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Application Temperature Influences Pyrithiobac (Staple(R)) Efficacy in a Semi-Arid Environment

Ginger G. Light, Peter A. Dotray and James R. Mahan


Weed control by Staple® has been inconsistent since its commercial introduction in 1996. The influence of temperature at the time of post-emergence Staple® applications was investigated as a source of the variability in field activity. Over two growing seasons, sixteen independent and random applications of Staple® were made to 2-4 inch Palmer amaranth (Amaranthus palmeri S. Wats). The herbicide was applied at a rate of 0.063 lbs. a.i./A in a carrier volume of 15 GPA with 1% crop oil concentrate using a backpack sprayer. Plant/soil scene temperature was monitored with an infrared thermometer and air temperature was monitored with a thermocouple. These temperatures were recorded at 15-minute intervals from mid-June to late September of both years. Field activity, expressed as a percentage of the non-treated dry weight, was quantified as the dry weight accumulated by treated plants for a 14 day period following herbicide application. Accumulated dry weight ranged from 0.1% to 71.5% when compared to non-treated Palmer amaranth. Field activity differences were correlated with plant/soil scene temperature at herbicide application. Applications at temperatures above 93° F resulted in poor activity.

To determine the source of the thermal limitations on Staple® efficacy, thermal dependence of the inhibition of the target enzyme, acetolactate synthase (ALS), was examined. A crude extract of acetolactate synthase was obtained from Palmer amaranth and utilized in microtiter assays. The herbicide concentrations where the enzyme was inhibited by 50% (I50 values) were obtained from 50 to 122° F in 9° increments. The lowest I50 value occurred at 86° F, indicating the most efficient inhibition of the enzyme. At temperatures below and above this optimal temperature, the enzyme inhibition was less efficient. Comparison of field activity against I50 values showed that both accumulated dry weight and I50 values increase rapidly above 93° F. Therefore, the thermal dependence of the ALS-Staple® interaction may contribute to inconsistent field activity.

Field activity was correlated with plant/soil scene temperature, a parameter not readily available to producers. However, analysis of temperature data showed that below 93° F, air temperature adequately represented plant/soil scene temperature. Based on this study, air temperatures of 68 to 93° F optimize Staple® activity. Applications made outside this range may provide inconsistent weed control.

Reprinted from Proceedings of the 1999 Beltwide Cotton Conferences pp. 747 - 748
©National Cotton Council, Memphis TN

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Document last modified Monday, Jun 21 1999