Submitted to: National Cotton Council Beltwide Cotton Conference
Publication Type: Proceedings
Publication Acceptance Date: 1/3/1999
Publication Date: N/A
Technical Abstract: Weed control by Staple has been inconsistent since its commercial introduction in 1996. The influence of temperature at the time of herbicide application was investigated as a source of the variability in field activity. Sixteen independent and random observations from field studies performed over 2 growing seasons identified efficacy differences that were correlated with environmental temperature. Field activity was quantified a the dry weight accumulated by treated plants for a 14 day period following a Staple a application expressed as a percentage of the non-treated dry weight. Accumulated dry weight ranged from 0.1%-71.5%. Accumulated dry weights of <10% were equivalent to good weed control while accumulated dry weights >10% were equivalent to poor weed control. Application temperatures above 34 degrees C(93degrees F) were the temperatures where poor control occurred. Based on the field observations, air temperatures of 20-34 degrees C optimize weed control by Staple. To determine the source of the thermal limitations on Staple efficacy, the thermal dependence of the inhibition of the target enzyme, acetolactate synthase(ALS), was examined. The concentrations of Staple where the enzyme was inhibited by 50%(I_50 values) were obtained from 10-50 degrees C in 5 degree increments. The lowest(I_50 value) occurred at 30 degrees C, indicating efficient inhibition of the enzyme. At temperatures below and above this optimal temperature, the enzyme inhibition becomes less efficient. Regression analysis of field activity against I_50 values showed the 2 data sets to be highly correlated (R=0.94). Therefore, the thermal dependence of acetolactate synthase may explain, in part, the inconsistent weed control observed in field applications of Staple.