Cover cropping strategies for year-round weed control on mixed vegetable farms in southern New England

Cover cropping strategies for year-round weed control on mixed vegetable farms in southern New England

Summary

The purpose of this project is to evaluate methods for improving soil health in high-intensity vegetable production systems without reducing yields or “idling” fields. We are focusing on reduction of tillage and addition of organic matter via cover crops as methods of improving overall soil health. This report covers the third year of the project.

In 2012 we continued to refine and test the effects of zone tillage, permanent beds, and interseeded cover crops on soil health and crop yields, with conventional tillage and winter cover crops as the standard for comparison. We also added an in-depth study of the weed control benefits of the four systems. The conventional tillage system resulted in the highest yields, and the lowest soil microbial activity. The zone tillage system had the highest soil microbial activity, but persistently low yields and high weed pressure. Both the permanent bed and interseeded cover crop systems showed potential.

We held four outreach events in 2012: a winter workshop, two twilight meetings on the research farm, and a twilight meeting at a grower’s farm. Through these events we reached 157 stakeholders. The results from 2011 were presented at the 2012 annual conference of the American Society of Horticultural Science, and the results of the 2012 weed study were presented at an undergraduate research conference at URI. The project also provided hands-on learning opportunities for 31 URI undergraduate students who contributed to the project as part of their coursework or as farm employees. Data analysis and drafting of reports continue, and are expected to result in publication of a thesis, multiple journal articles, and a set of extension publications in 2013. We will also be holding another winter workshop in February 2013, and a final twilight meeting in April 2013.

Objectives/Performance Targets

Twenty vegetable producers (10% of Rhode Island total) will adopt a combination of cover crops and reduced tillage to control weeds and improve soil quality on a total of 500 acres (25% of RI vegetable acreage). Quarterly twilight meetings and workshops will attract 200 growers over three years; participants will learn about new vegetable varieties and production methods they can adopt on their farms. Fifty University of Rhode Island students pursuing careers in agriculture will receive hands-on learning opportunities by assisting with cover crop trials as part of the vegetable production class or as summer interns.

• • 20-30 growers will attend each Twilight Meeting at the URI research farm. This milestone will be met repeatedly in years 1-3.
  o The April twilight meeting was attended by 46 people. The June meeting was held at a grower’s farm in eastern RI; it was attended by 50 growers, including many who do not normally attend twilight meetings held at the research farm. The August meeting was attended by 30 people.
  • At least 50 growers will attend each winter workshop. This milestone will be met repeatedly in years 1-3.
  o The 2012 winter workshop was attended by 59 people, mostly from Connecticut and Massachusetts. The 2013 meeting is scheduled for February 14.
  • 15 URI undergraduate students will participate in the cover crop research each year as part of the vegetable production class.
  o The cover crop research plots were used as teaching tools in both the spring and fall vegetable production classes; a total of 26 students were involved. In addition 5 graduate students, 10 undergraduate students, and 2 high school students worked on the project over the summer.
  • By the end of the second year 50 growers will have contacted us for more information on cover crops/reduced tillage and individual assistance in implementing these practices on their farms.
  o We have had difficulties in tracking the number of growers who contacted us for information and individual assistance. The university does not provide effective support for websites or for logging phone contacts. In addition, our extension agent left in December 2011 and the position remained vacant until August 2012.
  • The field research will generate new information on the relative efficacy of the four cover crop + tillage treatments for improving soil quality, controlling weeds, and maintaining profits.
  o A third year of data was collected on crop yields, soil health, weed levels, and labor inputs. Data for all three years is currently being analyzed, and the graduate student on the project will defend his thesis in the first half of 2013.

Accomplishments/Milestones

2012 was the third growing season for the project. No significant changes were made in project design relative to 2011, except that we grew the cabbage from transplants rather than direct seeding into the field. In addition to data on crop yields and input costs (including labor) we monitored weed populations and soil nitrogen levels throughout the growing season. The amount of fertilizer was increased, but we still encountered nutrient deficiency issues in some treatments.

Treatment A: Conventional Tillage
Our efficiency within the control plots continued to improve. This season we added another pass to our tillage schedule which lowered weed abundance when compared to last year. Use of a weed burner increased our success with the carrot and salad plantings, and further reduced our need for hand weeding.

