Development and Dissemination of a Cowpea Cultivar for Cover Crops
Cowpea cover crops are cost effective because they enrich the soil with carbon and more than 100 lbs per acre of nitrogen and can reduce pest populations. Adoption has been slowed by a lack of pest-resistant varieties adapted for the Western United States. We have identified new sources of resitance to nematodes, weeds, insects, and diseases. In the last year we were able to identify promising candidates for varietal release. The candidate cultivars produce high biomass and seed yields and have resistance to cowpea aphid, root knot nematode, and seed shattering. The ability to withstand drought was similar for all genotypes. An erect growth habit was best able to out-compete weeds. The candidate cultivars will be tested on farms in specific cropping situations to evaluate their suitability for release as cover crops. Information is being disseminated through talks, publications, and organic production training sessions and a manual.
Two major objectives were pursued in the first year of this three-year grant:
1) Identify a few cowpea breeding lines as potential cover crop cultivars.
2) Identify the characteristics of cowpea that make it competitive with weeds.
The completion of each objective is detailed below.
Irrigation treatment effects
The irrigation regimes imposed at both Shafter and Coachella produced significantly different levels of overall cowpea biomass production at these locations. At Shafter, reducing total water availability by half reduced biomass production by 57%. At Coachella, one-third and two-thirds reduction in total water applied reduced total biomass production by 17.5 and 33% respectively. The much greater reduction in biomass between the full and 50% irrigated treatment at the Shafter location compared to the reduction that occurred at Coachella between the full and two-thirds reduction in total water is likely due to the irrigation methodology, i.e. in the case of the furrow irrigated experiment at Shafter, water was applied at week to 10 day intervals for the full treatment and at 14 to 20 day intervals for the drought treatment, compared to 5 day a week drip irrigation for the full treatment vs. 2 times per week for the 33% irrigation treatment at Coachella. We did not detect strong evidence of genotypic differences in response to deficit irrigation among the genotypes involved.
Genotype and Genotype x Irrigation Level effects
At Coachella in 2003, analysis of variance indicated significant differences for Genotypes, but not for Genotypes x Irrigation Level.
Effect of Temik insecticide application
Unexpectedly, the application of Temik (applied at layby at 14 lbs/ac or double the label rate) to control cowpea aphid significantly reduced biomass compared to the untreated treatment.
Genotype x Location effects
A strong Genotype x Location effect was observed. In both 2002 and 2003, 01FCV-26 and 01FCV-110 produced significantly more biomass than Iron/Clay at Coachella, but Iron/Clay produced significantly more biomass than 01FCV-26, 01FCV-85, and 01FCV-110 at Kearney in 2003 (Table 2). (The 2002 trial at Kearney was infested by aphids and provided a good assessment of aphid resistance but biomass yields were not obtained in that year.). At Shafter, genotypes were not significantly different in terms of biomass production (under full irrigation) in either year.
Ranking the genotypes:
Two years of evaluation of biomass production at three sites indicates that genotypes 01FCV-26 and 01FCV-110 produce significantly more biomass than check cultivar Iron/Clay at Coachella in the low elevation desert but not in the Central Valley of California. These two genotypes also produced more biomass than 01FCV-85.
Seed yields of 01FCV-26 and 01FCV-110 over three seasons were higher than 01FCV-85 and similar to Iron/Clay, but the relative rankings varied considerably year to year (Table 3). Iron/Clay, but not the other genotypes being evaluated, exhibits seed shattering when the pods are dry and atmospheric humidity is very low. It is thought that the low seed yields of Iron/Clay in 2001 and 2002 could have been due in part to seed shattering that occurred in those years but not in 2003.
Table 1. Resistance and performance characteristics of commercial cowpea cover crop varieties and three new cowpea breeding lines developed at the University of California, Riverside.
M. incognita Cowpea Seed
Genotype avirul. virulent M. javanica Aphid Shattering
Chinese Red Sus S us Sus Sus Res
Iron/Clay Res S us Sus Sus Sus
01FCV-26 Res Res Res Res Res
01FCV-85 Res Res Res Res Res
01FCV-110 Res Res Res Res Res
*avirul. = avirulent, effectively controlled by gene Rk; virulent = not effectively controlled by gene Rk alone. Selections within genotypes 01FCV-26 and 01FCV-110 found to be resistant to M. incognita and M. javanica
Table 2. Biomass of selected cowpea cover crop cultivars at Coachella Valley Agricultural Research Station (CVARS) and Shafter and Kearney Research and Extension Centers (REC) in 2002 and 2003. Values for experiments conducted at CVARS and Shafter REC in 2003 are from fully irrigated treatment.
