California and Arizona desert vegetable growers can achieve increased yields, pest suppression, and reduced soil erosion and nutrient leaching by growing summer cover crops prior to planting winter crops.
Cover crops suppress pests and fix the nitrogen in legumes which enhances plant development especially in organic crop production where synthetic pesticides and nitrogen fertilizers are not permitted. Cover crops can offer similar benefits in conventionally-grown winter vegetables.
Close management is required to increase plant growth and yield since cover crops can change the nitrogen (N) availability in the soil. If a cover crop produces large amounts of biomass with a high-carbon nitrogen (C:N) ratio, the microbes consuming the cover crop residue can immobilize the soil nitrogen. In this case, extra N should be applied to the cash crop.
A soon-to-be-released Cooperative Extension paper summarizing summer cover crop use in low-desert vegetable production is authored by Guangyao (Sam) Wang, cropping systems specialist, UA Maricopa Agricultural Center, Maricopa, Ariz.; and Kurt Nolte, area Extension agent and county director, UA Cooperative Extension, Yuma County.
Wang shared the findings of the decade-long research conducted in the California and Arizona low desert during a preseason vegetable workshop in August at the UA Yuma County Extension office.
The findings are based on research conducted at the University of California (UC) Coachella Valley Agricultural Research Station (CVARS) in Thermal, Calif., (1999-2003); and at the University of Arizona's (UA) Yuma Agricultural Center (YAC) in Yuma (1998-2000) and the Maricopa Agricultural Center (MAC) in Maricopa (2008 to the present).
Wang worked on the CVARS project from 2001 to 2003 during his doctoral work at UC Riverside. He has led the UA MAC trials since 2008.
Wang says the summer cover crops cowpea and sudangrass and several newer species can increase yields in soils planted in winter vegetables and spring cantaloupe.
The CVARS research included four summer cover crop treatments: cowpea planted in July and cut and incorporated into the soil in September; cowpea cut and left on the soil surface as a mulch during the same time frame; sudangrass planted in July and cut in September for soil incorporation; plus a bare ground control plot.
“The incorporated cowpea cover crop improved conventional and organic fall lettuce and spring cantaloupe yields,” Wang said. “The sudangrass cover crop slightly reduced fall lettuce yields, but improved the spring cantaloupe yield.”
Fall lettuce was transplanted in October into double rows on a 60-inch bed with 10-inch in-row spacing for a December harvest. Spring cantaloupe was transplanted in February and March into a single row on a 60-inch bed with 10-inch row spacing and harvested in June.
Wang says the YAC trials generated similar results to the CVARS findings.
Lettuce and cantaloupe yields in the organic treatment in the first year (1999-2000) were 57 percent and 80 percent respectively compared to conventionally-managed crops.
In the last year (2003-2004) at CVARS, lettuce and cantaloupe yields in the organic treatment mirrored the conventional treatment. The soil nitrogen and weed population showed similar trends.
Wang explains that soil goes through fundamental changes during organic transition before reaching a new equilibrium in an organic system. Growers considering organic production can make the transition more smoothly by applying organic fertilizers earlier. A breakdown of the cover crops cowpea, sudangrass, and other potential warm-weather summer cover crops for desert use includes:
Cowpea, Vigna unguiculata, is the most common U.S. summer legume crop and well acclimated to the desert's drought and high temperature environment. Cowpea is day-length sensitive and begins to flower in early September in the desert.
“Cowpea can produce about 2 tons of biomass per acre in 75 days in central Arizona,” Wang said. “Cowpea has a favorable C:N ratio and breaks down rapidly.”
The summer cowpea trial at CVARS increased fall lettuce and spring cantaloupe yields.
Killing cowpea before October is critical to avoid setting seeds and volunteer plants in the cash crop. Wang says Iron Clay is a common variety for planting in a 20 to 30-inch row spacing at a 35 to 40-pound/acre seeding rate.
Sudangrass, Sorghum vulgare var. sudanense, can be planted in the spring or summer. If planted early, sudangrass can be harvested as forage in the early summer. The last re-growth can be incorporated as a cover crop. Sudangrass produces 3 to 5 tons of biomass in 70 days.
“In the CVARS study where sudangrass was followed by fall lettuce and spring cantaloupe, sudangrass decreased the lettuce yield due to the slow nitrification rate and possible allelopathic chemicals,” Wang said.
“However the sudangrass increased cantaloupe yield since the nitrogen released from the grass residue provided a more stable and higher rate of nitrogen.”
If planted as a cover crop or rotational crop, Wang says sudangrass residue should break down for at least one month. A seeding rate of 40 to 60 pounds/acre is recommended.
Lablab, Lablab purpureus, grows well in hot weather. The variety ‘Rongai' produces about 2 tons of biomass in 60 to 70 days, Wang says. Lablab grows vegetatively without flowering after October which offers growers more flexibility in planting and incorporation dates.
Wang recommends a 20 to 30-inch row spacing and a 50 to 60-pound/acre seeding rate. Lablab grows erect in the early growth stage.
Sesbania, Sesbania exaltata, unlike cowpea and lablab, does not require a herbicide in weedy fields for establishment. The crop grows well in arid desert climates. Sesbania covers the ground in four weeks in 15-inch row plantings at the MAC.
The crop grows 6 feet high in 50 days and yields about 2.5 tons of biomass. Sesbania flowers in early September and should be cut or incorporated into the soil before the end of the month. Wang suggests a 30 to 40-pound/acre seeding rate.
Pearl millet, Pennisetum glaucum, grows 4 feet to 5 feet high and produces 2 to 3 tons of biomass in 60 days. The crop is planted in June and flowers later in the month.
“Pearl millet has a deep root system that grows well with low nitrogen and water inputs,” Wang said. “When used in vegetable cropping systems, pearl millet can use residual nitrogen and reduce nitrate leaching. The recommended planting rate is 5 to 10 pounds per acre.”
Wang says other warm-season cover crops with a potential good fit in desert vegetable systems include: sunnhemp, Crotalaria juncea; velvet bean, Mucuna deeringiana; German (foxtail) millet, Setaria italica; and Japanese millet, Enchinochloa frumentacea.