Control of silverleaf nightshade in Roundup Ready cotton with Roundup Ultra

摘要

The second year of this field experiment was conducted to determine the most appropriate cotton growth stage or stages to apply Roundup Ultra~(R) (RU~(R)) herbicide to maximize the control of silverleaf nightshade in Roundup Ready~(R) cotton. Treatments consisted of a control (2 cultivations) and application of RU~(R) on 3 dates in various combinations. Date 1 applications were at the 4 leaf stage, date 2 applications were at 3 weeks past the 4 leaf stage, and date 3 applications were at 20 percentopen bolls. Each treatment was applied to the land areas as in 1999. In 2000, two weeks after the date 3 spray application, approximately 85, 77, 62, 90, and 98 percent weed control was achieved with spray date 1, date 1 plus 2, date 3, date 1 plus 3, and date 1 plus 2 plus 3, respectively when compared to the control. The percent weed biomass competition was highest with the control. The very hot and dry 2000 growing season severely affected lint yields. Lint yield in 2000 ranged from 38 to 57 lbs/A.Even though yield differences among treatments were significant, they were all too low to be meaningful. Silverleaf nightshade numbers were compared between the spring of 1999 (beginning of study) and the fall of 2000 (end of study). During this time silverleaf nightshade numbers had increased by more than 3 times in the control plots. Application of RU~(R) at the 4-leaf stage and/or 3 weeks past the 4 leaf stage resulted in more than a 65 percent reduction in stems/A whereas spraying once at 20 percentopen bolls resulted in only an 18 percent reduction. A combination of RU~(R) applied at an early stage and at 20 percent open bolls reduced nightshade stems/A by 90 percent. In 2000, cotton quality was inferior to normal quality due to heavy rainfall 2weeks prior to cotton harvesting. Regression analysis showed yield losses of 0.12 lbs/A for each increasing silverleaf nightshade stem per acre in 1999.