Treatment B: Zone Tillage
After noticing a low bulk density for our 2011 cover crop, and hearing from several other growers that our seeding rate was too low, we doubled our winter rye seeding rate. The increased density resulted in shorter plants in the spring, and didn’t change our fate with the weeds throughout the growing season. After rolling the winter rye we ended up with a mulch layer that was very similar to the previous year. Unfortunately, the winter rye is not obtaining the biomass others have suggested is necessary to successfully control weeds with this treatment. The mulch layer was enough to slow weed germination until the middle of July before we were forced to try and control them with mowing. Increased fertilization enabled us to improve yields on all crops except melon and cucumber, which do not respond well to the lack of black plastic mulch. However weeds, especially crab grass, continue to dominate the treatment and stunt the crops.

Treatment C: Raised beds and Perennial Mulch
In 2012 we did not attempt to create raised beds, but laid the plastic mulch by hand over level ground. This avoided the problems of disruption the perennial ryegrass/white clover mulch, but added substantially to the labor required. The perennial ryegrass and white clover living mulch successfully limits and smoothers broadleaf weeds, but high populations of potato leafhopper (Empoasca fabae) sent the clover into dormancy in August, enabling crabgrass to take over.

Treatment D: Intercropped Crimson Clover
A dry summer once again resulted in poor establishment of the crimson clover. Even after seeding the treatment twice we were unable to obtain the cover crop biomass we desired. It may be that successful establishment of an interseeded covercrop requires sprinkler irrigation, which we were unable to use due to the limits of our irrigation system. With neither cover crop nor cultivation we had high levels of both grassy and broadleaf weeds. Biweekly mowing was once again used to control excess weed growth, with the result that yields were comparable to those in the conventional tillage plots.

Cabbage
To avoid problems with establishment in some treatments associated with direct seeding this year we transplanted the cabbage crop. This turned out to be very beneficial in all treatments as our starts had a large advantage over the germinating weeds. While all crops established nicely, the weed pressure in treatments B and C lead to greatly reduced head size (figure 1), dramatically reducing yield. No statistical difference between treatments A and D was observed.

Melons
The mild winter combined with the decision to not do fall tillage in the 2011 winter squash fields resulted in very high levels of striped cucumber beetle in May and June of 2012. The beetle damage was particularly severe in the third replication of treatments A and D, as the plots were located directly downwind of a former squash field. While we were able to control the beetles on the other two replications, the damage done to the third replication lead to dramatic reductions in yield, and the damage gradient obscured much of the treatment effect.

Cucumber
While the cucumbers did not sustain as much damage as the melon crop, the damage was enough to cause reduced yields in the third replication. We continue to see the benefits of black plastic mulch. Treatments A, C and D had significantly higher yields than treatment B which continued to struggle without the plastic.

Lettuce and Carrots
Rather than planting the entirety of each plot on a single date, the lettuce and carrots were grown using weekly plantings. A 20-foot section of each plot was planted each week of the growing season. Weekly weed management of areas to be seeded led to nearly weed free plantings of salad, and greatly reduced the weed pressure within the carrot plantings. Treatment D had the highest yields, but the only statistically significant difference was with the lowest-yielding treatment, B. Treatment B’s total weight per carrot was significantly lower than treatments D, A, and C (figure 2). Reducing weed cover on the bed edges eliminated mouse and vole predation which damaged seeds and seedlings in 2011.

Tomatoes
Continued trellising and early control of a tomato hornworm infestation led to solid yields for all treatments. However, Treatments B and C, had significantly lower yield totals than treatments A and D. Despite even fertilization throughout all the treatments, the B and C tomatoes showed a significant amount of nutrient stress, which was likely due to weed pressure.

Soil Respiration with Solvita
We continued biweekly soil analysis with the Solvita soil CO2 respiration tests this season. Biweekly soil sampling was conducted thirteen times from April to October. Our conventional tillage treatment had significantly lower soil respiration rates than the other three treatments (figure 3). The highest rates of soil respiration occurred in treatment B, with lower but statistically similar rates in treatment C. Treatment A yields were consistently in the top group, while those of treatment B were significantly lower than the other treatments for all vegetables. These results suggest that while soil microbial activity is an indicator of the amount of plant-available nitrogen in the soil, the connection with yield is more complex. This complexity is likely related to competition for nitrogen between the crop and weeds and/or cover crops.

