Effects of glyphosate over-the-top applications on the reproductive growth of Roundup Ready cotton

Abstract

Roundup Ready technology is an additional tool that can be used against weed problems. However, previous studies have indicated that lint yield can be negatively impacted when plants are treated with over-the-top Roundup applications. Consequently, damage to reproductive organs by glyphosate has been proposed to be one cause of lint yield reduction. Deltapine 5690 RR was cultivated under field conditions using three different rates of Roundup Ultra applied at three different stages of plant growth; 4-, 8-, and 12-leaf. The rates used bracketed the single over-the-top recommended rate of Roundup Ultra of 2.37 L ha⁻¹ until the 4-leaf stage. Twelve hours after natural pollination, the degree of anther opening and the number of pollen grains attached to the stigma were determined for first position flowers. As the rate of glyphosate increased and the stage of application was delayed, a greater number of anthers remained closed and fewer pollen grains were attached to the stigma. This effect was accentuated in nodes 9-11. Pollen tube rate of growth, and distance traveled were measured at different times after artificial pollination. At eight hours after pollination no significant differences were found in pollen tube growth across all treatments. This data suggested that glyphosate has no inhibitory effects on pollen tube growth within the style. Plant mapping data indicated that, as the rate of glyphosate increased and the stage of application was delayed, less retention of first position fruit was observed within node range 6-10. This reduction in first position fruit retention was due to low pollen availability during pollination. Plants that produced fewer or no first position fruit within nodes 6-10, following glyphosate treatment, attempted to compensate yield losses by shifting fruiting patterns toward more distal positions. The closer the glyphosate application was to floral development, the greater the effect upon pollen availability. We concluded that Roundup Ready cotton yield was negatively affected mainly at the 8- and 12 -leaf stage of application due to low pollen availability in first position flowers within nodes 6-10. Over-the-top applications of Roundup showed no detrimental effects on lint yield when applied within the labeled rate.