Selecting and Managing Vineyard Cover Crops to Reduce Consumption of Net Basin Water

Final Report for OW14-032

Project Type: Professional + Producer
Funds awarded in 2014: $49,467.00
Projected End Date: 12/31/2015
Region: Western
State: California
Principal Investigator:
Fritz Westover
Vineyard Team
Co-Investigators:
Kris Beal
Vineyard Team
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Project Information

Abstract:

A decline in the California Paso Robles Groundwater Basin has increased awareness of water conservation practices in vineyards. This project investigated whether and to what degree the depletion of soil moisture over the winter by certain cover cropping practices might affect the quantity of groundwater pumped for irrigation to replace these losses. While cover crops have many benefits, the cost of those cover crops in terms of water use and associated pumping costs to replace soil moisture that may be depleted due to the cover crop is not well understood on the Central Coast. This study consisted of two experiments conducted over two seasons. One experiment investigated whether the timing and method of terminating a grass cover crop affected the depletion of soil moisture over the winter. The second experiment evaluated the effects of cover crop selection (species) on the depletion of soil moisture over the winter. Neither the timing and method of terminating a grass cover crop nor the selection of cover crop species differed consistently in terms of soil moisture depletion compared to clean cultivation or fallow/no-till controls. These outcomes suggest that growers have the freedom to choose cover cropping management practices based on factors other than potential soil moisture depletion in this area during low-rainfall years.

Introduction

Declines in the California Paso Robles Groundwater Basin and several years with very little rain have increased awareness of water conservation practices in vineyards. The planting of winter cover crops is a widely used management practice in vineyards in the Paso Robles area. Cover crops can improve water infiltration from precipitation while also preventing erosion and building organic matter in the soil. However, the cost of those cover crops in terms of water use is not well understood on the Central Coast. If cover crops substantially deplete soil moisture during the winter and early spring, then more water has to be applied through irrigation to make up for that loss.

 

In 2014, grant funding from Western Sustainable Agriculture Research and Education (Western SARE) was awarded to the Vineyard Team for a two-year study to evaluate cover crop species and management of grass cover crops to reduce net water consumption. The assumptions behind this study were:

 

  1. Different species of cover crops would deplete soil moisture at different rates during the winter and early spring. This would mean some types of cover crops are more desirable to use than others in terms of maintaining soil moisture before the beginning of the growing season.
  2. The timing and manner of terminating a grass cover crop would impact how much water was lost from the soil. This would mean some timings or techniques are preferable to others in terms of conserving soil moisture prior to the start of the growing season. This would be important as grasses, such as Blando Brome or cover crop seed mixes that contain a high proportion of grass seed, are popular cover crops in the Paso Robles area.

 

The outcomes of this study would allow growers to make more informed decisions about what cover crops to plant if they desire the benefits of cover crops while simultaneously conserving soil moisture. The same is true for when and how to terminate a cover crop. These outcomes could then be disseminated to the larger grower community through the existing education and outreach mechanisms and activities of the Vineyard Team.

 

Two experiments were designed and executed to investigate those assumptions and produce information about what may or may not be advisable cover cropping practices during years of low rainfall and limited irrigation water. The first experiment compared five species of popular cover crops against a clean cultivated control. This experiment was repeated at three different sites in the same general area. The experimental design was a randomized complete block design with three replications each of five treatments and a control. The experiment was executed twice, in consecutive years, using the same design – i.e. the same cover crop was grown in the same plot each year.

 

The second experiment compared various combinations of timings and methods for terminating a Blando Brome cover crop to an unplanted control. This experiment was also repeated at three different sites in the same general area. The experimental design was again a randomized complete block design with three replications each of the five treatments and a control. The experiment was executed twice, in consecutive years, using the same design- i.e. the method/timing was used in the same plot each year.

