Development and Dissemination of a Cowpea Cultivar for Cover Crops
Cowpea cover crops are cost effective because they enrich the soil with carbon and over 100 lbs per acre of nitrogen, and can reduce pest populations. Adoption has been slowed by a lack of varieties specifically adapted to production systems in the western USA. Cowpea varieties can be selected that grow prostract, semi-erect or erects and these characteritics have important implications with respect to weed competition and agronomic suitabiltiy. A new statistical method for determining whether one group of cultivars is distinct from another was developed and used to show that the erect growth habit trait consistently conferred greater weed resistance across cultivars. In the last year we were able to identify two additional promising candidates for varietal release. The existing and new candidate cultivars produce high biomass and seed yields and have resistance to root knot nematodes, and seed shattering. The ability to withstand drought was similar for all genotypes. The candidate cultivars are currently being tested on farms in specific cropping situations to evaluate their suitability for varietal release. Information is being disseminated through talks, publications, and organic production training sessions and a manual.
Identify cowpea cover crop cultivars that resist nematodes, cowpea aphid, Fusarium wilt and shattering when grown in the Western USA.
Disseminate seed of improved varieties and related information through the California Foundation Seed Service and commercial seed companies.
Demonstrate and optimize the merits of cover crops in specific cropping systems.
Disseminate information about cover crops and their advantages, and about seed production of cowpea as a new crop for limited resource and other growers.
Objective 1: Identify new cover crop cultivars. As reported last year, new cover crop cultivars have been developed that incorporate nematode resistance and other desireable agronomic traits.
Objective 2: Disseminate seed of improved varieties and related information: As reported last year, we have begun to increase seed toward widespread dissemination. We have begun to get novel cowpea genotypes to growers, and have been working with them to conduct meaningful on-farm tests in desert and inland valleys.
Objective 3: Demonstrate and optimize the merits of cover crops: On-farm experiments and grower contacts have identified biomass production and pest resistance as highly desirable traits for new cover crop varieties. A major focus of the breeding program has been selecting for genotypes with high biomass production and resistance to nematodes, aphids, and diseases. The three novel genotypes that we have recently developed, CC-85-2, CC-27, and CC-36, were selected based on their biomass production and enhanced nematode resistance.
Weed control is the greatest expense in producing many crops. Minimizing weed control costs is even more essential for cover crops than cash crops as there is not a direct financial reward for growing cover crops. If the cost of controlling the weeds exceeds the potential nitrogen and other benefits, then the cover crop is usually plowed under prematurely. However, breeding for resistance to weeds is a concept that has often been suggested, but seldom attempted. A major reason why no food crop or green manure had been specifically bred with traits that resist weeds is a lack of knowledge. Breeders do not know which genetic traits would confer weed resistance, or how to detect traits that could potentially help crops out-compete weeds.
In the many field trials of genotypes conducted prior to the start of W SARE funding, we observed how cowpea growth habits affect the ability of cowpea to shade the soil. Cowpea growth ranges from erect to prostrate. Initially we hypothesized that the rapidly spreading vines of the prostrate genotypes would have the greatest ability to shade out weeds. However, a preliminary experiment found that an erect genotype, Iron Clay, had the greater ability to compete with sunflower than semi-erect 288 or prostrate growing 779 (Wang et al. 2004). 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.
We used replacement series to compute the aggressivity index (AI) of six cowpea genotypes to confirm the most competitive growth habit. We also investigated the relationship between AI and growth parameters to identify the growth parameters associated with competitive ability.
