WINTER SHEEP GRAZING IN THE IRRIGATED SONORAN DESERT .2. SOIL PROPERTIES AND ALFALFA REGROWTH

Irrigated alfalfa (Medicago sativa L.) fields of the Sonoran Desert of the southwest USA produce high quality hay for 9 to 10 mo and also become winter pasture for sheep (Ovis aries). Presently, the common practice is to have the sheep graze until only bare ground and grazed alfalfa crowns remain. Our objective was to evaluate the effects of sheep stocking rates on first-cutting alfalfa regrowth and soil physical properties. Water infiltration rate and soil penetration resistance were measured following 0, 200, 400, and 600 head d/acre (equivalent to 40 lambs grazing one acre for 0, 5, 10, and 15 d, respectively) of grazing in December and January. Alfalfa yield was taken during the 20 April harvest. Water infiltration rates were high (> 0.2 in./h) and were not affected by grazing for this irrigated pasture system, which contradicts the results for rangeland. Hoof action by sheep only effected penetrability in the surface 2 in. (P = 0.024). Alfalfa yield was 5.4% greater (P = 0.089) for the grazed vs. the ungrazed plots. We conclude that intensive winter sheep grazing was not detrimental to plants and soils of the irrigated pastures of the Sonoran Desert.