A Comparison of Sheep Grazing with Herbicides for Weed Control in Seedling Alfalfa in the Irrigated Sonoran Desert

Abstract

A three year study was conducted in the irrigated Sonoran Desert to compare the effect of different weed management methods in seedling alfalfa (Medicago sativa L.) on crop stand and yield. Treatments included ; grazing with sheep (Ovis aries L.) when the crop was ready for the first harvest, a combination of preemergence and postemergence herbicides, postemergence herbicides only, and an untreated control where weeds were harvested with the hay. Weed management practice did not affect alfalfa yield in the first season, although the herbicide treatments reduced total forage (alfalfa plus weeds) yield compared with the grazed treatments and the untreated control. Crop density was not different between treatments. Herbicide treatments lowered forage yields at the first harvest by eliminating of weeds and because of crop injury in 2 of the 3 yr. At the third and subsequent harvests, there were no differences in forage yield for treatments. Plots were weed free after the second harvest. Lamb grazing selectivity in weedy seedling alfalfa was also quantified by analyzing esophageal extrusa. The lambs were selecting the weeds over the alfalfa as grazing progressed. This preference was consistent between lambs and plots, although there were year differences. Forage quality of the winter annual broadleaf weeds present in this study was comparable with the alfalfa. We concluded that grazing lambs are a good weed control method in seedling alfalfa during the winter grazing season in the irrigated Sonoran Desert.