Transgenic cotton with improved resistance to glyphosate herbicide

Glyphosate [N-(phosphonomethyl)glycine]herbicide can be topically applied twice at rates as high as 0.84 kg a.e. (acid-equivalent) ha(-1) to glyphosate-resistant cotton (Gossypium hirsutum L.) cultivars until the fourth true leaf stage, with the requirement of at least 10 d and two nodes of growth between applications. But, such cultivars are not reproductively resistant to glyphosate applied topically or imprecisely directed after the four-leaf stage because glyphosate can curtail pollen development and ovule fertilization, which potentially reduces yield. Extending glyphosate resistance past the four-leaf stage would provide growers with additional weed management options. Our objective was to test under field conditions glyphosate resistance of cotton germplasm transformed with gene constructs previously shown to impart extended glyphosate resistance in the greenhouse. Four or six transgenic cotton lines containing one of several constructs conferring extended glyphosate resistance, plus the current glyphosate-resistant control ('Coker 312'-1445), were tested at nine U.S. locations in 2001. Within locations, treatment designs consisted of cross-classified arrangements of transgenic lines and glyphosate rates [0, 1.68, and 2.52 kg a.e. ha(-1)]. Treated plots received glyphosate over-the-top of cotton at four growth stages (3-, 6-, 10-, and 14-leaf crop stages). Compared with Coker 312-1445, extended glyphosate resistance was expressed as higher yields when glyphosate was applied topically at the four growth stages. Mature plant mapping confirmed extended glyphosate resistance of the new transgenic cotton through similar fruit distribution and weight with or without glyphosate treatment. The capability to apply glyphosate topically to cotton later in crop development will facilitate weed management and could reduce dependence on directed herbicides.