Project Objectives:
  1. Evaluate five cover crop species for their impact on water infiltration, soil moisture retention, and use of plant available water in the vineyard.
  2. Evaluate treatments to suppress and terminate cover crops and their impact on water infiltration, soil moisture retention, and use of plant available water in the vineyard.
  3. Perform economic evaluation of vineyard floor management practices and document financial return on investment in terms of potential water savings.
  4. Promote and maximize the adoption of beneficial practices by producers from the findings of this cover crop project and extend through a well-established portfolio of outreach mechanisms including tailgate meetings, newsletters, email blast, website, social media, fact sheets, presentations, and trade publications.
  5. Measure adoption of water conservation practices identified in this project by surveys of the grower audience who participate in tailgate meetings, a webinar, or an online educational module.

Cooperators

Click linked name(s) to expand
  • Carl Bowker
  • Simon Graves
  • Anji Perry
  • Steve Vierra
  • Jason Yeager

Research

Materials and methods:

Objective 1: Evaluate five cover crop species for their impact on water infiltration, soil moisture retention, and use of plant available water in the vineyard.

 

Experimental Design

Three sites for the experiment were selected in vineyards owned by J. Lohr Vineyards and Wines, Opolo Vineyards, and Niner Wine Estates. All three site were along the “46 East” corridor encompassing the border between the Paso Robles Estrella District and Paso Robles Geneseo District AVAs east of Paso Robles, California. The experimental designs were randomized complete block designs consisting of five treatments plus a control in three blocks (See Figure 1 for an example of the experimental design). The treatments were different species of cover crops planted at the recommended rate per acre within the experimental plot (Table 1). The control for comparison was “clean cultivation” which was executed as disking the plot at the same time the other plots were prepared as seed beds, but nothing was planted. These plots were not disked again until after data was collected. It was not practical to disk the control plots multiple times during the winter, so there was weed growth. This does reflect what a grower practicing a clean cultivation program would do as they would not cultivate during the winter. The species of cover crops planted were Blando Brome, Barley (UCD 937), Triticale (Trios 888), Peas (Dundale), and Medic (Parragio). All seed and planting services were provided by Helena Chemical Company.

 

Experimental plots consisted of the middles (or “tractor avenues”) on either side of a vine row. The width of the plot including the vine row was 20 feet. The length of the plot was nine vines equaling either 45 ft. or 54 ft. depending on vine spacing. The total area of the experiment equaled about 0.40 acres at each site. All data were collected from the area around the middle three vines of the experimental plot. Experimental plots were marked in the field using flagging tape. The same cover crops were planted in the same plots in November 2014 and November 2015. Samples for measuring plant biomass and soil moisture were collected in late April/early May of both years.

 

Data Collection

It was not possible to collect data from one of the sites in either 2014 or 2015. In the first year, the area was mowed before data could be collected. In the second year, the cover crops did not germinate. At another site it was not possible to collect data from the experiment in 2016 because the area was mowed and disked before samples could be collected. Therefore, there is data for analysis and comparison from two sites in 2015 and one site in 2016. In April 2015 and April 2016 samples were collected from the experimental plots at the available sites using the same protocols providing comparable data, and therefore, comparable statistical results.

 

Plant samples were collected from each plot using a 20” X 20” quadrat thrown at random into the plot. All plants originating from within the quadrat we clipped at approximately 1” above the soil surface and collected in paper bags. This was done three times in each plot yielding three subsamples per plot. The bags containing the plant material were dried at 100°C in a forced air drying oven for 24 hours. Samples were then weighted to determine dry matter reported in grams per square meter.

 

Soil samples were taken at 18” and 36” on one side of the vine row near the halfway point of the length of the plot. Samples were placed in plastic bags. Forty grams of soil were weighted into a sample can and were dried at 100°C in a forced air drying oven for 24 hours. Samples were then weighted to determine gravimetric soil moisture reported as a percentage.

 

To determine if there were any effects of the treatments on the growth of the vines in the experimental plots, cane counts and cane weights were taken in January of 2016. Of the three cover crop species experiment sites, two had pre-pruned the experimental area before data could be collected leaving one site for cane data collection.

