Cropping Systems for Intensive Desert Vegetable Production

Objective 1. Evaluate the effectiveness of cover crops in reduced tillage (surface mulch) and conventional tillage (incorporated) production. The three main treatments are: (1) summer cowpea used as mulch in the fall, (2) summer cowpea incorporated into the soil in the fall, and (3) summer fallow (Figure 1). Each main treatment was subdivided into two sub-treatments (management systems) of either conventional or organic management (Figure 2).

1.a. Improvement of soil temperature regime.
In the low desert, fall vegetable planting may be delayed until high soil temperatures decline to allow seed germination and crop growth. We found that cowpea mulch buffers soil temperature fluctuations (Hutchinson and McGiffen 2000). At a depth of up to 6 inches, cowpea mulch reduced the daily maximum temperature and increased the daily minimum temperature. For fall planted lettuce, decreasing maximum soil temperature may allow growers to plant earlier and therefore target markets with better prices.

1.b. Non chemical weed suppression.
Weed control is a major component of vegetable production inputs in the desert. Weed control is generally conducted manually and therefor constitutes a big constraint to large-scale production. We found that either incorporated into the soil or used as mulch, cowpea cover crop provided better weed suppression than summer fallow or sudangrass cover crop for the fall planted lettuce (Hutchinson and McGiffen 2000; Ngouajio et al In Press). At Coachella, this effect was carried over from fall lettuce to cantaloupe planted in winter/spring. The organic crop management system also resulted in fewer weeds than the conventional system during the cantaloupe growing season at Coachella (Figure 5). The Yuma, AZ site had a low weed population prior to the start of the experiment, and weeds were generally low throughout the study in Yuma.

1.c. Insect population control.
Insect populations were similar whether the plot was previously fallow, grown with cowpea or sudangrass and residues incorporated or grown with cowpea and residues used as mulch. Cover crops do not appear to increase insect populations by serving as an alternative summer host.
The crop management system affected insect populations. The organic system had more leaf feeding insect crop injury than the conventional system. This was probably not due to the inherent ability of organic system to attract insects, but more likely to a reduced activity of insecticides used in organic plots. More effective insect control is an obvious need for organic production.

1.d. Crop growth and yield.
The cowpea cover crop significantly boosted lettuce yield, but the sudangrass cover crop decreased yield (Figure 3). All plant growth characteristics including average lettuce head weight, head diameter, leaf weight, leaf area, dry biomass, and yield were smaller in sudangrass plots. Parallel to the low yield, lettuce plants from sudangrass plots showed low midrib nitrate-N content (Figure 7). This result indicates either nitrate immobilization during the decomposition of sudangrass residues or release of allelochemicals by sudangrass residues, which may interfere with nitrate uptake by the crop. However, effect apparently declined during the lettuce growing season. Cantaloupe planted in the same plots the subsequent spring had the greatest fruit yield in the summer sudangrass plots in Yuma and similar to yield in fallow plots at Coachella. In both sites, sudangrass residues were completely decompose by the end of the lettuce season. Severe cantaloupe yield reduction (40 to 60%) was observed in cowpea mulch plots. Due to rapid decomposition, the beneficial effects of the mulch were eliminated, and soil compaction was readily apparent. Reduced tillage systems may require the use of conventional plowing after each crop.
For all crops and sites, the organic management system resulted in lower crop growth and nitrate content as well as smaller yields when compared to the conventional or the integrated system in the first year (Figure 7). However, the yield penalty was within acceptable limits in Coachella, although significantly less than the conventionally managed plots in Yuma. In the second year, yields for conventional and organic management systems were equivalent when pest populations were below damage thresholds for Coachella. . In Yuma, AZ, lettuce and melon yields were always less for the organic treatments compared to the conventional treatments. For lettuce both insects and soil fertility limited yields in the organic plots. For melons the limiting factor in the organic plots was always insects.

Plant nitrogen content was less stable when no cover crop was planted. Lettuce leaf nitrate content rose dramatically in the summer fallow treatments, declining just before harvest. Leaf nitrate content was relatively stable for either cover crop regime throughout the growing season, especially when cowpea was incorporated prior to lettuce planting. However, leaf nitrate levels were dramatically lower in the bareground (summer fallow) treatments during the critical first month of lettuce growth.
The bareground treatments also had large fluctuations in soil nitrate content. Early season soil nitrate content was tenfold greater in the bareground treatments than either cowpea cover crop treatment (Figure 8). However, soil nitrate reserves declined sharply in the summer fallow treatments throughout the growing season, while soil nitrate levels in lettuce following either cover crop system remained relatively unchanged. As a result, while soil nitrate levels in following summer fallow was greatly lower by the end of the season, soil nitrate content was similar across all cover crop regimes at harvest time.

Objective 2. Develop cost studies for desert ICM and organic based vegetable production systems and compare those costs to conventional production systems.

These studies are currently underway. Preliminary results indicated that the number of handweedings and insecticide applications are smaller in the ICM and organic systems than in the conventional system. These findings will likely guaranty greater net benefit for the ICM system since its yield was comparable to that of the conventional system. Even though yield was slightly reduced in the organic system, low inputs and higher market prices of organic produces may also counterbalance yield penalties.

Objective 3. Extent current knowledge of cover crop systems to vegetable growers, Soil Conservation Service, and other interested parties. We have gotten excellent response to our results, including many requests for talks and papers. The project has led directly to many extended collaborative ventures that follow up our research and extension projects.

Researchers from the University of California Cooperative Extension and USDA have been experimenting with cover crops as a way to maintain soil quality in many cropping situations (Cavanaugh 1998). But vegetable producers plant two or more high value crops per year to recover high land and water costs. It seemed impractical to invest additional land, labor, and other production costs into the uncertain payback of cover crops.
But growers began to experiment. They found a gap between spring and fall plantings when the land was usually fallow (Ngouajio and McGiffen 2000). A cowpea cover crop was inexpensive, required little water and no fertilizer, and produced abundant biomass and nitrogen (Aguiar et al 1998). Cowpea cover-crop acreage went from virtually none in 1997 to over 1,000 hectares in 1999. Some of this increase was driven by the increased demand for organic produce. Many limited resource farmers view organic vegetables as a niche market that allows them to remain competitive (Vischer 1994). Since production costs and market value are generally higher for organic produce, some growers wondered whether the costs of growing organically certified produce was worth the benefit (Brumfield et al 2000). But many farmers also recognized that cover crops offer many advantages in vegetable production, including N fixation, recycling nutrients, reducing soil erosion, and adding organic matter to the soil (Gaskell et al 2000; Stirzaker et al., 1992; Stirzaker et al., 1993). Strong grower interest led to increased breeding and pest management research to develop cover crop cultivars with enhanced resistance to nematodes (Matthews et al 1998), weeds (Hutchinson and McGiffen 2000), and other pests (Hall et al 1997). Because nematode, weed, and other pest populations decline after planting resistant cowpeas, it appears possible that novel cover crop varieties could replace fumigation and other chemical pest control. Cover crop cultivars may eventually be developed to solve specific problems from conventional agriculture, such as the loss of methyl bromide and other pesticides.