Twelve replacement series experiments were conducted with sunflower or purslane competing with one of six cowpea genotypes of similar vegetative vigor and maturity but different growth habits. These were selected because earlier evaluations (Ehlers, unpublished data) showed them to be promising cover-crop cowpea genotypes with similar maturity and vegetative vigor. Common purslane, a short statured weed, and common sunflower, a tall species, were selected to represent cowpea competitors with different growth types. Five proportions of two species (cowpea with sunflower or cowpea with purslane) were used: 100:0, 75:25, 50:50, 25:75, 0:100
The experiment was a randomized complete block design with four replications. Cowpea and weed seeds were planted on July 10, 2003 and Aug 16, 2004, then thinned to the desired density and proportion five days after germination. All plants were harvested when cowpea had reached their maximal pre-flowering size at 690 degree-days after planting (single sine method (Zalom et al. 1983) using an 8.5 C base temperature (Hall, 2001)), i.e. on Aug 18, 2003 or October 1, 2004.
Shoot biomass was removed at the soil level and plants were separated by species. Dry weights of each species were obtained by leaving plants at 70 C with ventilation until a constant weight was reached. To compare the competitive ability of cowpea genotypes against sunflower or purslane, relative yield total (RYT) and aggressivity index (AI) were calculated. Data on plant dry weight RYT, and AI were subjected to analysis of variance (ANOVA) and treatment means separated using Duncan’s test at the 0.05 probability level. An ANOVA of treatment and year showed no significant treatment and year interaction, so plant growth data for the two years were combined. Because the ANOVA showed that weed species affected cowpea growth and cowpea genotypes affected weed growth, separate comparisons were derived for each weed species and cowpea genotype. To test if the same order exists in this glasshouse experiments with more cowpea genotypes, we used the isotonic regression method presented by Robertson (1988).
When grown with sunflower, neither cowpea genotype or growth type affected the RYTs. When grown with purslane, cowpea genotype did affect the RYTs, but the differences disappeared when averaged over cowpea growth types. The overall averages for six cowpea genotypes with sunflower or purslane were very close to 1 (0.98 for cowpea and sunflower; 0.97 for cowpea and purslane), indicating that cowpea used the same resources as sunflower or purslane.
When grown with either sunflower or purslane, there were significant growth type, genotype, and proportion effects (all p<0.001) on AI, but the interaction of growth types and proportion or genotypes and proportion was not significant (all p>0.19). When grown with sunflower, erect genotypes and semi-erect genotypes had higher AI than prostrate genotypes, indicating that erect and semi-erect cowpea genotypes were more competitive with sunflower than prostrate genotypes. When grown with purslane, erect and prostrate genotypes had higher AI than semi-erect genotypes, indicating that erect and prostrate cowpea genotypes were more competitive with purslane than semi-erect genotypes.
The relative yields of cowpea were averaged by growth type, then graphed with those of competing sunflower or purslane. When competing with sunflower, the relative yield of prostrate cowpea genotypes decreased faster than that of erect or semi-erect genotypes, and the relative yield of sunflower increased faster when competing with prostrate cowpea genotypes. This indicates that the erect and semi-erect cowpea genotypes were more competitive to sunflower than prostrate cowpea genotypes. When competing with purslane, the relative yield of semi-erect cowpea genotypes decreased faster than erect and prostrate genotypes. The relative yield of purslane increased faster when competing with semi-erect cowpea genotypes. Erect and prostrate cowpea genotypes were more competitive to purslane than semi-erect genotypes. The relative yield results are consistent with the statistical ranking of AI cowpea growth types when competing with sunflower or purslane. It appears that the competitive advantage purslane gained by emerging one day earlier than either cowpea or sunflower (five days after planting) was insufficient to overcome purslane’s relatively short stature. Cowpea’s ability to compete with sunflower may have been related to an ability to tolerate shade.
The isotonic regression confirmed the results of Wang et al. (2004) that the order of competitive ability for cowpea genotypes erect> semi-erect > prostrate when grown with sunflower, and erect > prostrate > semi-erect when grown with purslane (full discussion of statistical analyses available upon request).