 

To collect the data, three vines in the middle of each plot were chosen as data vines. All canes originating from the cordon or trunk of the vine at or above the head were counted and recorded. Then all cane material from those points of origination was collected and weighed. This information was used to calculate average cane weights.

 

Statistical Analyses

Data were analyzed using ANOVA in SPSS Version 24. Treatment and block were included in the models and Tukey’s HSD at the 0.05 level of significance was used to separate significant means.

 

Objective 2. Evaluate treatments to suppress and terminate cover crops and their impact on water infiltration, soil moisture retention, and use of plant available water in the vineyard.

 

Experiment Design

Three sites for the experiment were selected in vineyards owned by J. Lohr Vineyards and Wines, Derby Wine Estates, and Treasury Wine Estates. All three sites were along the “46 East” corridor along the southern border of the Paso Robles Estrella District AVA east of Paso Robles, California. The experimental designs were randomized complete block designs consisting of five treatments plus a control in three blocks (See Figure 2 for an example of the experimental design). The treatments were different timings and methods of terminating a Blando Brome cover crop in the spring (Table 2). The control for comparison was “clean cultivation” which was executed as disking the plot at the same time the other plots were prepared as seed beds, but nothing was planted in the same manner as the control plots in the cover crop species experiment described above. The plots of the no-till treatment were not disked at any time during the period of the experiment. For the other treatment plots a seed bed was prepared and they were planted with Blando Brome at 25 lbs./per acre. All seed and planting services were provided by Helena Chemical Company. The treatments in the experiment were 1) No-till, 2) mow 30 days after bud break, 3) mow at bud break, 4) mow and disk at bud break, 5) chemical mow with glyphosate at bud break, and 6) the unplanted control.

 

Experimental plots consisted of the middles (or “tractor avenues”) on either side of a vine row. The width of the plot including the vine row was 20 feet. The length of the plot was nine vines equaling either 45 ft. or 54 ft. depending on vine spacing. The total area of the experiment equaled about 0.40 acres at each site. All data were collected from the area around the middle three vines of the experimental plot. Experimental plots were marked in the field using flagging tape. The same treatments were executed in the same plots in 2014 and 2015 with the cover crop being planted in November and the treatments being executed in late March/early April of each year. Samples for measuring plant biomass and soil moisture were collected in late April/early May of both years.

 

Data Collection

It was possible to execute the experiment and collect data from all three sites both years. In April 2015 and April 2016 samples were collected from the experimental plots at the three sites using the same protocols providing comparable data, and therefore, comparable statistical results.

 

Plant samples were collected from each plot using a 20” X 20” quadrat thrown at random into the plot. All plants originating from within the quadrat we clipped at approximately 1” above the soil surface and collected in paper bags. This was done three times in each plot yielding three subsamples per plot. The bags containing the plant material were dried at 100°C in a forced air drying oven for 24 hours. Samples were then weighted to determine dry matter reported in grams per meter.

 

Soil samples were taken at 18” and 36” on one side of the vine row near the halfway point of the length of the plot. Samples were placed in plastic bags. Forty grams of soil were weighted into a sample can and were dried at 100°C in a forced air drying oven for 24 hours. Samples were then weighted to determine gravimetric soil moisture reported as a percentage.

 

To determine if there were any effects of the treatments on the growth of the vines in the experimental plots, cane counts and cane weights were taken in January of 2016. Of the three cover crop termination experiment sites, two had pre-pruned the experimental area before data could be collected leaving one site for cane data collection.

 

To collect the data three vines in the middle of each plot were chosen as data vines. All canes originating from the cordon or trunk of the vine at or above the head were counted and recorded. Then all cane material from those points of origination was collected and weighed. This information was used to calculate average cane weights.

