Response of wheat (Triticum aestivum L.) cultivars and downy brome (Bromus tectorum L.) to metribuzin and ethyl metribuzin Public Deposited

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  • Downy brome (Bromus tectorum. L.), a serious grass weed in eastern Oregon can be partially controlled in wheat by metribuzin [4-amino-6-tertbutyl- 3-(methylthio)-as triazin -5(4H)-one]. However, cultivars of many crops Lave been reported to differ in their level of tolerance to metribuzin. This lack of tolerance has led to the investigation of other compounds, such as the ethylthio analog of metribuzin [4-amino-6-tert-butyI-3-(ethylthio)-as-triazin- 5(4H)-on], for selective control of downy brome in wheat. Greenhouse and growth chamber experiments on wheat, growing in soil or nutrient solution showed that: 1) GR50 values for Stephens, Hill 81, Yanahill, and Malcolm in soil were 0.41, 0.23, 0.25, and 0.44 kg/ha, respectively, for metribuzin and 1.89, 1.29, 1.37, and 1.97 kg/ha, respectively, for ethyl metribuzin; 2) GR50 values for these cultivars in nutrient solution were 3.26, 1.17, 1.32, and 3.87 uM, respectively, for metribuzin and 12.69, 7.20, 7.19, and 13.41 uM, respectively, for ethyl metribuzin; 3) Uptake of both herbicides.; by wheat was through the roots; 4) Cultivars did not show any differences in herbicide absorption from the nutrient solution. Therefore, differences in varietal tolerance may be are due to biochemical or physiological reasons, but are not due to differences in herbicide uptake. Increased growth at lower concentrations of these herbicides was observed in all cultivars studied. Metribuzin was five times more active than ethyl metribuzin. Both of these herbicides can be applied as an early postemergence treatment at the 3 leaf stage of wheat for effective control of downy brome. Yield components of the four cultivars were measured in response to metribuzin and ethyl metribuzin at two ecologically different sites. The susceptible cultivars (Hill 81 and Yamhill) showed greater injury in the lighter soils with high pH and lower organic matter content. Visual estimates of crop injury were generally higher than the actual reduction in final yield. Crop injury ratings up to 25% generally did not show a significant reduction in final grain yield. A positive correlation between protein content and concentration of these herbicides was noted. A study on the joint action of these two herbicides in the field and greenhouse showed that the combination of the two, increased the control of downy brome at lower rates. Their combined action was studied by using Colby's method and isobole method of Tammes. The reaction was calculated as additive. Downy brome was 8, 5, 5, and 9 times more sensitive to metribuzin and 6, 4, 4, and 6, times more sensitive to ethyl metribuzin than were the cultivars Stephens, Hill 81, Yamhill, and Malcolm, respectively. The use of these two herbicides to control downy brome in the cultivars Hill 81 and Yamhill should be avoided unless careful attention is given to soil properties and environmental conditions.
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