1995 Annual Report for SW95-012
A Cover Crop System for Sustainable Grape Production in California - Beyond the Transition Phase
1. Compare the impact of a legume grass cover crop system on weed abundance in row middles and vine berms; and its value as a substitute for soil applied herbicides.
2. Determine the impact of this cover crop system, on soil fertility and vine nutritional status, and its value as a substitute for synthetic chemical fertilizers.
3. In a separate experiment, compare the impact on soil fertility and vine nutritional status by early and late plow-down of the legume-grass cover crop; and its value as a substitute for synthetic chemical fertilizers.
4. Determine the role of the cover crop systems, described in objectives 1 and 3, in the natural control of leafhoppers and spider mites, and their role in reducing the use of insecticides and miticides.
5. Determine the effects of the cover crop systems, described in objectives 1 and 3, on soil and vine water status, and their potential impact on water use.
6. Determine the effects of the cover crop systems, described in objectives 1 and 3, on vine growth and development, and grape yield and quality.
7. Determine the costs and benefits of the cover crop systems, described in objectives 1 and 3, in terms of overall chemical inputs, yield, labor requirements and farm profitability.
8. Develop guidelines and integrated demonstrations of cover crop systems in vineyards for insects and mites, weeds, vine-nutrition management, and water demand.
This report describes the results of a project evaluating the long term impact of several cover-crop based vegetation management systems on weeds, vine nutrition, water use, insect pests, plant parasitic nematodes, and grape yield and quality in California wine grape and raisin grape vineyards. Our findings to date indicate that rye-vetch or oat-vetch cover crops can have a substantial but variable impact on several elements of grape production. The impact depended largely on cover crop management, water use, and vineyard age. Where the cover crop was used as dry mulch for weed control in the vine rows, weed populations were maintained at acceptable levels with mulch and one late winter application of postemergence herbicides. By the fourth year (1996), mulch in vine rows in combination with postemergence herbicides reduced the cumulative negative effect of summer weeds on grape yield and quality in bare and non-mulched plots that had been treated only with postemergence herbicides. To maintain vines at similar water status, the mulch (Kearney I) and reseed systems (Madera vineyard where cover crop not used as mulch but plowed down at senescence) utilized approximately 30 percent more irrigation water than the clean-cultivated or green-manure (Madera vineyard where cover crop plowed down in April) systems. The mulch system provided the greatest relative increase in soil fertility, soil microbial biomass, and vine nitrogen levels compared with the clean-cultivated system. The green manure and reseed systems maintained vine nitrogen at levels similar to a clean-cultivated system that received a fall-application of compost; but the reseed system resulted in slightly improved grape yield and quality. Water and nutrient stresses occurred in the reseed system in a young vineyard (Napa) when supplemental water was not provided to the vines.
Densities of leafhopper pests depended on cover crop management and vineyard age and their resulting impact on spider abundance and vine water and nitrogen. In the ‘young’ Kearney I vineyard, leafhopper problems were exacerbated at the end of the season in two out of four years in the cover crop and mulch system, primarily due to higher nitrogen levels and low resident spider populations. In contrast, a reseed system of rye and vetch (without added nitrogen) maintained vine nitrogen at similar levels to a clean-cultivated system (but higher than a rye alone system), but resulted in substantially higher spider densities and lower leafhopper densities than in the clean-cultivated plots. We have previously determined that a spider assemblage similar to that found in the Madera and Kearney II vineyards can cause significant reductions in leafhopper abundance.
The effects of vegetation management on operating costs represented a trade-off in water, fertilizer, pesticide and resource use. The use of cover crops (despite greater water demand) significantly reduced operating costs where savings were realized by reducing chemical (pesticide, herbicide, and nitrogen fertilizer) inputs. These savings were greatest where the use of cover crops (primarily as reseed with and without mulch) increased grape yield and/or quality. Savings may also increase if we were to include the potential costs where environmental contamination or increased health risk to humans and wildlife are likely to occur as a result of the use of selected pesticides, herbicides and synthetic fertilizers.
Finally, dissemination of our findings has apparently resulted in increased interest and adoption of cover crops in grape production in California. Information developed from our studies is being disseminated to both the academic community and farmers through reports and meetings. All of the major participants in this project regularly participate in industry training programs for farmers. In addition, we are co-authors of the University of California’s Pest Management Guidelines for Grapes, and have authored sections of the University’s Grape Pest Management manuals. Quantifiable results obtained through these studies will be incorporated into the guidelines.
Our research provided practical benefits to several aspects of grape production including insect, weed, and nutrition management. Our findings also contributed to the general understanding of the feasibility of developing and implementing sustainable crop previously observed higher levels of leafhoppers in association with reseed rye-vetch systems.
In addition, our research demonstrated for the first time that extrafloral nectar-bearing plants in the vineyard floor can be used to exclude ants from the grape mealybug-infested vine canopy, in turn improving biological control of the grape mealybug. These results have broad implications for crops that are infested with honeydew-producing insects (e.g., aphids, scales and mealybugs) and are attended by ants which can potentially disrupt biological control of the honeydew producing insects. Providing ants with alternative resources on the extrafloral nectar-bearing plants may be used as non-chemical alternative to ant exclusion and control on many crops infested with honeydew producing insects.
Our research has shown that if managed properly cover crops can directly and indirectly reduce pest and weed densities, and (potentially) minimize the use of insecticides and herbicides, in turn resulting in significant reductions in operating costs.
Farmer Adoption and Direct Impact
Perhaps the greatest contribution of our on-farm research has been the willingness of growers to take greater risks and reduce pesticide usage on their farm. This is clearly indicated in the testimonials of the cooperating growers. The growers have also been closely involved in the design of our cover crop system. We have made every available effort to modify the design to facilitate the adoption of this system under various management conditions. Preparation of seed bed, timing of planting, and irrigation practices have been some of the elements that were modified for the various farms used in our studies. Most importantly, our research has helped our cooperating farmers to adapt farming practices that incorporate cover crops in the management of their vineyards. Their adoption of this practice has also encouraged others to do the same.
This summary was prepared by the project coordinator for the 1998 reporting cycle.