Delayed Glyphosate Application for No-Till Fallow in the Driest Region of the Inland Pacific Northwest

Abstract

Farmers typically use three applications of glyphosate to control weeds in no-till fallow. Some are now experimenting with an unconventional modification to this widely used approach. This modified approach is based on an intentional delay in the time of the first spraying. Farmers delay their first spraying because they want to rely on competition from winter annual grasses to suppress the growth of Russian thistle and eliminate the need for a third application. Optimism for this kind of weed-control program is tempered by concerns related to soil water storage. The objective of this research was to evaluate effects of delayed control of downy brome and volunteer winter wheat on the plant-available water content of, and loss of water from, no-till fallow. Treatments, applied to plots arranged in a randomized complete block design with four replications, were distinguished by the time of the initial glyphosate application.The initial early-season treatment was applied as soon as possible after emergence of downy brome and volunteer winter wheat. Initial mid-season and late-season treatments were applied 4 and 6 wk later, respectively. The amount of plant-available water in the soil profile ranged from 71.8 to 153.7 mm in May and 16.5 to 80.9 mm in September. Water loss was usually minimized in plots treated with the initial early-season treatment. An exception to this trend occurred at a site where the density of downy brome and volunteer winter wheat was greater than average. Abated water loss from the initial late-season treatment, at this site, may have been a consequence of reduced evaporation caused by a decrease in near-surface wind speed and deflection of solar radiation away from soil. Estimated impacts of water loss on grain yield of winter wheat, produced the year after fallow, range from 269 to 600 kg ha−1.