Conservation Tillage of Cover Crops as a Means of Improving Carbon Sequestration and Diminishing Greenhouse Gas Emissions in California Vineyards Soils

Reducing greenhouse gas emissions from agriculture and other sectors of California’s economy has become one of the most important environmental concerns of State and Federal regulatory organizations. This is the result of 1) the passage of the California Global Warming Solutions Act, Assembly Bill 32 (AB32) in June of 2006, 2) the US EPA?s recent endangerment finding for GHGs of CO2, N2O and CH4 (http://www.epa.gov/climatechange/). This project represents the culmination of an overarching effort to accurately quantify CO2, N2O and CH4 emissions from vineyards, soil carbon (C) sequestration, and above- and belowground annual net primary productivity of C. The project coordinates with efforts by Dr. William Salas (Applied Geosolutions LLC), Dr. Changsheng Li (University of New Hampshire, Institute for the Study of Earth, Oceans and Space) and Alison Jordan of the Wine Institute to calibrate the DeNitrification DeComposition model (DNDC). The model will be embedded into a decision support system (DSS) for use by grape growers and other practitioners for carbon assessments (http://www.wineinstitute.org/ ghgprotocol). The modeling exercises will allow us to test multiple management practices in order to lessen (mitigate) N2O emissions from California vineyards. The data is being made available to Dr. Alissa Kendall and Sonja Brodt of the Department of Agricultural and Environmental Engineering and Agricultural Sustainability Institute to assemble life cycle analyses for carbon footprints of vineyards and orchards. We are quickly approaching that objective for a conventionally managed vineyard and a vineyard managed under minimum-till conditions to increase soil C sequestration. There is currently tremendous uncertainty concerning the quantity of GHGs produced and consumed in vineyards (Carlisle et al., 2008). It is estimated that in vineyard land managed under no-till, soil carbon sequestration would greatly increase (Kroodsma and Field, 2006) but little data exists to support this contention. Our research led to working budgets of GHGs and C sequestration in a Napa Valley vineyard being managed with three tillage treatments. These treatments were: (1) minimum-tillage with a barley cover crop (no-till); (2) conventional tillage with the same barley cover crop; and, (3) conventional tillage of weeds, where conventional tillage refers to disked and disked and rolled twice yearly in spring and fall (2 x 2 passes). The major research success of this project concerns a comprehensive carbon footprint for a Napa Valley vineyard under minimum tillage (no-till) conditions. The deliverable results included a number of important observations that emerged from quantifying variables needed for a footprint assessment. These are: (1) long-term increase of soil carbon under the minimum tilled cover crop; (2) slightly higher rates of N2O loss from minimum tillage cover crop; (3) apparent loss of soil carbon in the conventional tilled cover crop; (4) soil carbon sequestration potential observable at depth (1.2 meters); (5) methane oxidation and therefore GHG offsets under all treatment conditions; (6) a shift in root distribution under minimum tillage that resulted in higher carbon deposition; (7) successful spatial modeling of N2O emissions in the area around N¬fertigation drip emitter to increase accuracy of scaling emissions; (8) temporal constraint of N2O emissions to an annual budget for assessing offsets; and, (9) observation of desirable vine de¬vigoration under the minimum tillage treatment.

