Influence of Early‐Season Nitrogen and Weed Management on Irrigated and Nonirrigated Glyphosate‐Resistant and Susceptible Soybean

Abstract

Field studies were conducted on Sharkey clay soil (very‐fine, smectitic, thermic Chromic Epiaquert) at Stoneville, MS (33°26′ N lat). The objectives were to determine the effect of application of 0 and 35 kg N ha−1 applied early in the growing season to glyphosate‐resistant (GR) and non‐GR soybean [Glycine max (L.) Merr.] cultivars using two weed management systems in irrigated and nonirrigated environments. Weed management systems were (i) pre‐emergent followed by postemergent weed management using nonglyphosate herbicides applied to both GR and non‐GR cultivars (PRE + POST) and (ii) postemergent weed management using glyphosate on GR cultivars and nonglyphosate herbicides on non‐GR cultivars (POST). Applied N had no effect on weed management in or yield from soybean and lowered average net returns by $28 to $50 ha−1. Average seed yields from the highest‐yielding GR cultivar in 1999 and 2000 were 135 and 270 kg ha−1 more than 1999 and 2000 yields from a non‐GR cultivar in the nonirrigated environment (all net returns were negative and yields <1500 kg ha−1). In the irrigated environment, use of a non‐GR cultivar compared with a GR cultivar resulted in a significant 200 and 250 kg ha−1 greater yield and greater profits in 2 of 3 yr. Use of PRE + POST compared with POST‐only was not necessary for achieving greatest yield or net return with either non‐GR or GR cultivars. Use of postemergent glyphosate always resulted in the cheapest weed control ($43 to $81 ha−1), even with the greater cost for seed of GR cultivars included. There was no measured effect of glyphosate compared with nonglyphosate herbicides on GR cultivar yield.