Prune Refuges and Cover Crops to Facilitate Low-Input Production of California's Raisin, Table, and Wine Grapes

Prune Refuges and Cover Crops to Facilitate Low-Input Production of California's Raisin, Table, and Wine Grapes

Summary

Objectives
1.Determine the impact of a legume-grass cover crops on leafhoppers, mites, and their natural enemies.
2. Determine the value of the cover crops as substitute for chemical fertilizers and its effect on the long-term, overall nutritional status of grapevines.
3. Determine the feasibility of cover crops as substitutes for herbicides in annual weed management in row berms.
4. Determine the costs and benefits of the cultural practices studied in terms of overall chemical inputs, yield, labor requirements and farm profitability.
5. Develop guidelines and integrated demonstrations of cover crop systems in vineyards for pest, weed, and vine-nutrition management.

Abstract of Results
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 weed suppression to reduce herbicide use. The other cover crop system was cut and left in row middles, but as in the clean-cultivated system, weeds on berms were controlled chemically. 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 an Experiment Station vineyard; and to compare cover crops used as green manure and re-seed to a clean cultivated system. The cover crop systems were developed to increase the sustainability of grape production by reducing inputs of chemical pesticides and fertilizers. We determined the impact of these systems on vine-nutrient status, weed suppression, and arthropod pests and beneficials. We also developed relative cost budgets for each management system.

Our findings to date indicate that, if properly managed, winter annual, legume/grass cover crops can reduce the reliance of grape growers on insecticides and miticides used to control leafhoppers and spider mites.
If sulfur dust (used for disease control) was used sparingly in late Spring and early Summer, the presence of these cover crops increased early season activity of predatory mites, resulting in reduced spider mite infestations. Similarly, 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 were attributed to enhanced activity of certain groups of spiders, which consistently attained higher densities in the presence of cover crops compared to the clean-cultivated systems.

Leafhoppers were also utilizing the cover crops as non-host crops which may have resulted in less time spent on vines. These differences in pest abundance could not be attributed to changes in plant quality, as these were not sufficiently strong to have significantly produced the observed effects on leafhopper densities. However, although it remains to be seen, we expect in the long term that cover crop use will affect plant nutrient and water status to an extent that will impact the performance and the abundance of insect and mite pests, further reducing the reliance on insecticides and miticides.

Cover crops can also produce positive effects on vine-nutrient status by the second or third year, but may produce negative effects if the cover or vineyard are poorly managed. The positive effect is usually delayed, and is best illustrated by the results from the one test site where, by the third year, the cover crop had contributed in excess of 75 lbs/A of nitrogen. These effects may take even longer in table grape culture, where the cover crop (as grown in our systems) was not incorporated until the Fall, while weeds were allowed to grow in row middles. In table grape culture, apparently much of the nutrient content of the cover is used to grow the resident vegetation during the summer. Potassium levels were also enhanced by cover crops by the third year, and in some cases where cover crops were grown with great care (to produce high levels of biomass), increased potassium (and nitrogen) levels was observed by the second year.

The amount of dry biomass produced by cover crops for weed suppression varied between vineyards. During the spring and early summer, the mulched berms received 1800 to 8,726 1bs. of dry biomass, with a total nitrogen content of 33 to 109 1bs/A. To date, the results from the north coast site 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.

Economic Analysis (and Yields)
The effects of cover crops on grape yield and operating costs depended on grape culture (raisin versus table), and represented a trade-off in water, fertilizer, pesticide and resource use. Although significant differences in yields have not been realized in the commercial vineyards, raisin yields in the experiment station vineyard were increased by 900 lbs./A which is equivalent to an increase of approximately $450/A in gross returns by the second year of the experiments where cover crop biomass was used as dry mulch for weed suppression in row berms. Furthermore, the use of cover crops (despite greater water demand) would significantly reduce operating costs if savings are realized by reducing chemical inputs for mites and leafhoppers. 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 usage.

Potential Contributions
A key feature of our on-farm research is that it simultaneously sought to develop and implement sustainable agricultural practices in grape production. It was conducted in large scale replicated trials on commercial farms, and was therefore visible to other farmers. 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 production practices. This is information that crosses commodity lines. Our research also generated new hypotheses related to the interaction between vegetational diversification of agroecosystems, plant age, predator-prey interactions, and weeds ecology.

Grapes are the leading crop grown in California, with a total value of over $1.7 billion. Most growers incur direct costs in grape production by applying synthetic insecticides, acaricides, herbicides and nitrogen fertilizer. Our on-farm research sought to develop cost effective, non-chemical alternatives for the management of vineyard pests, weeds and vine nutrition. Our research has shown that, if managed properly, cover crops directly and indirectly reduce pest and weed densities, and (potentially) minimizing the use of insecticides and herbicides, in turn resulting in significant reductions in operating costs. By the third year, the system showed early signs of significant positive contribution to vine nutrient status, and increases in grape yield. Should these studies continue beyond this transition phase, we expect a substantial reduction in the need to apply synthetic chemical fertilizer, and insecticides and herbicides, while maintaining (or even increasing) grape yield and quality, and profitability of farming grapes.

Farmer Adoption
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. The growers have also been closely involved in the design of our cover crop system. We have spent 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.

Operational Recommendations
Cover crops which produce sufficient biomass will reduce weed growth in the row berms. However, they require the growers to increase their management level to successfully produce the cover cover crop in addition to the grapes. Timing of planting, water requirements, and cover crop selection are all considerations that will have to be taken into account for this method to be effective and accepted. For example, planting on raised beds that are bordered by irrigation furrows can substantially reduce water use, while maintaining the same quality cover crop.
Reported in 1995
Final Results