Perennial weed management in glyphosate- and bromoxynil-tolerant cotton

Texas Southern High Plains cotton (Gossypium hirsutum L.) producers lose yield to weeds each year. Crop losses caused by weeds are a direct result of competition with weeds for light, water, and mineral nutrients, as well as harvest problems and lowering of crop quality because of contamination of crop seed by weed seed (Janick et al. 1981). The presence of problem weed species result in reduced cotton lint yields (Bridges and Chandler 1984). Across the Cotton Belt, weeds still cause yield reductions that total $188 million annually or 5.2% of the total crop value in 1985 (Chandler and Cooke 1992). Weed management systems currently used in cotton can cost from $55 to $165 per hectare for a total cost of approximately $410 million annually in the United States (Chandler and Cooke 1992). Annual weed species, which complete their life cycle in one growing season, are controlled by preplant incorporated and preemergence herbicide treatments. Perennial weeds live for three or more years and propagate either by seeds or by roots, rhizomes, stolons, tubers, corms, and bulbs. Preplant and preemergence herbicides provide little control of perennial weed species. Perennial weeds are among the most difficult to control simply because of their extensive underground reproductive system. Some perennials also produce viable seed in the soil, which allow for new populations in future years. The high weed-seed populations of agricultural soils make weed control a continual and integral part of crop culture (Janick et al. 1981).