Coachella Valley Cover Crop Research
Jose Aguiar, Jeff Ehlers, Walt Graves, Bill Matthews, Phil Roberts, Vince Samons
UC Cooperative Extension, Riverside County
and Botany & Plant Sciences Dept., UC Riverside
The Coachella Valley produces an exceptionally diverse variety of vegetable crops, including leaf lettuce, celery, artichokes, carrots, bell peppers, and seedless watermelon during the fall and spring. Late summer plantings of vegetable crops in the Coachella Valley account for a significant portion of total US production of these crops during the winter months. Production of all these high value crops is achieved through the generous use of fertilizers, pesticides and irrigation water.
There are many problems associated with the intensive crop production systems of the Coachella Valley. The sandy, low organic matter soils are easily leached of most forms of synthetic nitrogen fertilizers and such leaching can contaminate surface and groundwater. Air pollution and loss of topsoil result from fields left bare during the summer fallow period. The combination of sandy soils, high temperatures, and intensive cultivation of nematode susceptible crop varieties can lead to severe nematode problems that reduce yields. Weeds also quickly build up under these amply irrigated, high fertility systems. Control of both nematodes and weeds can require expensive chemical remediation. An alternative system that improves soil fertility and appears to address all of these problems is cover cropping during the summer fallow period.
Growers in the Coachella Valley have expressed interest in finding a cover crop that fits into current production cycles. A cover crop screening trial was conducted in the summer of 1996, at the Univ. of California, Riverside (UCR) Coachella Valley Agricultural Research Station (CVARS) near Thermal to select warm season annuals that could provide soil cover, weed suppression, and organic nitrogen. The cover crop had to perform well during the typical fallow summer period between spring and summer vegetable crops.
A range of potential cover crops was selected for evaluation based on their reported adaptation to hot weather. These included cowpeas, crotalaria, guar, lablab, sesbania, and soybeans. The evaluations were based primarily on production of biomass and ability to produce ample and quick ground cover.
Cowpea, Vigna unguiculata, was clearly the best choice. Cowpeas are well adapted to our sandy desert soils and the species can tolerate heat and drought better than almost any other crop. Fortunately, if growers are keen to try a cowpea cover crop, seed of a few acceptable varieties of cowpea, such as ‘Iron-Clay’, and the appropriate inoculant (type "EL"), are already available from seedsmen. Even better varieties should be available soon, as our results show. The variety ‘Chinese Red’ should be avoided because it is highly susceptible to root-knot nematodes, and combine cowpeas should also be avoided because it flowers early resulting in low biomass production.
Cowpeas grown as a 70-day cover crop can fix 200 pounds of nitrogen or more per acre and add substantial amounts of organic matter to the soil. Nematode resistant varieties of cowpea can substantially reduce soil populations of root-knot nematodes, which help to protect the cowpeas, and also benefit the succeeding susceptible crops grown in rotation.
In 1997 we conducted a cowpea cover crop variety trial to select the variety with the highest potential for biomass production and weed suppression. After looking through the database of the extensive (5,200 accessions) cowpea collection maintained at UCR, a subset of 500 accessions with the key characteristics of small seed size and photosensitivity were selected for this trial. Photosensitive cowpeas are desirable for cover cropping because they will not flower until the days become short in the fall and plants that remain in the vegetative growth stage will produce more biomass and fix more nitrogen per day than those that transition quickly to flowering and setting pods. At CVRS, the 500 accessions were also planted in the fall and assessed for seed yield potential, as a cover crop variety has to be a good seed producer, in addition to being a good biomass producer, so that seed costs for the cover crop will be low.
About 140 accessions were judged to have adequate seed production. Of these, 75 were found to be resistant to the root-knot nematode species most common in the Coachella Valley, Meloidogyne incognita. Many of these were also found to have adequate resistance to M. javanica. This "broad-based" resistance to root-knot nematodes greatly simplifies management decisions because it is not necessary to identify which species is present in a particular field.