 

Statistical Analyses

Data were analyzed using ANOVA in SPSS Version 24. Treatment and block were included in the models and Tukey’s HSD at the 0.05 level of significance was used to separate significant means.

Research results and discussion:

Objective 1. Evaluate five cover crop species for their impact on water infiltration, soil moisture retention, and use of plant available water in the vineyard.

 

Results showed no significant differences in the depletion of soil moisture between the species and varieties of cover crops in this experiment compared to the control or compared to each other. At the Opolo site this was true for the winter of 2015 (no data available for the winter of 2016) and both the 2015 and 2016 winters at the J. Lohr site. 

 

There were statistically significant differences in biomass production at the J.Lohr site (F=7.425, p = <.001) and the Opolo site (F=13.843, p = <.001) in 2015. Tukey’s HSD means separation found that Dundale peas produced more biomass than the other treatments at the J. Lohr site and both Dundale peas and UC 937 barley produced more biomass than the other treatments at the Opolo site. These results are presented in Table 3 and Figure 3. There were no significant differences between treatments in terms of biomass produced at the J. Lohr site in 2016.

 

There were no statistically significant differences in gravimetric soil moisture content in either year at either location.

 

Vine Growth

There were no significant differences between treatments in terms of cane counts or average cane weight at the one site where data could be collected.

 

Objective 2. Evaluate treatments to suppress and terminate cover crops and their impact on water infiltration, soil moisture retention, and use of plant available water in the vineyard.

 

Termination Experiments

There were statistically significant differences between treatments in terms of gravimetric soil moisture content (F=4.084, p = .0028) at the J. Lohr site in 2015. Tukey’s HSD means separation found the unplanted control to be different from the late mow treatment. The unplanted control had the highest level of gravimetric soil moisture content and the late mow treatment had the lowest. Clean cultivation was not different than the other four treatments and late mow was not different than the other four treatments. These results are presented in Table 5 and Figure 4.

 

There was also a significant difference in plant biomass as dry matter at the J. Lohr site in 2015 (F=5.373, p = .001). Tukey’s HSD means separation found the no-till treatment to have produced more biomass than the other treatments. These results are presented in Table 6 and Figure 5.

 

Although the experiment at the Derby Wine Estates site showed no statistically significant differences between treatments in terms of soil moisture depletion either year (see Table 5), there was a highly significant difference in the amount of biomass between treatments (F=14.489, p=<.001) in 2015 with Tukey’s HSD means separation finding the no-till treatment producing more biomass than the other treatments. There was a statistically significant difference between treatments again in 2016 (F=6.591, p = .006) in the production of biomass. In 2016, the chemical mow treatment produced less biomass than the early mow plus disk treatment, the early mow treatment and the unplanted control.  These results are presented in Table 6 and Figure 6.

 

At the Treasury Wine Estates site there were no significant differences between treatments in terms of gravimetric soil moisture content in 2015, but there was a highly significant difference in biomass production (F=30.52,  p=<.001). Tukey’s HSD means separation found the no till treatment to have produced significantly more biomass than the other treatments. These results are presented in Table 6 and Figure 7.

 

In 2016, there were significant differences between treatments in terms of soil moisture depletion at the J. Lohr site (F=4.112, p=.027). Tukey’s HSD means separation found the early mow plus disk treatment to be significantly different than clean cultivation, but not the other four treatments. The early mow plus disk treatment had the highest gravimetric soil moisture content and the unplanted control had the lowest. These results are presented in Table 5 and Figure 4.

 

Of the five experiments in this study, only one showed a significant difference in gravimetric soil moisture at the 36 inch depth in either year. At the Treasury Wine Estates site there was a significant difference (F=3.72, p=.035) between treatments at the 36” level in gravimetric soil moisture content in 2016. The no till treatment had the lowest gravimetric soil moisture and the early mow plus disk treatment had the highest. These results are presented in Table 5 and Figure 8.

