Weed management in transgenic, herbicide-tolerant crops

Weed interference and subsequent reduction in crop yield, harvest efficiency, and quality are major concerns for producers. Although standard herbicide programs are effective in managing weeds, considerable losses from weeds continue to occur. Use of genetically engineered, transgenic, herbicide-tolerant crops may enhance weed management and economic returns in crop production systems.;Weed management, yields, and net returns from standard and transgenic systems in corn, cotton, and soybean were compared in field studies from 1996 to 1998. Transgenic, herbicide-tolerant cultivars included glufosinate-tolerant corn, glufosinate- and glyphosate-tolerant soybean and bromoxynil- and glyphosate-tolerant cotton.;In corn, standard systems of metolachlor plus atrazine preemergence or nicosulfuron plus atrazine postemergence controlled goosegrass, morningglory, common lambsquarters, and smooth pigweed at least 70% late-season. Glufosinate alone controlled all weeds less effectively than standard systems. However, metolachlor preemergence followed by glufosinate postemergence or glufosinate plus atrazine postemergence controlled weeds similar to or more effectively than standard systems. With the addition of ametryn at layby, weeds were controlled at least 96%.;In soybean, glyphosate controlled rhizomatous johnsongrass, goosegrass, and smooth pigweed more effectively than glufosinate. In contrast, glufosinate controlled morningglory more effectively than glyphosate. Sequential applications of either glufosinate or glyphosate and soil-applied herbicides in conjunction with glufosinate or glyphosate controlled weeds at least 87% late-season. Standard systems utilizing imazaquin plus dimethenamid preemergence followed by chlorimuron postemergence controlled all weeds, except johnsongrass, similar to the more effective glufosinate or glyphosate systems.;Cotton grows more slowly and is less competitive with weeds than corn or soybean. Therefore, standard herbicide systems often include pendimethalin preplant incorporated, fluometuron preemergence, fluometuron plus MSMA early postemergence-directed, and cyanazine plus MSMA late postemergence-directed. This standard system controlled broadleaf signalgrass, large crabgrass, morningglory, and smooth pigweed at least 99% late-season. Weeds were controlled similarly in transgenic, herbicide-tolerant cultivars utilizing bromoxynil systems in BXN cotton and glyphosate systems in Roundup Ready cotton.;Bromoxynil does not control grasses, and grass can escape soilapplied herbicide programs used in BXN systems. Producers would prefer to mix postemergence graminicides with bromoxynil for control of both broadleaf and grass weeds simultaneously. Field, greenhouse, and laboratory studies suggest that bromoxynil can antagonize postemergence graminicides labeled for use on cotton and this loss in activity may be due to a reduction in graminicide absorption.;Even though both non-transgenic and transgenic systems can be successful, each system offers advantages and disadvantages. Herbicides used in transgenic, herbicide-tolerant crops allow broad spectrum weed control and the convenience of postemergence over-the-top applications, usually with excellent crop safety. Additionally, these systems offer the potential to reduce or eliminate prophylactic soil-applied herbicides which may reduce total herbicide use and risk of herbicide carryover. However, transgenic, herbicide-tolerant crops are not without potential disadvantages including increased seed costs and technology fees as well as ethical and public concerns.;Our work demonstrated that when transgenic, herbicide-tolerant cultivars are treated with transgenic systems, crop yields were similar to or exceeded yields from non-transgenic or transgenic, herbicide-tolerant cultivars treated with standard systems. (Abstract shortened by UMI.)...