Tolerance of Pasture Grasses to Deficit Irrigation in Northern California.

Irrigation water is a highly valued and limited resource in the irrigated West.  The long-term availability of irrigation water is a major concern of forage producers. Next to alfalfa, irrigated pasture is the highest water-use crop in the intermountain area of Northern California. Research was needed to determine the effect of early-season irrigation termination on grass yield and persistence and to identify pasture grass species that are better able to withstand drought. A trial was established in the spring of 2005 to evaluate deficit irrigation of pasture grasses.  After a year of uniform irrigation for grass establishment, three different irrigation treatments were imposed: 1) Normal full-season irrigation; 2) Irrigation cut-off on June 1^st; and 3) Irrigation cut-off on July 15th.  Irrigation treatments were the main plots and pasture species/variety was the subplot treatment.  Irrigation treatments were imposed in 2006 through 2008 and the plots were harvested three times each year.  All plots were harvested once in June 2009 to assess the residual effect of the irrigation treatments. Twenty six different grasses were evaluated including 10 tall fescue varieties, 7 orchardgrass varieties, 4 brome species, 3 wheatgrass species, and a festulolium and Harding Grass variety.  Dramatic differences were found in the yield and persistence of the different grass species. Unlike previous results for alfalfa, grass yield dropped significantly for the cutting following irrigation termination. Brome species and wheatgrasses were more drought tolerant than tall fescue or orchardgrass species.  However, these species did not tolerate full season irrigation and when fully irrigated their stand was dramatically reduced. Orchardgrass varieties tolerated a single season of deficit irrigation but over years the stand did not persist as well as tall fescue varieties.  Overall, tall fescue appeared to be the species best adapted to the combination of full irrigation and deficit irrigation.