Practices for Improving Weed Management and Soil Nitrogen Retention

The goal of this research is to develop alternative “in-row” weed management practices that provide effective weed management, do not negatively impact grapevine growth and juice, and improve soil nitrogen (N) retention by minimizing inorganic N leaching and nitrous oxide (N2O) emissions. The establishment of “in-row” treatments was initiated in October 2009. Greenhouse gas emissions (N2O and carbon dioxide, CO2) were collected bimonthly, and C and N dynamics in response to events that increase greenhouse gas emissions were measured from October 2009 to October 2011. Findings indicate that ?pulse? events such as cultivation, compost addition, fertigation, irrigation and rainfall are major periods of greenhouse gas emissions. When cultivation occurred just after compost addition in May of year 1 (Nov. 2009-Oct. 2010), N2O and CO2 emissions increased and were greater than cover crop and herbicide treatments. When rainfall occurred immediately after this cultivation, N2O and CO2 emissions were also greater than the other treatments, highlighting the interactive effects of management (i.e., cultivation and compost) and rainfall on greenhouse gas emissions. When treatments were irrigated in year 1, N2O and CO2 were greatest from the cultivated treatment, followed by cover crop and herbicide treatments, respectively. Irrigation in the year 2 (Nov. 2010-Oct. 2011) caused increases in CO2 and N2O emissions in all treatments. Cumulative N2O emissions over the two year study were greatest in the cultivated treatment followed by the cover crop and herbicide treatments, respectively. CO2 emissions over the two year period were highest in the cover crop followed by the cultivated and herbicide treatments, respectively. The cover crop and herbicide treatments tended to have greater nitrate leaching than the cultivated treatment in year 1. Temporal dynamics of leaching differed among treatments in year 1 but not year 2. In year 1, total nitrate leaching from the cover crop, herbicide, and cultivated treatments was 129.34±47.70, 104.37±55.45, and 50.83±3.20 µg NO3-N/g resin, respectively. In year 2, total leaching from the cultivated treatment tripled to 158.80±32.55 µg NO3-N/g resin. Increases in leaching were not observed in the herbicide treatment (138.98±28.87) or cover crop treatment (145.23±24.26). This study identified interactions among weed control practices, greenhouse gas production, nitrate leaching, and vine nitrogen status. Few differences among treatments were detected at this location, suggesting that over a short-term period (i.e., 2 years), no management practice appeared to be significantly better than any other. We anticipate that cover crops would emerge as a desired “in-row” practice due to improved nutrient retention, as observed in Steenwerth and Belina (2010). Other reasons to choose one weed control over another, such as organic certification, would weigh more heavily on choosing a weed

The Impact of Cover Crop Management on Rooting Profiles, Mycorrhizal Colonization and Mycorrhizal Community Structure of Young Grapevines

The purpose of this project is to better understand how cover crops and management of their residue affect the development of vine root systems and arbuscular mycorrhizal fungi (AMF) in a young vineyard. This project is an extension of ongoing work, where we are monitoring the impact of winter annual cover crop use and management of the above-ground mowed residue (simulating ?a mow & blow? approach) on vine growth and development, vine mineral nutrition, water stress, weed suppression, soil moisture conservation and other soil quality attributes. Work on the root and AMF aspect of the project has just begun, since the graduate student who will be working on this project was not available until the spring term of 2011 and because funds did not arrive at OSU until January of 2011. However, soil samples were collected from the research plots as planned (from the vine row and alleyway at three depths within each location), and preliminary root data has been collected and partially analyzed. We examined more plots than originally proposed by including the 3X mulch residue treatment in our analysis. Fine roots were collected from the plots and samples were safely stored for future analysis. These will be used to assess AMF colonization of roots and to amplify AMF in order to identify the specific fungi forming mycorrhizas. Analysis of the root fresh mass data collected to date indicates that the use of cover crop residue as an in-row mulch has already altered the spatial distribution of roots in this vineyard. The density of total root mass (woody + fine roots) in both the 1X and 3X mulched plots was greater in-row (under the mulch) than in the alleyway, but this was not true in the mowed-in-place or in unplanted control plots. In addition, total root mass was greater in the 3X mulched treatment rows than in either the mowed-in-place or control treatment rows, and the 1X mulched plots being intermediate between these divergent groups. The higher density of in-row roots of the mulched plots was associated with greater soil moisture and reduced soil compaction in the mulched plots compared to the control and mowed-in-place treatments. Vine shoot lengths and leaf SPAD values were also greater in both the 1X and 3X mulched plots. These early results indicate that the use of cover crop residue as an in-row mulch will have a dramatic impact on the growth potential and rooting patterns of young vines and may lead to a more sustainable approach to establish vines on dry-land sites.

Conservation Tillage of Cover Crops as a Means of Improving Carbon Sequestration and Diminishing Greenhouse Gas Emissions in California Vineyard Soils

