Evaluation of transgenic cotton lines tolerant to glufosinate, bromoxynil, and both glufosinate and bromoxynil
Becker, William David
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The adoption of weed control systems that use biotechnology-derived, herbicide-tolerant crops has increased rapidly since their commercial introduction in 1997. The limited availability of effective, broad-spectrum, post-emergence herbicides has been a contributing factor in the widespread use of these weed control systems in cotton production. The prospect of multi-transgene-stacking of herbicide tolerances provides opportunities for improved weed resistance management, enhanced weed control, increased flexibility, and decreased weed control costs. Cotton varieties have been previously developed that are tolerant to bromoxynil or glufosinate-ammonium. For the purposes of this study, experimental cotton lines were developed that are tolerant to bromoxynil or glufosinate-ammonium and to both bromoxynil and glufosinate-ammonium. The lines were developed in order to evaluate the effects of the bxn and fcar genes singularly and in combination on cotton growth, agronomic characteristics, and fiber properties. Four crosses were made between a bromoxynil-tolerant cotton line and plants from a glufosinate-ammonium-tolerant population. F2 derived lines were developed from each of these crosses that express either: (1) bromoxynil tolerance, (2) glufosinate-ammonium tolerance, (3) tolerance to both herbicides, or (4) no herbicide tolerance. Sixteen treatments (four crosses and four herbicide traits) were evaluated in a randomized complete block design with a factorial arrangement of treatments. The trials were conducted over two years, with one location being used in 2002 and three locations being used in 2003. There were no significant main effect differences observed among herbicide traits for either year, except for maturity across locations in 2003, and plant height in one location in 2003. Results from the 2002 experiment showed significant interaction effects between cross and herbicide trait for storm resistance, plant height, and lint percentage, as well as the fiber properties of length, strength, micronaire, and uniformity. The 2003 results showed a significant interaction effect across locations between cross and herbicide trait for storm resistance, lint percentage, lint yield, fiber length, strength, micronaire, and uniformity. The inconsistent differences observed over crosses among herbicide traits for each of the characteristics evaluated may be due to a relatively small effect of the expression of the herbicide tolerance genes when compared to the inherent variation typically observed in F2 derived cotton lines. The variation observed in each of the characteristics evaluated, among the experimental lines, was within the normal range of what would be expected from non-transgenic cotton lines developed from F2 derived cotton populations. Thus, the expression of the bar and bxn genes singularly or in combination in these tests did not substantially affect cotton growth, agronomic characteristics, or fiber properties. Transgene efficacy tests using experimental lines tolerant to both bromoxynil and glufosinate-ammonium showed no detrimental effects on early season cotton growth when both herbicides were applied in combination.