Abstract
Giant ragweed (Ambrosia trifida L.) is competitive with agronomic crops and can cause significant yield losses. Rapid adoption of glyphosate-resistant (GR) crops and a concomitant increase in the reliance on glyphosate for weed management has led to the evolution of GR giant ragweed in Ontario, Canada. Field studies were conducted to evaluate the level of resistance in giant ragweed biotypes from Ontario, and to evaluate the effectiveness of various postemer-gence (POST) herbicides in soybean (Glycine max L.). The effective dose (ED) to provide 50%, 80% and 95% giant ragweed control was up to 1658, 9991 and >43200 g?a.e.?ha–1 4 weeks after application (WAA), respectively. For effective control, growers would need to apply glyphosate 18 times greater than the recommended field application dose. Glyphosate applied at the recommended field dose of 900 g?a.e.?ha–1 provided up to 57% control and resulted in soybean yield equivalent to the weedy check. Cloransulam-methyl applied POST provided up to 99% control, reduced giant ragweed density 98%, reduced giant ragweed shoot dry weight 99% and resulted in soybean yield equivalent to the weedfree check. Chlorimuron-ethyl, fomesafen, imazethapyr and imazethapyr plus bentazon applied alone or with glyphosate did not provide adequate control of GR giant ragweed. Based on these results, some GR giant ragweed biotypes from Ontario have evolved a high level of resistance to glyphosate. Cloransulam-methyl applied POST was the only herbicide that provided adequate control and suggests that additional weed management tactics will need to be implemented in order to effectively manage GR giant ragweed.
Highlights
Glyphosate [N-(phosphonomethyl)glycine] was developed by John E
Cloransulam-methyl applied POST was the only herbicide that provided adequate control and suggests that additional weed management tactics will need to be implemented in order to effectively manage GR giant ragweed
One set of experiments evaluated the response of giant ragweed to with Postemergence Herbicides varying doses of glyphosate, and another set evaluated various herbicides registered for POST application in soybean (POST herbicides)
Summary
Glyphosate [N-(phosphonomethyl)glycine] was developed by John E. By 1974, glyphosate was commercially introduced in several markets as a postemergence (POST), non-selective herbicide for the control of weeds prior to crop planting [1,2,3]. Glyphosate is translocated from foliage to the roots, rhizomes, and apical tissues and controls hard-tokill perennials such as Canada thistle Prior to the mid 1990’s, the use of glyphosate was limited in field crop production because it killed treated crops [6]. The introduction of glyphosate-resistant (GR) crops, mainly soybean (Glycine max L.), canola (Brassica campestris L.), cotton (Gossypium hirsutum L.) and corn (Zea mays L.) between 1996 and 1998 allowed growers to apply POST applications of glyphosate to the crop for the control of emerged weeds without crop damage [3,7].
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