Reducing greenhouse gas emissions from agriculture and other sectors of Califorrnia?s economy has become one of the most important environmental concerns of State and Federal regulatory organizations. This is the result of 1) the passage of the California Global Warming Solutions Act, Assembly Bill 32 (AB32) in June of 2006, 2) the US EPA?s recent endangerment finding for GHGs of CO2, N2O and CH4 (http://www.epa.gov/climatechange/). This project represents an overarching effort to accurately quantify greenhouse gas emissions from California vineyards and currently focuses on CO2, N2O and CH4, quantifiable estimates of soil carbon (C) sequestration and above and belowground annual net primary productivity of C. The project coordinates with efforts by Dr. William Salas (Applied Geosolutions LLC), Dr. Changsheng Li (University of New Hampshire, Institute for the Study of Earth, Oceans and Space) and Alison Jordan of the Wine Institute to calibrate the DeNitrification DeComposition model (DNDC). The model will be embedded into a decision support system (DSS) for use by practitioners for carbon assessments (http://www.wineinstitute.org/ghgprotocol). The modeling exercises will allow us to test multiple management practices in order to lessen (mitigate) N2O emissions from California vineyards. The data is being made available to Dr. Alissa Kendall and Sonja Brodt of the Department of Agricultural and Environmental Engineering and Agricultural Sustainability Institute to assemble life cycle analyses for carbon footprints of vineyards and orchards. This second collaboration involves the use of our comprehensive GHG budget. We are quickly approaching that objective for a conventionally managed vineyard and a vineyard managed under minimum-till conditions to increase soil C sequestration. There is currently tremendous uncertainty concerning the quantity of GHGs produced and consumed in vineyards (Carlisle et al., 2008). In a recent report it was estimated that in vineyard land managed under no-till, soil carbon sequestration would greatly increase, (Kroodsma and Field, 2006) but little data exists to support this contention. Our research aims to construct working budgets of GHGs and C sequestration in a Napa Valley vineyard being managed with a cover crop under ?conservation? and conventional tillage practices. The GHGs being studied are those defined by the International Panel on Climate Change?s 2006 Assessment (IPCC, 2006) and consist of carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4). It is essential that we improve the understanding of vineyards? role in emitting or absorbing these gases. Little research is so far available on these topics for California vineyards (Carlisle et al., 2008).

Conservation Tillage of Cover Crops as a Means of Improving Carbon Sequestration and Diminishing Greenhouse Gas Emissions in California Vineyard Soils

Recent legislation, specifically, The California Global Warming Solutions Act of 2006, or AB 32, as well as marketing demands, have placed a tremendous emphasis on sustainable production practices, including those defined in terms of diminishing carbon footprints in vineyards. A carbon footprint can be defined as a comprehensive measure of the amount of greenhouse gases (GHGs) we are producing and consuming, and provides an indication of whether or not we are contributing to the increase of GHGs in the atmosphere and therefore to global climatic change. There is currently tremendous uncertainty concerning the quantity of GHGs produced and consumed in vineyards (Carlisle et al., 2008). In a recent report it was estimated that if vineyard land was managed under no-till or conservation tillage conditions, soil carbon sequestration would greatly increase, and thus diminish the overall C-footprint (Kroodsma and Field, 2006). Our research aims to construct working budgets of GHGs and carbon sequestration in a Napa Valley vineyard being managed under ?conservation? and conventional tillage practices, with cover crops. The GHGs being studied are those defined by the International Panel on Climate Change?s 2006 Assessment (IPCC, 2006) and consist of carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4). It is essential that we improve understanding of vineyards? role in promulgating or absorbing these gases. Little research is so far available on these topics for California vineyards (Carlisle et al., 2008). Our definition of conservation tillage (minimum-till) consists of surface disking (2.5 cm) in fall when needed to prepare a seed bed for cover crops, and deep tillage (15-20 cm) each 5th year to alleviate issues related to de-vigoration of vines. We are currently in the first year of work on a block with three five-year-old tillage treatments. These treatments are: (1) Minimum-Till with a barley cover crop; (2) Conventional Till with a barley cover crop; and (3) Conventional Till with resident vegetation. The Minimum-Till treatment was disked heavily for the first time during October of 2008 (year 5), adhering to decisions that many viticulturalists might make with a long-established conservation-tilled cover crop. Several important results have emerged from our measurements, which require further experimentation. These are: (1) vine devigoration in Min-Till rows; (2) apparently low carbon sequestration in the Conventional Till-cover cropped treatment; and (3) Deep carbon sequestration potential observable at depth (1.2 meters).