Weed abundance and suppression
As in 2011, we sampled for weed species diversity using a 50-square 1 X 0.5m grid. We collected data four times over the course of the season. In 2011, treatment C had a significantly lower number of observed weed species while treatment B had the highest. In 2012, treatment B continued to have the greatest number of weed species but no significant difference was observed between treatments A, C, and D.
In 2012 we added an undergraduate project that focused on weed abundance, measuring the dry weight mass of grasses, broadleaf weeds, and cover crops present per treatment (figure 4). We found that there was no significant difference between the dry mass of treatments A and C. Treatments B and D also showed no significant difference between them. Both treatments A and C had lower weed abundance than treatments B and D. When comparing just broadleaf weeds, the dry mass weight of treatment C was significantly lower than treatments A, B, and D. Even though treatment B displayed the poorest ability to prevent weeds, the cover crop dry weight mass of the treatment was significantly greater than all other cover crop treatments. Due to treatment D’s poor germination, its cover crop dry weight mass was equal to that of treatment A.

Lessons learned for 2012
Weekly use of a weed burner in carrot and salad plots continued to be the only way we could successfully cultivate directly seeded crops. Overall, crab grass continues to cause the majority of problems associated with reduced yields, especially in treatments B and C. Even when the planting rows were weeded the crab grass would grow in from the aisles, creating a lot of competition for the crops. Our Solvita soil respiration rates suggest that treatment A has the lowest amount of microbial activity and little organic matter, yet it is yielding equal or significantly greater quantities of all crops we grew. These results suggest that something besides limited nutrients is causing the yield reductions, most likely weed pressure. This season we added an additional tillage pass to treatment A to further help with weed control. The additional pressure from weeds in treatments B and C resulted in smaller fruit size and reduced yield. Ironically, treatment D where we had very poor establishment of our cover crop, crimson clover, yields similar to the conventional treatment were recorded despite the weed elevated pressure. This could be linked to an initial tillage we performed in the treatment in an attempt to create a better seedbed for the cover crop. We are currently analyzing soil nitrate levels from the soil samples we collected during the Solvita testing hoping this will further our understanding of yield differences. However, it appears that we have not yet identified a way to improve soil health without sacrificing our ability to control weeds, and that poor weed control depresses yields more than healthy soil can stimulate them. 2012 was the last field season for this project, but we will continue to experiment with cover crops and tillage alternatives in vegetables.

Attendance was strong at our twilight meetings and winter workshop, despite not having an extension agent for most of 2012. Kudos are due to our state IPM specialist, Heather Faubert, who provided assistance with planning and promoting the outreach events even though she is not officially connected with this project. Our growers are expressing interest in cover crops. Twenty-five of the attendees at the winter workshop completed evaluation surveys; 21 indicated that they currently use cover crops in the winter. Fewer respondees use cover crops during the summer or for intercropping, nutrition, weed control, or suppression of soil pathogens. However, most growers who were not currently using the practices indicated that they planned to implement them during 2012. Similarly most attendees at the April twilight meeting indicated that they were already using cover crops, but they also indicated that their knowledge of how to use cover crops increased after attending the meeting. We have not seen much interest in the use of zone tillage. This does not surprise us, for several reasons. First, we have not had good success on our research farm using zone tillage without herbicides. Second, the majority of our growers lack not only zone building equipment, but tractors with sufficient horsepower to pull a subsoiler. Finally, the greatest erosion control benefits from zone tillage are seen on sloping fields, but in RI such fields tend to be stony and the rocks create problems for the zone building equipment.

• students weeding lettuce
• Figure 1
• Figure 2
• Figure 3
• Figure 4

Impacts and Contributions/Outcomes

The 2011 research results were presented at the 2012 American Society for Horticultural Science meeting in August, and were published as an abstract in the conference proceedings. A copy of the poster is attached. The results of the weed abundance study were presented at the URI College of the Environment and Life Sciences annual undergraduate research conference; the poster is attached. Data is still be analyzed on the impacts of the extension component. However, it is clear that this project will have a lasting effect on URI’s research and extension support for food production agriculture. While we still have only one ag extension agent for the entire state, the position is now being funded directly by the Experiment Station/Cooperative Extension, rather than through competitive grants. This has enabled us to use competitive grant funding to hire additional FTEs of extension time. We have also been able to hire a second staff person at the research farm, and we now have food production agriculture options for both our graduate and undergraduate degrees.

• graduate research poster
• undergraduate research poster

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