 

In 2015, at the Treasury Wine Estates site, there was a statically significant difference between treatments in the production of biomass with the no-till treatment producing more biomass than the treatments (F=30.52, p = <.001). This was also true in 2016 (F=11.606. p =.018). These results are presented in Table 6 and Figure 7.

 

There were statistically significant differences between treatments at the Derby Wine Estates site in 2015 where the most biomass was produced by the no-till treatment (F=14.489,  p <.001)

Results from 2016 were also significant (F=6.591, p =.006), but with very different rankings. The no–till treatment was not different from any of the other treatments. The chemical mow treatment produced the least amount of biomass and was significantly different than the unplanted control, the early mow plus disk treatment, and the early mow treatment which all produced more biomass. These results are presented in Table 6 and Figure 6.

 

There may have been differences between treatments in 2016, but not 2015, because there was more rain during the 2015-2016 season, although it was still dry. The total precipitation for the 2014-2015 season was 7.45 inches. The total precipitation for the 2015-2016 season was 10.28 inches. Average precipitation for the area is 14.11 inches.   

 

Vine Growth

There were no significant differences between treatments in terms of cane counts or average cane weight at the one site where data could be collected.

Participation Summary

Educational & Outreach Activities

Participation Summary

Education/outreach description:

Objective 4. Promote and maximize the adoption of beneficial practices by producers from the findings of this cover crop project and extend through a well-established portfolio of outreach mechanisms including tailgate meetings, newsletters, email blast, website, social media, fact sheets, presentations, and trade publications.

 

The first outreach activity was the March 26, 2015 Tailgate Meeting (43 attendees representing 30,402 acres) (http://www.vineyardteam.org/events/past-events.php?id=151). Attendees were taken to one of the sites where both experiments were being performed to see how they were designed, executed, and developing.

 

Once results from the 2015 data collection were available, we posted a fact sheet to our web site. A mid-project fact sheet was prepared when the preliminary results from the 2016 data collection were in and made available through our regular e-newsletter (65 individual clicks) and through our website (http://www.vineyardteam.org/files/Newsletter%20Sites_Dead%20Page/2016%20W%20SARE%20Fact%20Sheet_For%20E-Blast2.pdf).

The results of the 2015 data collection were also presented in our April 27, 2016 Tailgate Meeting (38 attendees) (http://www.vineyardteam.org/events/past-events.php?id=181).

 

Two fact sheets were produced during the course of the experiments as annual updates (described above). Two final fact sheets have been completed- one specifically for each experiment. The first fact sheet summarized the results of the first year for both experiments as soon as the first round of data collection and analysis was complete. The second fact sheet summarized the results of the experiments for the two-year length of the project and replaced the first fact sheet at the link above.

The two final fact sheets contain the same findings, but presented as a standalone fact sheet for each experiment (http://www.vineyardteam.org/files/resources/FACT%20SHEET_Cover%20Crop%20Species%20Experiment.pdf and http://www.vineyardteam.org/files/resources/FACT%20SHEET_Cover%20Crop%20Termination%20Experiment.pdf). These fact sheets were peer-reviewed by a committee of growers and an academic before release. A poster for display at future educational events was produced from the final content of the two peer-reviewed fact sheets (http://www.vineyardteam.org/files/resources/W-SARE%20Poster%208.5%20X%2011.pdf).

 

The first part of the online outreach and education component of this project is an online educational module. Material from the fact sheets, presentations at the Tailgate Meetings, grower surveys, and grower interviews were used to produce the online educational module (http://www.vineyardteam.org/virtual-tailgates/managing-cover-crops/). The module will be promoted through our e-newsletter (over 2,500 sends) and is available on our website.

 

The second part of the online outreach and education component of this project is a webinar. The webinar will consist of a presentation of the findings of the study followed by a discussion of possible interpretations and implications. The findings of the study will be placed in the context of common grower practices, financial costs and returns, and existing knowledge about the costs and benefits of cover cropping in general. Several of the cooperating growers will participate as commentators sharing their interpretations and answering questions. The webinar is scheduled for December of 2016.