Evaluation of the Impacts of In-row Vineyard Floor Management Practices on Soil and Water Erosion, Vine Growth and Productivity, Weed Control, and the Economics of Wine Grape Production

One large-plot experiment and a small plot experiment have been established to evaluate the impact of in-row barley cover cropping on soil and water erosion, vine growth and productivity, weed control, and the economics of wine grape production. A 3.5-acre experimental site is evaluating three in-row management practices. The in-row treatments evaluated are:

  1. Standard, pre-emergent herbicide using flumioxazin (Chateau®) at 0.38 lb a.i./acre + 2{aed9a53339cdfc54d53cc0c4af03c96668ab007d9c364a7466e3349a91bf0a23} glyphosate with follow-up post emergence applications of 2{aed9a53339cdfc54d53cc0c4af03c96668ab007d9c364a7466e3349a91bf0a23} lyphosate applied as needed in the summer to control escaped weeds, row middles have a barley cover crop.
  2. Standard + bare middles.
  3. Inrow cover crop using barley that was allowed to develop to 12 inches and then burned back with a 2{aed9a53339cdfc54d53cc0c4af03c96668ab007d9c364a7466e3349a91bf0a23} glyphosate application + 0.38 lb a.i./acre Chateau. A small plot experiment was established, to evaluate the influence of timing of burn-back herbicide sprays to in-row covers.

Rainfall totaled 9.8 inches at the trial site between December 2007 and May 2008. This was double the rainfall of the 2006-2007 winter. Three storm events resulted in 1 to 2 inches of daily rainfall. Run-off amounts for the in-row cover and standard treatments were much less than the amounts measured in the bare plots, which suggest that the cover crop greatly reduced storm run off during the winter. Cover crops reduced nutrient and sediment concentration in the run-off as well as total volume of run-off which effectively reduced sediment, total P, and total N loads by 98{aed9a53339cdfc54d53cc0c4af03c96668ab007d9c364a7466e3349a91bf0a23}, 94{aed9a53339cdfc54d53cc0c4af03c96668ab007d9c364a7466e3349a91bf0a23}, and 96{aed9a53339cdfc54d53cc0c4af03c96668ab007d9c364a7466e3349a91bf0a23}, respectively, compared to the bare plots.

Soil moisture levels were lowest in the bare plots prior to winter rain events. During spring and early summer soil moisture between the large plot treatments was similar. As observed in 2007 a higher soil moisture level was measured in the in- row cover treatment compared to the standard and the bare plots during late summer and fall. The higher soil moisture in the in-row treatment was attributed to the straw-mulch reducing evaporation on the berm and improving infiltration under the dripper. The timing of herbicide applied to the in-row cover significantly affected soil moisture content of the profile with cover crops killed at 24-inches having the lowest soil moisture during March through June.

As observed in 2007 weed impacts were not an issue in 2008 and no significant differences occurred between the treatments. At younger ages the cover crops are more succulent and tend to breakdown more quickly. However, even cover crops sprayed at a young age (i.e. 6 inches tall) still providing significant ground cover into early May.

Soil organic matter in the row was increased with the use of in-row covers. Soil microbial activity in the row was also shown to be increased with in-row cover cropping. Greater soil microbial activity was seen when the in-row covers were allowed to develop higher biomass.

The use of an in-row cover was shown to reduce nitrogen and increase potassium and phosphorus bloom time petiole levels of the vines. The longer the in-row cover is allowed to develop the greater the reduction in nitrogen levels.

The use of in-row covers had no negative effect on the growth and productivity of vines in a fourth leaf vineyard when the cover was burned back at 12 inches in height. A reduction in shoot weight and total vine prunings for the in-row covers allowed to grow for an extending time demonstrated the potential competitive effects of this practice.

A Cover Crop System for Vineyard Pest, Weed and Nutrition Management

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).

