A System Approach for Improved Integration of Green Manure in Commercial Vegetable Production Systems

A System Approach for Improved Integration of Green Manure in Commercial Vegetable Production Systems

Summary

Initial results showed that use of Sunn hemp reduced N-fertilizer requirements of subsequent vegetable crops by 33-66%. In S-Florida the benefits of the use of a summer cover crop appeared to be greatest since vegetable crops are planted during the fall. In locations further north, a substantial fraction of the nitrogen accumulated by summer cover crop is lost during the winter. Introduction of a second cover crop, may address this problem. However, most leguminous winter cover crops show slow initial growth. Therefore the use of alternative green manure crops that make more efficient use of residual nitrogen may be desirable.

Objectives/Performance Targets

1) Determine which (combination of) green manure crop(s) when used with minimum tillage will result in optimal nitrogen supply to subsequent vegetable crops for different regions; 2) Assess the amount of supplemental N fertilizer required for optimal yields for green manure based production systems and compare their yields with conventional systems; 3) Determine nitrogen uptake efficiencies and N leaching for green manure-based cropping systems in comparison with conventional vegetable cropping systems; and
4) Develop a regional research and outreach program for improved integration of green
manure crops in commercial vegetable systems in close collaboration with local growers. Despite late arrival of funding we have made excellent progress in attaining the program objectives.

Accomplishments/Milestones

In Citra (N-Florida) we will complete our second annual cropping cycle towards the end of June of 2003. In Tifton (S-Georgia) and Boyton beach (S-Florida), the first annual cropping cycle will be completed around May and June of 2003, respectively. Initial results for Sunn hemp (our summer cover crop) appeared to be very promising. In Citra, Sunn hemp produced 8 and 12 MT/ha of dry weight in 12-14 weeks during the 2001 and 2002 growing season, respectively. Its use reduced inorganic nitrogen fertilizer requirements of sweet corn by 33% and decreased weed growth by 38 and 78%, respectively. In Tifton, Sunn hemp produced 7 MT/ha in 10 weeks. On-farm work in S-Florida showed that Sunn hemp produced 4 MT/ha in two months. The participating grower indicated that it was undesirable for plants to become excessively large since plants would get too “woody”. She stated that this could potentially interfere with bed preparation of subsequently grown mulched tomato/pepper crop. Keeping the cover crop small by mowing, resulted in smaller plants with less stems, yet proportionally more leaves. Growing Sunn hemp prior to tomato or pepper, reduced inorganic nitrogen fertilizer requirements of these vegetable crops by 33-67%, and resulted in similar or higher yields compared to the conventional treatment. The greater efficiency of the Sunn hemp residue in replacing inorganic N-fertilizer in S-Florida compared to N-Florida was related to tomato and peppers being grown directly after Sunn hemp. In N-Florida and Georgia, most vegetable crops are grown during the spring season, so substantial amount of nitrogen released from summer cover crops may be lost during the 4-5 months prior to planting. In this case, use of “woody” (more recalcitrant) crop residue may be beneficial since its nitrogen release patterns may better match crop nitrogen requirements of a subsequent corn crop. During the first year (2001) we examined the use of Lupin as a winter cover crop to minimize N-losses and/or to fix additional nitrogen. Lupin produced about 4 MT of dry biomass per hectare. Although Lupine appears to benefit from the Sunn hemp crop residue, combining it with Sunn hemp did not result in an additional yield increases compared to Sunn hemp by itself nor did it reduce fertilizer N-requirements of a subsequent sweet corn crop. Based on Dr. Phatak’s positive experience with Cahaba white vetch, we included this winter cover crop also in N-Florida during the second growing season. However, due to unfavorable weather conditions, overall growth was relatively poor and/or inconsistent in both Georgia and N-Florida. Final growth data are pending. Depending on these results, we may opt to include alternative winter cover crops such as radish and/or crimson clover. Rapid initial growth of radish may result in more efficient use of residual soil-N from Sunn hemp residue. Via its deep rooting system it may also counteract soil compaction below the plow zone. Soil analysis of the upper six inches of the soil showed after the first annual cycle showed that total soil nitrogen and organic matter were slightly higher in cover crop treatments compared to conventional treatments. We will continue to monitor soil properties and we expect that differences will become more pronounced over time, especially in the topsoil layer. Soil analysis also showed increased root-knot and lesion nematodes numbers after a sweet corn crop. Use of Sunn-hemp, on the other hand, reduced nematode populations compared to the conventional treatment. We have installed suction lysimeters in the N-Florida location. This will allow us to determine nitrogen release and leaching patterns from crop residues and/or inorganic fertilizer over time. These results will be essential for the development of decision support system that will assist growers to better match nitrogen supply with crop nutrient requirements for cover crop based production systems. We will recruit a second graduate student during the fall of 2003. This student will evaluate the economic aspects of cover crop use in vegetable cropping systems. We have collected detailed information on the breakdown of crop residues and we have started the development of computer modules that will be integrated into web-based decision support system. A post-doctoral researcher will be hired to continue this work during the course of the second year.

Impacts and Contributions/Outcomes

Corey Cherr, the graduate student participating in SARE program, presented two scientific presentations during the national annual meetings of the American Society of Agronomy/Crop Science Society of America/Soil Science Society of America in November 2002. Monica Cooper (University of Florida, Doctor of Plant Medicine Program) and Amy Van Scoik (University of Florida, BSc program Agronomy Department) completed internships working on special studies that were part of this program during the spring and summer of 2002, respectively. On January 24th a group of thirty farmers and extension agents from Columbus and Robeson counties (North Carolina) visited the project site in N-Florida during a field tour. The main focus of this group was to explore alternatives for conventional agriculture and improved use of green manures in agriculture. Some of the visiting farmers were interested in Sunn hemp. We sent Milton Parker, the county extension agent and group leader 10 lbs of seed to evaluate this crop under local growing conditions. On February 4th, we presented results from our program to the Junior Science, Engineering and Humanities Symposium (JSEHS) at the University of Florida to participating high school students to increase their awareness of the importance of sustainable agriculture via improved use of cover crops in conventional production systems. Johan Scholberg, the project coordinator, was invited to participate in a roundtable conference sponsored by SARE in Statesboro, GA to assist organic farmers and researchers with the development of future research strategies. During this workshop, concepts develop within the context of this SARE grant were shared with growers and scientist from several states throughout the southeastern US. During the second week of April research trials will be shown to 30-40 students participating in the southern regional student activities subdivision of the ASA/CSSA/SSSA. In this manner we anticipate to further increase the program impact and general awareness of students and scientists of our cover crop program.

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