 

The execution of outreach and educational activities and materials was hampered by the harvest of 2016 which delayed both input from and participation of growers at the end of the project.

 

We are considering writing two manuscripts based on this project for submission to several journals which publish applied agricultural experiments.

Project Outcomes

Project outcomes:

This study was inspired by the concern that cover cropping practices common in the Central Coast region were responsible for depleting soil moisture that had been replaced by winter rains, thereby requiring additional irrigation water during the growing season. The lack of rainfall over the last seven years coupled with additional pumping of groundwater due to increased vineyard development caused growers to question whether the benefits they realize from their cover cropping practices were worth the potential cost in terms of water that would have to be replaced by irrigation during the growing season.

 

What was learned is that although there may be seasonal and site specific differences in biomass production and cover crop water use, these differences are not consistent even on the same site and are minimal in their impact on soil moisture.

 

The result at the J. Lohr site serves as an example of the outcomes from the cover crop species experiment. In the first year at the J. Lohr site the treatment with a cover crop of peas produced 31% more dry matter than any other treatment without a statistically significant difference in soil moisture depletion compared to the other crops. Similarly, peas produce 17% more biomass than barley which produced 72% more biomass than any of the others species without a statistically significant depletion of soil moisture compared to the other treatments. These differences were not found the following year, suggesting that 2015 was a better year for growing a pea cover crop than 2016 or there is some other factor affecting the success or failure of that crop. All of the project growers and many other growers stated that two years was not long enough to know if differences were real or not and certainly not long enough for them to change or modify their practices based on that information. For example, it does not make intuitive sense that a pea crop with 15%-30% more biomass than neighboring crops would not draw more water out of the soil than the neighboring plant types. And yet this is what the analysis showed.

 

In the cover crop termination experiments there were also contradictory results and lacks of difference which are counterintuitive or not convincing enough for cooperating growers to consider changing practices. One concern several growers expressed at the end of the project was that the clean cultivation treatment would end up with more soil moisture in the spring than the cover crop regardless of termination method. This would encourage other growers to turn to cultivation as their sole means of floor management, increasing erosion and decreasing soil health in the region as a whole. But the history of the region has been one of increasing adoption of cover cropping as part of vineyard floor management practice. Indeed, at the J. Lohr site the cultivated, unplanted treatment did have more soil moisture than the other treatments (all Blando brome, different terminations), but the opposite was true in 2016. Again, this was the only case of statistically significant differences in soil moisture between treatments across the three sites of the termination experiment and the two sites of the species experiment.

 

An encouraging finding was that the no till treatment produced significantly more biomass than the other treatments in the termination experiment at all three sites in 2015 and at two sites in 2016. The growers at these sites manage their floors as fallow/no till systems, and this finding supports the practice, as it produced the most biomass with no difference in soil moisture depletion.

 

In exit interviews cooperating growers remarked on the fact that overall there were no meaningful or consistent differences in soil moisture depletion between treatments in any of the experiments. The consensus on the experiments was that growers are free to choose cover cropping practices based on factors other than soil moisture depletion- at least in dry years.

 

In the exit interviews several cooperating growers said there are too many other factors and variables in the environment affecting cover crop success and behavior. Several of the growers pointed out that conditions in the Paso Robles area do not favor growing cover crops based on their experiences and frustrations. Paso Robles is too hot or too cold depending on who you ask. The soils are hard clay soils which are difficult for seedlings to root in. Even preparing seed beds is difficult or impractical. At one site we disked the plots four or five times and were left with a 1”-2” layer of dust to plant into. The lack of rain in the fall both years was also identified as a problem by the growers, but they also admitted that this was not unusual. The cooperating growers said they would continue with their current practices which are either annual cover crops for the purpose of increasing soil organic matter or a no till strategy with reseeding every five to eight years as required.