A Cover Crop System for Vineyard Pest, Weed & Nutrition Management

This report describes the 1993 results of continuing, large replicated trials in three San Joaquin Valley vineyards where we are studying the impact of three vineyard floor management systems on pests, weeds, and vine nutrient status. The experiments consisted of three main treatments: (1) an oat/vetch (two vineyards) or a merced rye/vetch mixture (one) vineyard planted in row middles, with row berms treated with preemergence herbicides, (2) the same as in 1, but the cover was cut and blown on berms for weed suppression (with only postemergence herbicides applied during late winter), and (3) cover crops were not planted, but row middles were clean-cultivated (control), and berms were treated with herbicides for weed suppression. An additional trial was established on a two-acre vineyard at the UC Kearney Agricultural Center. This vineyard included only treatments 2 and 3, and a treatment where we attempted to evaluate the impact of berm mulching on weed suppression without the use of preemergence and postemergence herbicides. During the 1993 season, the presence of cover crops did not affect the number of leafhoppers on vines in any of the four vineyards, but there were large differences between vineyards in the size of leafhopper infestations, levels of leafhopper-egg parasitism, and abundance and species composition of spiders. In general, two vineyards (7 and 10 years old) supported larger numbers of leafhoppers (11-15 nymphs per leaf) compared to the other two vineyards (more than 30 years old) where leafhopper numbers were lower than one nymph per leaf during the peak of the second generation. We do not really know the cause of these differences, but it appears that factors associated with vine age may have a significant impact on leafhopper abundance. The proportion of leafhopper eggs parasitized by Anagrus were also higher in the ‘young’ compared to ‘old’ vineyards, probably resulting from higher densities of leafhopper eggs in these vineyards. Parasitism in cover plots was similar to control plots, but there was a trend toward higher egg parasitism in cover plots during the second leafhopper generation. There were also large differences between vineyards, and between cover treatments within vineyards, in the abundance and species composition of spiders occurring on grapevines. In the ‘older’ vineyards, there were significantly higher populations of spiders on vines in cover crop and mulch plots compared to vines in control plots. Leafhoppers were too low in these vineyards for any effect to be observed during the 1993 season. The differences in spider abundance between cover and control treatments were only obtained in the old’ vineyards where spiders were predominantly of the web-building species. In the two young’ vineyards, where spiders were predominantly of the non-web-building (or hunting) species, we did not observe a significant effect of cover crops on spider abundance on vines. Initial 16 results from marking studies indicated that the cover crops are increasing the spider’s food abundance, and are providing an alternative foraging habitat for the spiders, but it is possible that not all spider species are similarly affected in this manner by the cover crops. Further results of these marking are pending. Results from experiments on weed suppression with dry mulch are variable. However, our studies and those conducted by C. Elmore in north coast vineyards indicated that yearly accumulation of biomass in vine rows should provide sufficient weed suppression to minimize the use of herbicides. The data on the nutritional status of vines showed that petiole tissue analysis for nitrate-nitrogen and potassium varied considerably between vineyards. In this first year of comparing fertilized and unfertilized plots (in the two ‘old’ vineyards) in the presence and absence of cover crops, we did not observe any significant impact of cover crops on nitrate-nitrogen status; but there was a trend toward higher potassium levels during veraision in the cover plots compared to control plots in the two ‘old’ vineyards. In the two ‘young’ vineyards, nitrate-nitrogen levels were reduced in the presence of cover crops, but no changes occurred in potassium levels. These effects were probably caused by factors related to the cover crop stand and weeds growing on berms. It is also possible that young’ vineyards are less competitive with cover crops than are ‘old’ vineyards. We are continuing to monitor changes in vine nutrient status which may not be reliably detected until the following year. Our initial budget for the three floor management systems indicates that the lowest cost is incurred in the absence of cover crops and when the commonly applied herbicides are used to control weed in the vine rows. We expect, however, that the overall benefit of using cover crops in vineyard production will be increased the cost of insecticides and fertilizers are incorporated into the cost/benefit analysis. Our findings are being disseminated to the scientific community and farmers in meetings and field days.

A Cover Crop System for Vineyard Pest, Weed and Nutrition Management

Large scale replicated trials were initiated in the fall of 1991 on three farms in the San Joaquin Valley. The first year’s data was collected during the 1992 season. In general, we observed an increase in the activity of natural enemies, especially spiders which resulted in a suppression of leafhopper numbers in some vineyards. The numbers of leafhoppers during the 1992 season were too low to observe a strong effect of cover crops on their numbers. Whole-vine spider exclusion and spider caging with leafhoppers indicated that the most common spiders in vineyards are important predators of leafhoppers. Continued monitoring of our vineyards is necessary to determine the long term effect of cover crops on the numbers of pests and their biological control agents. Our results on weed suppression with dry mulch is variable. However, our studies and those conducted by C. Elmore in north coast vineyards indicate that yearly accumulation of biomass in vine rows should provide sufficient weed suppression to minimize the use of herbicides. In table and wine grape vineyards, cover crops left to dry in row middles can suppress weeds, conserve soil moisture, decrease mowing costs, and reduce dust problems. The data on the nutritional status of vines did not show any treatment differences. This is not surprising, however, since the effect of cover crops on mineral nutrition of vines is a delayed effect, often not detectable until the following year. Our initial budget for alternative floor management systems indicates that the use of cover crops for weed management may increase the cost of grape production, primarily due to the added cost of cover crop seeds. This increased cost, however, should turn into savings when insecticide and fertilizer costs are included in the enterprise budget. It is anticipated that our cover crop system will reduce insecticide, herbicide, and fertilizer inputs. In the long term, seed costs should also be reduced, since the cover crops used in our studies are self-seeding.