In 1998, we conducted two cover crop trials: an advanced trial with large plots that included the standard California blackeye bean variety ‘CB46’ and three entries thought to be most promising (‘Iron Clay’, IT89KD-288, and IT93K-503-1), and a screening trial of the 75 nematode resistant accessions. In the advanced trial, both African accessions IT89KD-288 and IT93K-503-1 had nearly double the biomass yield of either ‘Iron-Clay’ or ‘CB46’. In the screening trial, the top four accessions were UCR730, IT90K-284-2, UCR671, and IT89KD-288. These accessions had more than double the biomass yield of ‘CB46’ and were significantly higher in biomass production than ‘Iron-Clay’. UCR730, UCR671, and IT89KD-288 appear to have all the desirable characteristics of an ‘ideal’ cover crop variety for the desert, and are clearly superior to presently available varieties (Table 1).
Also in 1997-98, field experiments demonstrated that root-knot nematode resistant cowpea genotypes (including ‘Iron Clay’) significantly reduced soil population densities of M. incognita compared to wet fallow and susceptible cowpea treatments (Table 2). These experiments also showed that this reduction in levels of nematode provided substantial benefits to the succeeding tomato crop grown in rotation (also in Table 2). A similar trial is underway for the 1998-99 season to further compare the effects of resistant cowpeas versus a susceptible cowpea check and wet and dry fallows.
Table 1. Cowpea Cover Crop Varieties
| Characteristics | Varieties | |||||
| Ideal Variety | CB46 | Iron Clay | UCR671 | UCR730 | IT88KD-288 | |
| Biomass & Nitrogen1 | V. High | Low | High | High | V. High | V. High |
| Seed Yield2 | V. High | V. High | High | High | High | High |
| Resistance to Root-knot Nematodes3 |
Yes+ | Yes | Yes | Yes+ | Yes+ | Yes+ |
| Seed Size | Small | Large | Small | Small | Small | Medium |
| Photosensitivity | Yes | No | Yes | Yes | Yes | Yes |
| Growth Habit | Spreading | Erect | Semierect | Spreading | Spreading | Spreading |
| Origin | --- | Calif. | South-USA | Kenya | Botswana | Nigeria |
| Status4 | NR | A | UE | UE | UE |
1When grown as a cover crop in summer, V = very.
2When grown to produce planting seed in the fall or spring in the low-desert valleys.
3Yes = Resistant to avirulent isolates of Meloidogyne incognita root-knot nematode but not to virulent isolates of M. incognita or to aggressive M. javanica. Yes+ = resistant to all above root-knot nematodes.
4Status: NR = Not recommended; A = Presently available commercially; UE = Under evaluation
Table 2. 1997-98 Cowpea Cover Crop Yield and Bioassay Trial on Root-knot Infested Soil
Tomato Bioassay (6/98) |
||||||
| Treatment | 1pi (6/27/97) | 1pf (9/26/97) | Cowpea Yield(gms/plot) (10/9/97) |
1pi (1/29/98) | Fruit weight gms/plot | Root gall index2 |
| UCR7793 | 564 | 14622a | 626 d | 2196a | 15123 c | 7.16a |
| UCR430 | 694 | 26 b | 5562a | 20 b | 33101a | 2.60 bc |
| Iron Clay | 668 | 26 b | 5946a | 16 b | 32872a | 2.52 c |
| H8-8-27 | 873 | 43 b | 2111 c | 38 b | 30324ab | 3.54 bc |
| CB46 | 922 | 153 b | 3575 b | 140 b | 30670ab | 4.62 b |
| Wet fallow | 769 | 119 b | --- | 78 b | 25349 b | 4.49 bc |
| lsd (p=0.05) | N.S. | 6444 | 1177 | 560 | 6993 | 1.87 |
1#2nd stage larvae per 250cc soil; pi = Initial population; pf = final population
2 0-10 scale
3Susceptible check
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