This report describes a continuing on-farm project evaluating the use of two cover crop-based systems for pest, weed, and vine nutrition management in California vineyards. In two large vineyards, from 1992 to 1994, we compared two systems that used a winter annual, oat/vetch cover crop to a system that used clean-cultivation and conventional methods of chemical soil amendments and weed control. In one cover crop system, we used the cover as dry mulch by cutting the cover biomass and placing it on row berms for the suppression of annual weeds without the use of soil-applied herbicides. In the other cover crop system, the cover was cut and left in row middles, but as in the clean-cultivated system, weeds on berms were controlled chemically. Another vineyard was used during the 1992 season to determine the relative impact of spiders and other abiotic factors on leafhopper biology and abundance. We also initiated two additional experiments in 1993 using a merced rye/vetch cover crop, to compare a cover/mulch system with clean cultivation in a 2-acre vineyard located at the Kearney Agricultural Center. A large commercial vineyard located near Madera was used to compare clean cultivation to cover crops used as green manure or as reseed. We determined the impact of these systems on vine-nutrient status, weed suppression, and leafhopper pest and their natural enemies. We also developed operating cost budgets for each management system. The effect of cover crops on leafhoppers and their natural enemies varied between years and vineyards. Our findings to date indicate that, if properly managed, winter annual, legume/grass cover crops can reduce the use of insecticides for leafhopper control. Where leafhopper numbers were not very low and cover crops were properly maintained through early July, the presence of cover crops resulted in reduced infestations of leafhoppers. These reductions may be attributed in part to enhanced activity of certain groups of spiders, which were consistently found at higher densities in the presence of cover crops compared to the clean-cultivated systems. Trace-element marking of cover crops indicated that cover crops may also influence leafhopper populations by serving as non-host vegetation which interferes with their movement patterns and perhaps other aspects of their life history. Although cover crops may affect vine physiology (e.g., through changes in soil fertility, soil-water status, etc.) which may in turn affect leafhopper biology, the changes we have observed in vine conditions to date were not sufficient to explain the differences in leafhopper abundance between the cover crop and clean-cultivated treatments. Cage data indicated that leafhopper reproduction and survivorship were not affected by cover crop treatment The effect of cover crops on vine-nutrient status varied between years and vineyards. Cover crops produced positive effects on vine-nutrient status by the second or third year if the cover crops were managed well, but produced negative effects if the cover or vineyard was poorly managed. The positive effects were usually delayed, and were best illustrated by the results from our Fowler site where by the third year, N levels (nitrate-N in petioles) in unfertilized cover-cropped vines were similar to N levels in fertilized clean-cultivated vines, independent of the type of weed management in row berms. Potassium levels were also enhanced by cover crops by the third year at the Fowler site. We have not observed these effects in the Earlimart vineyard where the cover crop was not incorporated until Fall, while weeds were allowed to grow in row middles during the Summer. Under this ground cover management, apparently much of the nutrient content of the cover is either lost by volatilization, or used to grow the resident vegetation during the Summer. At the Kearney and Madera sites (second year), although we have not detected significant changes in vine-nutrient status in the cover crop treatments compared to clean-cultivated treatments, we have observed trends toward higher levels of nitrogen and potassium in vines associated with cover crops compared to clean-cultivated vines. 21 The amount of dry biomass produced by cover crops for weed suppression varied between vineyards. During late winter and early spring, the mulched berms received 1800 to 8,726 lbs of dry biomass per acre, with a total nitrogen content of 33 to 109 lbs per acre. To date, the results from the north coast (by C. Elmore et. al.) and from the San Joaquin valley indicate that with sufficient levels of biomass production, berm mulching should reduce the use of pre-emergence herbicides. The mulch, however, will not control all weeds equally. Perennial weeds such as field bindweed were not controlled, and we do not have enough data on yellow nutsedge to determine if mulching will be effective. We expect that in the long term, yearly accumulation of the dry mulch should incrementally increase the level of weed control resulting in substantial reductions in the use of soil-applied herbicides. ;The effects of cover crops on grape yield and operating costs depended on grape culture, and represented in a trade-off in water, fertilizer, pesticide and resource use. Although significant differences in yields have not been realized in the Earlimart (third year) and Madera (second year) vineyards, berry size and raisin yields were increased significantly higher by the second year at the Kearney site where cover crop biomass was used as dry mulch for weed suppression in row berms. Berry weight was also significantly greater by the third year in cover crop compared to clean-cultivated treatments at the Fowler site. Greater berry weights should have translated into greater raisin weights, but this effect could not be measured as the raisins at the Fowler site were badly damaged by early fall rain.. The partial cost budget indicated that the use of cover crops (despite greater water demand) may significantly reduce operating costs if savings were realized by reducing chemical inputs for insects and mites. These savings are expected to increase if cultural methods (e.g., raised beds with adjacent furrows for irrigation are used instead of flood irrigation) are modified to maintain satisfactory cover crop growth while reducing water use. Despite the encouraging results from the transition phase, several critical questions remain to be addressed in order to assess the long term impact of cover crops on several elements of grape production. At present we do not know what impact our cover crop systems will ultimately have on several elements of soil fertility and water use in vineyards. For example, we do not know if the increased yield at the Kearney and Fowler sites were due to the greater amount of water used to grow the cover crop. In this case, water usage should be controlled as an experimental variable so we can understand why higher yields were obtained in the presence of cover crops. Studies of more than three-year duration are needed to adequately determine the complex relationships between cover crops, arthropod pests, and weeds, and to evaluate their impact on soil fertility, vine nutrition, and vineyard water use. Our hypothesis is that benefits of cover crops to grapevines will increase incrementally through time, and can be measured. We are continuing our research in the Fowler, Kearney, and Madera sites, and we also initiated similar studies in Napa. We are expanding our multidisciplinary expertise to include a soil scientist (Dr. R. Miller) and a grape physiologist (Dr. L. Williams).