Economic Analysis

Objective 3 of this project was to perform an economic evaluation of vineyard floor management practices and document financial return on investment in terms of potential annual energy and water savings.

 

As there were no meaningful or consistent differences in the depletion of soil moisture between the type and timing of termination of the grass cover crop or between the different species of cover crops, in this study, there is no difference in return on invest (ROI) between the treatments in terms of annual energy use or water savings.

 

There are, however, differences in costs between the practices as shown in Tables 7 and 8.

 

During the exit interviews with growers after the completion of the experiments there was a general consensus that the benefits of cover crops cannot be captured solely in financial terms. The most common motivations given for cover cropping were to build soil organic matter, prevent erosion and reduce dust. Another theme that emerged during the interviews was that growers all have different reasons for and approaches to cover cropping. Therefore, different growers are willing to spend different amounts of money on their cover cropping program depending on their individual goals and beliefs.

 

This suggested to the growers, as well as the authors, that with no demonstrated savings in energy use or water use growers need not include those variables in their decisions, but can instead design a cover cropping program based on other benefits and at an acceptable cost.

Farmer Adoption

Objective 5. Measure adoption of water conservation practices identified in this project by survey of the producer audience who participate in tailgate meetings, webinar, and online educational module.

 

The assumption upon which this project was based is that cover crops growing in the winter deplete some of the soil moisture delivered by rainfall. The secondary assumption was that different species of cover crops would deplete more or less soil moisture and the timing and manner of terminating a grass cover crop would show differences in the depletion of soil moisture over the winter. Therefore, certain species or manners of termination would be preferable than others because less water would have to be added back to the system through irrigation, saving water, energy, and money. Those practices would have then been promoted.  With no consistent differences or patterns in soil moisture depletion between the practices in this study, there is little to be done in terms of farmer adoption related to this project.

 

It was not possible to obtain an accurate snapshot of grower practices on a regional scale before the start of the project. It was possible to get a sense of what grower practices were, are, and likely will be in the near future, however.

 

In formal interviews, informal interviews, and questionnaires distributed at two Tailgate Meetings we asked growers about their floor management practices. We then asked them if those practices had changed in the last three years. After a discussion of what their practices are and why they do them, we asked them if they planned to change anything about their floor management program. Only about a third indicated that they had or would make any changes in their floor management practices and these changes were minimal. Few of the growers at our meetings reported having made changes, even fewer reported planning to make changes.

 

The areas where the growers in our meetings said they might make a change fell into two categories: tillage and cover crop species. The results of this study do not suggest that any one termination practice or cover crop species is preferable in terms of reducing soil moisture depletion. Therefore, decisions about changes to tillage and cover crop selection would not be affected by the results of this study.

Recommendations:

Areas needing additional study

During the course of this project several areas of uncertainty were identified both through execution of the experiments and by grower observations.

 

  1. Effect of low rainfall

In tailgate meetings, interviews, and personal conversations with us, growers repeatedly expressed doubts about the generalizability of the finding because of the lack of rainfall in both years of the experiments. We discussed the question of whether more winter rains would have affected the outcomes with individual growers and with a group of growers at a tailgate meeting. A long-term field experiment would be required to answer this question and, again, the variations in rainfall from season to season would still be an issue for generalizability.

 

  1. Difficult area for growing cover crops

All of the cooperating growers commented on the general difficulty of growing cover crops in the local area due to soil conditions and weather patterns. Soils in the area tend to be very hard until they are wetted due to low organic matter and high clay content. Weather patterns also pose an obstacle as wet weather and warm weather usually do not coincide. When discussing the differences in the cover crop stands between our plots and the rest of his vineyard, one cooperating grower suggested that timing was the difference. He planted two weeks earlier than we did before the weather turned colder. Another cooperating grower commented that in his experience the Paso Robles area was simply a difficult place to grow cover crops compared to his experiences in Northern California. In an area where cover crops are more likely to flourish, the outcomes of the same experiments may have been different.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.