Weed management in strip- and conventional-tillage non-transgenic and transgenic cotton.

Studies were conducted to evaluate weed management systems in non-transgenic, bromoxynilresistant, and glyphosate-resistant cotton in stripand conventional-tillage environments. Tillage did not affect weed control, cotton lint yields, or net returns. Early season stunting in strip-tillage cotton was 5% or less, regardless of herbicide system or cultivar and was transient. Excellent (>90%) control of common lambsquarters, common ragweed, Ipomoea species including entireleaf, ivyleaf, pitted, and tall morningglories, and prickly sida was achieved with systems containing bromoxynil, glyphosate, and pyrithiobac early postemergence (EPOST). Glyphosate systems provided better and more consistent control of large crabgrass than bromoxynil and pyrithiobac systems. Bromoxynil and pyrithiobac EPOST did not control sicklepod unless applied in mixture with MSMA and followed by (fb) a late postemergence-directed (LAYBY) treatment of prometryn plus MSMA. Palmer amaranth was controlled (>90%) with all glyphosate and pyrithiobac systems and the bromoxynil system that included a broadcast soil-applied herbicide treatment. Bromoxynil systems without a broadcast soil-applied herbicide treatment controlled Palmer amaranth 87% or less. Herbicide systems that included glyphosate EPOST controlled sicklepod with or without a soil-applied herbicide treatment. The highest yielding cotton included all the glyphosate systems and bromoxynil systems that contained a soil-applied herbicide treatment. Non-transgenic systems that included a soil-applied herbicide treatment yielded less than soilapplied treatment plus glyphosate EPOST system. Net returns from glyphosate systems were generally higher than net returns from bromoxynil or pyrithiobac systems. INTRODUCTION Historically, cotton has been grown in a conventional-tillage environment using primary and secondary tillage. Prior to the registration of postemergence (POST) herbicides with over-the-top selectivity, cotton producers used intensive soil-applied herbicide treatments and high rates of relatively non-selective herbicides and specialized equipment for postemergence-directed (PDS) applications (Buchanan, 1992; McWhorter and Bryson, 1992; Wilcut et al., 1995, 1997). Strip-tillage cotton acreage is increasing across North Carolina and the Southeastern Coastal Plain (Anonymous 2002). There are several advantages for utilizing strip-tillage production systems. These advantages include: (1) water conservation and reduction of sand blasting of cotton on sandy soils, (2) reduced tillage operations and the number of trips made across the field, and (3) improvement in soil tilth and water-holding capacity over time (Bradley, 1995). Strip-tillage production systems work well in soils that develop a hardpan or plow layer that impedes root growth (Sholar et al., 1995). Low weed control has been cited as the major limitation to adoption of cotton in conservation-tillage cotton production (McWhorter and Jordan, 1985). Soil-applied herbicides do not provide seasonlong weed control in cotton, therefore proper selection of POST herbicides and other inputs are crucial for maximum weed control, cotton yield, and economic returns (Crowley et al., 1979;