Cover Crop Biomass Production and Water Use in California' San Joaquin Valley.

The value of using cover crops to improve the efficiency and productivity of cropping systems while also minimizing adverse environmental impacts has been widely documented.  Although surveys on cover crop use have not been conducted in California's semi-arid, highly productive Central Valley (CV), estimates of current cover crop use in the state's annual cropping systems are quite low due not only to farmer concerns about opportunity costs involved in forgoing cash crop income, but also to questions about the cost and availability of additional water that will be needed to grow a cover crop particularly during periods of drought.  We took advantage of a unique, long-running cropping systems comparison study that has been underway in Five Points, CA since 1999, to quantify cover crop biomass production under winter rainfall and limited supplemental irrigation, to determine effects of winter rainfall variability, prior crops and tillage management on cover crop dry matter accumulation in the CV, and to compare changes in soil water under cover crops and winter fallowed soil.  Over the fifteen years of the study, a total of 56.2 MT ha^-1 of biomass (dwt) from a cover crop consisting of rye (Secale cereale), triticale (Triticosecale) and pea (Pisum sativum) was produced with winter rainfall and 20 cm of supplemental irrigation in three of the years.  Soil volumetric water content in the surface 90 cm during the November through March winter cover crop growing window averaged 5.3 cm higher in the fallow soil than under a cover crop.