Coachella Shafter Kearney
Entry 2002 2003 Mean 2002 2003 Mean 2003*
01FCV-26 9157 7265 8211 5525 7653 6589 6993
01FCV-110 8616 7264 7940 5365 7595 6480 6627
01FCV-85 7153 6700 6927 5459 7367 6413 6972
Iron/Clay 6843 5562 6203 5515 7186 6350 7623
LSD(0.05) 1015 262 NS NS 605
Table 3. Seed yields during fall season at Coachella in 2001-2003.
Seed yield ————————————
Entry 2001 2002 2003 Mean Seed size
———–kg/ha—————— g/100 seeds
01FCV-26 1912 2087 1774 1924 13.6
01FCV-110 2334 1872 1511 1906 11.7
01FCV-85 1817 1620 1264 1567 11.8
Iron/Clay 1776 1785 2035 1865 10.8
Mean 1960 1841 1646 1816 12.0
LSD(0.05) 498 497 291 0.8
Competitive ability of erect, semi-erect, and prostrate cowpea genotypes with sunflower and purslane.
In mixed-species plant populations, height is a very important determinant of competition. Crop height correlates strongly with the competitive ability of many crop species (Berkowitz, 1988; Nangju, 1978). Since sunlight attenuates exponentially within a plant canopy with cumulative LAI, the upper leaves of taller plants intercept more direct sunlight, and a taller stature can compensate for lower LAI of a species in terms of competitive ability (Kropff and van Laar, 1993). We found that the tall, erect genotype IC has a greater ability to compete with sunflower than semi-erect and prostrate growing 288 and 779. IC biomass was less affected by sunflower, received more light when growing with sunflower, and caused a greater decline in sunflower biomass and leaf area than 288 or 779. Taller IC gained a competitive advantage by capturing more light than semi-erect 288 or prostrate 779.
779 and IC have a greater ability to compete with purslane than 288. The consistently low leaf area throughout both growing seasons reduced the ability of 288 to absorb light before light could reach low-growing purslane. 779 and IC have similar LAI and light interception ability, and thus a similar competitive effect on purslane. However, the taller genotype, IC, is less affected by purslane. When competing with purslane, the erect cowpea genotype with higher LAI shades purslane and has less reduction in biomass than more prostrate genotypes. LAI and height are important factors when a crop competes with weeds (Nangju, 1978; Callaway, 1992). Our results with cowpea also suggest that LAI and height appear to be key cowpea varietal characteristics for competitive ability with purslane and probably with other low-growing weeds.
The genotypes with larger canopy widths could have an advantage over genotypes with narrower canopies when competing with weeds growing between rows. In our experiments, purslane and sunflower were planted in the cowpea seed lines. Any weeds growing in the furrow and on the shoulders of the beds were removed by hoeing. The prostrate genotype 779 could not take advantage of its wider canopy in terms of weed suppression. However, early-season weeds not in crop lines could be easily controlled by mechanical cultivation in grower fields. Rapid development of canopy width may be an important trait only in systems where it is not possible or economical to use mechanical cultivation.
Impacts and Contributions/Outcomes
Breeders have selected for resistance to nematodes, insects, and diseases for most crops. Breeding for resistance to weeds is a new idea that has yet to be applied. Our research toward selecting weed-resistant cover crops furthers a new idea that should reduce dependence on pesticides, as well as develop improved varieties. Our previous work to introduce broad-based nematode and other pest resistance has led to new genotypes that will reduce pest populations during the cover crop season and in subsequent cash crops.
We have identified two genotypes that appear to be pest resistant and yield well. Another year of field evaluations for biomass production in the low-elevation desert and in the Central Valley should enable us to make a final determination as to whether 01FCV-26 and/or 01FCV-110 should be recommended for release through the California Crop Improvement Association.
Associate Research Specialist
University of California
Department of Botany and Plant Sciences
Riverside, CA 92521-0124
Office Phone: 9097874332
Professor and Nematologist
University of California
Department of Nematology
Riverside, CA 92521
Office Phone: 9097877291