Adoption of permanent cover crops and no-till systems is considered integral to achieving the California state air quality standards regulating airborne particulate matter, as indicated in Senate Bill 656. Imposition of such management techniques in a vineyard could create competition with vines for limited water resources. We evaluated the effects of cover crops that were either tilled or just mowed on vineyard floor composition, weed populations, vine water relations and growth, and fruit composition over three years within a mature commercial Merlot vineyard subjected to deficit irrigation in Lodi, San Joaquin Valley, California. The vineyard floor in this experiment supported resident vegetation that was tilled (standard grower practice), an oat cover crop, or a legume/oat cover crop. The two planted cover crops were either tilled or mowed (i.e., no-till). Biomass of cover crops, weeds, and legumes varied by year and treatment, but consistent effects among treatments were not observed. Weed species composition and cover segregated with the presence or absence of tillage rather than cover crop type and the weed species composition of the resident vegetation was distinct from those in the cover crop treatments. Some treatments, like 'Oats/Legumes + NoTill', 'Oats + NoTill', 'Oats/Legumes + Till', and 'Resident Vegetation + Till' reduced soil water content (circle minus(v)) in at least one of the three shallow soil layers spanning 0 to 30 cm, 30 to 60 cm, and 60 to 100 cm in 2008, and 'Oats/Legumes + NoTill' also dried the two upper layers in 2009, but these differences had no consistent influence on plant water status. Distinctions in circle minus(v) between years were attributed partly to the cessation of rainfall two months earlier in 2008 than in 2009, despite similar total annual quantities. Significant reductions in circle minus(v) imposed by the 'Oats/Legumes + NoTill' treatments reduced vine vegetative growth in two of three years, but these effects did not manifest in yield and fruit composition. Values for the Ravaz index for two of the three years indicate that the vineyard was overcropped for all treatments, but maximizing production in this region is a common practice. Weak competitive effects of the cover crops for water were likely associated with the use of a well-established mature vineyard and demonstrated that these management strategies could be employed to improve air quality to meet California air quality regulations with limited effects on vine water status and production.USDA-ARS Sustainable Viticulture Production Systems (NP305 Crop Production) 2032-21220-006-0

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