Experiments were conducted in 2002 and 2003 at the University of Arizona Maricopa Agricultural Center to evaluate the toleranceof RR Flex cotton to topical Roundup Weathermax (glyphosate) applications and to study weed management programsin RR Flex cotton. RR Flex cotton demonstrated excellent tolerance to glyphosate as measured by pollen viability andyield. The presence of the RR gene did not affect the yield of genotype pairs that were identical except for the presence or absenceof the RR Flex genetic construct. In the weed management study, delaying the first topical glyphosate application resultedin larger, more difficult to control weeds and reduced cotton yield by allowing greater early season competition betweenweeds and cotton. The best weed control programs included early (1 to 2 leaf growth stage) topical applications atrates greater than 0.75 lb ae/A and a second Roundup application after the first post-planting irrigation (10 node growthstage). The data also suggested that there may be significant value in making a layby, directed-broadcast application that includesa residual herbicide such as Caparol (prometryn) at layby.
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