Results of the replacement series were related with data from growth analysis by multiple correlation and stepwise regression. Plant growth was characterized by directly measuring parameters (seed weight, plant weight, height, and leaf area) or deriving parameters (relative growth rate (RGR), unit leaf rate (ULR), leaf area ratio (LAR), specific leaf area (SLA), leaf weight ratio (LWR), and plant height growth rate (HGR)) using functional methods (Chiariello et al, 1991). Degree-days were calculated using the single sine method (Zalom et al. 1983), with base temperatures of 8.5 C for cowpea (Hall 2001), 7 C for sunflower (Robinson 1971), and 10 C for purslane (Zimmerman 1977). The overall AI was the dependent variables, and the independent variables included mean values of RGR, ULR, LAR, SLA, LWR, HGR, plant dry weight, plant height, and initial seed weight. Correlations matrices of all parameters were calculated, and stepwise regressions were performed.
Correlation and regression were performed to determine which growth parameters best predicted the aggressiveness of cowpea genotypes and weeds. When grown with sunflower, the parameter least correlated with AI was RGR. This is consistent with the study by Roush and Radosevich (1985). Plant height had the strongest relationship with AI. Stepwise regression showed that SLA, plant height, and seed weight best explained the variation of AI, indicating larger initial size, higher position and larger leaf area per unit leaf weight to capture more light were the more important determinants of competitive outcome. The equation was AI = 0.50 – 62.48 * SLA + 0.01 * Height – 0.22 * Seed_weight. The R2 for the equation was 0.996.
When grown with purslane, the least correlated parameter was plant height, and the most correlated was ULR. The equation selected by the stepwise procedure included ULR, SLA, and biomass, indicating the efficiency to produce new growth, larger leaf area per unit leaf weight, and plant size were the most important plant growth parameters in determining competitive ability. The equation was 1.37 – 1.30 * ULR –78.49 * SLA + 0.063 * Dry weight. These three variables explained 97.0% variation of AI.
Intercom Model Simulations
INTERCOM was used to simulate how increasing sunflower density decreased cowpea growth in 2003 and 2004. The hyperbolic yield loss equation was fitted to the simulated data and compared with the observed biomass loss data. INTERCOM accurately predicted cowpea yield loss for genotype 288, but slightly under-predicted biomass loss at low weed densities and slightly over-predicted biomass loss at high weed density for genotype 779 and IC.
The INTERCOM model was used to compare the effect of growth habit on crop competitive ability with weeds. Simulation results suggest that the erect genotype has larger biomass than semi-erect and prostrate genotypes when cowpea is grown alone. When grown with sunflower, the erect genotype has a greater competitive advantage than semi-erect and prostrate genotypes. The erect genotype produces more cowpea biomass and suppress sunflower to less biomass than semi-erect and prostrate genotypes. The semi-erect genotype is more competitive than the prostrate genotype, but the difference is small. As sunflower density increases from 1 plant/m2 to 4 plant/m2, the differences of erect growth habit and the other two growth habits are smaller. This suggests that weed density also affects the response of cowpea and weed biomass to cowpea growth habit.
Plant breeders would be most interested in what growth characteristics to enhance to make cowpea more competitive. Replacing only height growth or leaf area distribution in the above constructed theoretical cowpea genotypes showed that changing height growth or leaf area distribution from semi-erect to erect increased cowpea biomass and decreased sunflower biomass, while changing height and/or leaf area distribution from semi-erect to prostrate had opposite effects. Cowpea leaf area distribution had similar effect on cowpea biomass production with cowpea height growth when grown with sunflower. However, cowpea leaf area distribution had much smaller effects on sunflower biomass production comparing to cowpea height growth.
Impacts and Contributions/Outcomes
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. We are proposing CC-85-2 as a new cover crop variety. We will need another year of on-farm and other tests to verify their performance in different cropping systems, but CC-85-2 are a substantial improvement over previous releases in terms of pest resistance and yield. In addition, we will be testing two other advanced selections, CC-27 and CC-36, that appear to have higher biomass production than even CC-85-2, based on limited testing in 2005. Both of the new selections also have resistance to root-knot nematodes and Fusarium wilt.
