Managing Cover Crop and Conservation Tillage Systems To Enhance Vegetable Crop Yields, Economic Returns and Environmental Quality

Managing Cover Crop and Conservation Tillage Systems To Enhance Vegetable Crop Yields, Economic Returns and Environmental Quality

Summary

Cover crop species and mixtures were evaluated in western Oregon organic broccoli production systems for the potential to increase yield, reduce fertilizer inputs, and improve soil quality. A split plot design was used to study the interaction of cover crops with four rates of nitrogen fertilizer, using feather meal as a source of nitrogen. The phacelia-vetch cover crop mixture increased broccoli yield compared to the fallow, no cover crop treatment by an average of 1.3 tons per acre, worth $2,370 per acre. The oat-vetch cover crop mixture also increased average broccoli yield by 0.6 tons, worth $1,090 per acre. Broccoli yield was severely reduced by oat cover crops at the zero and 100 lb N rate, requiring up to 200 lbs N/acre of supplemental fertilizer to overcome the yield loss.

Objectives/Performance Targets

1. To evaluate biomass production and N accumulation from selected cover crop treatments.

2. To evaluate the effects of cover crops and organic fertilizer amendment (feather meal) on N availability and broccoli yield.

Accomplishments/Milestones

In a replicated trial near Corvallis OR, five cover crop treatments were compared to a no cover crop control. Cover crop, planted in October 2007, included: oats, phacelia, common vetch, oats plus vetch, and phacelia plus vetch. To insure accurate seeding rates, cover crops were hand planted. A split plot design was used to study the interaction of cover crops with four rates of nitrogen fertilizer (0, 100, 200, and 300 lbs/acre), using feather meal as an organic source of nitrogen. Cover crop biomass was sampled prior to tillage, and laboratory analysis conducted to determine percent carbon and nitrogen in the tissue. Broccoli transplants were greenhouse grown using organic methods, and mechanically transplanted to the field in late May, 2007. Broccoli yield was determined in five cuttings over a two-week period in mid August. Data were analysed using the SAS system.

Impacts and Contributions/Outcomes

The advantages of legume-based cover crop mixtures over sole crops are apparent in this study, both in terms of increasing broccoli yield, but also accumulating more biomass for increasing soil organic matter. The phacelia-vetch mixture increased broccoli yield compared to the fallow treatment by an average of 1.3 tons per acre (1.5, 0.8, and 1.5 tons broccoli/acre at zero, 100 and 200 lb N). Based on our actual market price paid for organic broccoli of $0.91/ lb ($1,820 /ton), the economic value of the crop yield increase for phacelia-vetch cover crops was $2,370 per acre.

The oat-vetch cover crop mixture also increased broccoli yield, but only at the 100 lb N rate. Broccoli yields were virtually identical at the other three N rates. At the 100 lb N rate, however, the oat-vetch treatment produced the highest broccoli yield, 5.8 tons/acre, which was 1.9 tons higher than the no cover crop, fallow plots. Economically, this increase was worth $3,460/acre. Averaged across the three N rates, however, the average yield increase was only 0.6 tons, worth $1,090 per acre.

Both the oat-vetch and the phacelia-vetch mixtures at 100 lbs fertilizer N per acre produced similar yields to the fallow treatment at 200 lbs N/acre, suggesting a 100 lbs of “N fertilizer equivalency” or fertilizer replacement value. The cost of the organic fertilizer used in this experiment (feather meal) was $2.33 per pound of N ($ .28/lb meal @ 12% N). Therefore the fertilizer replacement value of the cover crop mixtures (100 lbs N/ac) was estimated to be $230/acre. Broccoli yield was severely reduced by oat cover crops at both the zero and 100 lb N rates, requiring up to 200 lbs N/acre of supplemental fertilizer to overcome the yield loss. This yield reduction was most likely due to nitrogen immobilization by the oat residue, which had a C:N ratio of 68:1.

The increased adoption of legume-based cover crop mixtures by organic and conventional vegetable growers can provide a quadruple (or more) win for the grower and the environment, with increased crop yields, reduced fertilizer inputs and reduced nitrogen leaching, and improved soil quality. Replacing off-farm sources of nitrogen (conventional fertilizers, manure, or other organic sources) with on-farm nitrogen production through legumes will also greatly reduce the energy inputs to the farming system.

Collaborators: