single application, but not on a species such as wild radish that have later flushes or emerge throughout the season. Glyphosate applied as a single application at the late second to third trifoliate stage can provide optimum weed control and crop yield with lower herbicide cost than an alternative herbicide treatment. Key words: Herbicide timing, narrow-row soybean, soybean cultivars, 2601R, 2701R, S14-M7