Our research on the competitiveness of cowpea with weeds indicated that erect types were better competitors with weeds than cowpeas with semi-erect or prostrate growth habit. As a result of these findings we have selected aggressive erect growing genotypes that should be even better competitors with weeds.
Crop varietal differences in competitive ability with weeds demonstrate potential for breeding highly competitive varieties that resist yield losses from weed competition and suppress weed biomass and seed production. Competitive cultivars can reduce crop yield loss and herbicide use. However, breeding competitive varieties requires an understanding of crop-weed competition and ranking of competitive ability for a given trait. The INTERCOM Model simulations provide a valuable approach to guide crop breeding decisions.
Competitiveness is a key component of cover crop varietal value. From our work, it appears that development of cowpea cover crops with erect stature would be recommended where weed competitiveness is important. The erect genotype has taller stature and the relative height at which maximum leaf area density occurs is higher; the net result is a canopy that intercepts more light when competing with weeds. Varieties with erect growth habits have other practical advantages, including late season cultivation without disrupting the crop canopy. Other leguminous cover crop species also have erect to prostrate growth habit, and it would be interesting to see if erect types of these species are also generally more competitive than prostrate varieties.
The effect of cowpea growth habit on the biomass of cowpea and sunflower decreases as sunflower density increases. This suggests that a competitive cowpea cover crop may out-compete a tall competitor at low density, but may require supplemental control measures when weed density is high.
Acreage devoted to cowpea cover crops continues to grow. The increased interest in reducing the environmental impact of agriculture (TMDL’s, dust abatement) is spurring increased use of cover crops for both crop and environmental management tools.
Cowpea cover crop use is growing and currently exceeds several thousand acres. The improved genotypes should increase the acreage of use, and encourage new that include using cover crops instead of pesticides.
The increased demand for cover crops must be met with increased seed production. Cowpea cover crop seed is now being produced in the Southeast. Shipping charges to western areas adds significant cost to cowpea cover crop seed. The new genotypes created by this project will allow the production of cowpea cover crop seed in the low-elevation desert. This could create new economic opportunities for growers and seedsmen of these largely depressed farming communities.
We have completed cost studies that compare the economic return of vegetable production systems with or without summer cover crops (Ogbuchiekwe et al. 2004). Yield and net return were greatest when cantaloupe and lettuce were planted after the incorporation of a cowpea cover crop. Profits depended upon whether lettuce and cantaloupe were grown organically, and the price paid growers for their crops. The new pest resistant cover crop varieties developed by our current research should increase profitability by decreasing reliance on synthetic pesticides and fertilizers.
We have developed a course and a training manual on Organic Vegetable Production that will disseminate results from this project (In Press). Our first training session in Salinas had over 100 attendees. We continue to give grower talks and notify grower groups of our progress.
Ngouajio, M. and M.E. McGiffen, Jr. 2004. Sustainable vegetable production: effects of cropping systems on weed and insect population dynamics. Acta Hort. 638:77-83
Ogbuchiekwe, E.J., M.E. McGiffen, Jr., and M. Ngouajio. 2004. Economic return in production of cantaloupe and lettuce is affected by cropping system and management practice. HortScience 39:1321-1325.
Wang, G., J.D. Ehlers, E.J. Ogbuchiekwe, E.J., S. Yang, and M.E. McGiffen, Jr. 2004. Competitiveness of erect, semierect, and prostrate cowpea genotypes with sunflower (Helianthus annus) and purslane (Portulaca oleracea). Weed Science 52:815-820.
Ogbuchiekwe, E.J., M. Ngouajio , and M.E. McGiffen,. 2005. Economic return in production of cantaloupe and lettuce is affected by cropping system and value of hand weeding. HortScience 40:1007.
