Impacts of Weed Resistance to Herbicides on United States (u.S.) Cotton (Gossypium Hirsutum) Production

Because of diversity in climate, weed flora, and cotton production practices among the localities that span the nearly 5000-km, arc from Virginia to California that comprises the US Cotton Belt; the weed resistance found in the fields, areas, and four, commonly-recognized US cotton production regions differ in resistant species, their competitive effects, the management practices implemented against them, and their economic impacts. Each field generally has a collection of resident weed species. Many US cotton fields have one or more weed species that singly or in combination express resistance to one or more herbicide mechanisms of action (MOAs). The effects of such incidences of resistance on the composition of the weed flora, the practices and relative success of cotton production, and on its financial returns vary. The cotton industry historically recognizes four growing regions, the Far West – California, Arizona, and New Mexico; the Southwest – Texas and Oklahoma; the Mid-South – Arkansas, Louisiana, Mississippi, the Boot Heel of Missouri, and western Tennessee; and the Southeast comprising Alabama, Florida, Georgia, the Carolinas and Southeastern Virginia. The arid Far West is 100% irrigated, and generally furrow-irrigated land in alluvial valleys. To this point the Far West is the region least touched by weed resistance. The Southwest, chiefly comprising Texas, is by far the largest cotton growing area, and is itself quite diverse. It has high-yielding, mostly center-pivot irrigated cotton, and large tracts of non-irrigated, so-called 'dry land' cotton that tends to be grown with few inputs. Glyphosate-resistance in certain species, chiefly Palmer amaranth (Amaranthus palmeri) is found in the Southwestern region, but not yet to the full extent that it occurs in the humid Southeast and Mid-South regions. The Southeast is an area where many agronomic and horticultural crops, including fruit and nut orchards are grown in relatively small fields (many approx.10–25 hectares). The growing season is long (> 150–225 frost free days) and the weed pressure is intense and continuous (> 7 months for summer crops). The Mid-South region includes both alluvial soils with intense weed pressure in the Mississippi flood plain and loessial hills and fertile bottoms in the Uplands of Arkansas, Mississippi and Tennessee. Each region is different from the others in geomorphology, soils, cotton production practices, weeds, weed management, and as a consequence the type of herbicide selection and the relative presence or prevalence of weed resistance. The reasons for the changes in weed control practices are discussed leading to the conclusions that the major post emergence herbicides used in row crops in the United States are enabled by transgenic herbicide resistance traits in their respective crop cultivars. Selection of a cultivar is now also selection of a weed management system. No new herbicide mechanism of action has been registered in the United States since 1993 (the HPPDs), and it shows. Following the demise of glyphosate to weed resistance, great selection pressure has been placed on the remaining effective post-emergence mechanisms of action, the PPOs and glufosinate and now the auxins, 2,4-D and dicamba. The recent failure of the PPOs in the upper Mid-South is poised to create a crisis in weed management in soybean that is only partially being solved by the auxins. To this point, the new dicamba formulations have dominant market share for in-crop use vs. 2,4-D, but dicamba has been plagued by extensive off target movement. Cotton production now depends on the use of overlapping pre-emergence herbicides and hopeful use of post-emergence herbicides augmented by mechanical means – cultivation, and in certain areas, hand weeding. Cotton yields could increase because of improved genetics. Cotton weed control remains good because of increased herbicide use and increased supplementation by mechanical control. However, the costs of weed control in tillage, trait costs, increased herbicide use, and hand weeding have greatly increased with respect to those of 10 years ago. Such costs would be adversely affecting cotton hectares planted, were not the situation for weed control in soybeans in the Mid-South not as bad or worse than that of cotton.