Wang, G., J.D. Ehlers, P.A. Roberts, E.J. Ogbuchiekwe, and M.E. McGiffen, Jr. 2005. Weed resistant cowpeas: experiments and methods. HortScience 40:1024.
Wang, G., M.E. McGiffen, Jr., and J.E. Ehlers. 2007. Competition and growth of six cowpea (Vigna unguiculata) genotypes, sunflower (Helianthus annuus), and purslane (Portulaca oleracea). Weed Science In Press.
Wang, G., M.E. McGiffen, Jr., J.E. Ehlers, and E.C.S. Marchi, . 2006. Competitive ability of cowpea genotypes with different growth habits. Weed Science 54:174-181.
McGiffen, Jr., M.E. Editor. 2006. Organic Vegetable Production Manual. University of California, Agriculture and Natural Resources. In Press.
Ngouajio, M., M.E. McGiffen, Jr., and C.M. Hutchinson. 2003. Effect of cover crop and management system on weed populations in lettuce. Crop Protection 22(1):57-64
Ngouajio, M., and M.E. McGiffen, Jr. 2002. Going organic changes weed population dynamics. HortTechnology 12:95-99.
Wang, G., J. Ehlers, E. Ogbuchiekwe, and M.E. McGiffen, Jr. 2003. Competition Between Cowpea Cover Crop Varieties and Weeds. 2003. Proceedings of the California Weed Science Society 55: 150-151.
McGiffen, Jr., M.E., M. Ngouajio, D. Crowley, J. Borneman, C.M. Hutchinson. 2002. Soil organic amendments change low organic matter agroecosystems. International Horticultural Congress and Exhibition 26:289.
Ngouajio, M. and M.E. McGiffen, Jr. 2002. Sustainable vegetable production: Effects of cropping systems on weed and insect population dynamics. International Horticultural Congress and Exhibition 26:279-280.
Ogbuchiekwe, E.J., M. Ngouajio, and McGiffen, M.E. 2003. Economic return for lettuce and cantaloupe is affected by cropping system and management practice. Weed Science Society of America Abstracts 43:9.
Wang, G., Ehlers, J., Ogbuchiekwe, E.J., and McGiffen, M.E. 2003. Economic return for lettuce and cantaloupe is affected by cropping system and management practice. Weed Science Society of America Abstracts 43:9.
Tedeschini, J., B. Stamo, H. Pace, B. Huqi, M.E. McGiffen, Jr., and L. Ferguson. 2003. Organic methods of vegetation management. 4th National Integrated Pest Management Symposium p. 78.
Wang, G., J. Ehlers, E. Ogbuchiekwe, and M.E. McGiffen, Jr. 2003. Cowpea varietal resistance to weeds. 4th National Integrated Pest Management Symposium p. 85.
Wang, G., J. Ehlers, E.J. Ogbuchiekwe, and M.E. McGiffen, Jr. 2003. Competition between cowpea cover crop varieties and weeds. Weed Science Society of America Abstracts 43:8.
Melon weed control options. Melon Research Board, San Diego, CA, January 6, 2006.
Crop ecology and cover crops. Desert Valleys California Association of Pest Control Advisors Continuing Education Seminar, La Quinta, CA, September 8, 2005..
Compost uses and the organic effect. Compost Solutions Workshop, UCR Extension, sponsored by ACP-UCCE-CIWMB, September 12.
Organic vegetable handbook, organic working group, and related extension activities, Vegetable Crops Continuing Conference, Davis, CA, November 30, 2005.
Desert organic vegetable production, Desert Vegetable Crops Conference, Holtville, CA, December 7, 2005.
Organic Vegetable Production, presentation and field tour. Malaysian Organic Production Delegation, Riverside, CA, March 21, 2005.
Pests and organic production. Specialty and Organic Crops Seminar, Coachella, CA, April 20, 2005.
Cover crops and crop ecology. Pesticide Applicator Professional Association Continuing Education Seminar, Santa Maria, CA, June 15, 2005.
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