Fall, 1996 (v8n4)

Technical Reviews

Evaluation of five cover crop species or mixes for nitrogen production and weed suppression in Sacramento Valley farming systems.

Diana Friedman, Luciano Gristina, Miriam Volat, Steve Temple, Carol Shennan and Don Stewart

Ingels, Chuck (Editor). University of California Cover Crop Research & Education Summaries. University of California, Sustainable Agriculture Research and Education Program, Davis, CA. 1996.

Of particular concern in low-input and organic farming systems in the Sacramento Valley is selection of cover crops to follow tomatoes, which are generally harvested between late July and late August. A 30-ton harvest of tomatoes can leave as much as 100 pounds per acre of residual biomass nitrogen, which can be mineralized and lost over the winter if there is no crop to capture it. If the subsequent cah crop is corn or safflower (planred the following spring), then the intervening crop must also have vigorous fall growth in order to have substantial production by early spring and to compete successfully with weeds. To maximize economic returns, the cover must also be able to establish with minimal land preparation and supplemental water.

The objective of this particular research was to determine the best cover crop species or mix to follow a mid-summer harvested crop such as tomatoes and precede an early spring crop such as safflower or corn. Each species or mix was evaluated based on total biomass production, competition with fall and spring weeds and contribution of total nitrogen added in the cover crop biomass at incorporation.

Procedures

In the fall of 1993, five cover crop treatments were planted: 1) fava bean/Lana vetch, 2) Lana vetch, 3) purple vetch, 4) cowpea/Lana vetch and 5) sorghum-sudangrass. In 1994, the treatments were identical, except that berseem clover was seeded into the sorghum-sudangrass in late October. All treatments were established with supplemental irrigation. However, 1993-94 was a very dry year, while 1994-95 was one of the wettest years in recent history. Differences in the seasonal rainfall had a significant impact on cover crop growth.

Results and Discussion

In both years, all treatments had good germination and stands through the fall, until late November, when the sorghum-sudangrass and cowpeas died as expected. In 1993, this allowed for large amounts of spring weeds to enter the sorghum-sudangrass plots. In 1994-95, the clover was able to emerge in the spring, although it never produced enough biomass to shade weeds, so in 1994 this treatment still had the highest quantity of weeds at incorporation (Table 1). Because the fall of 1993 was so dry, the sorghum-sudangrass only produced 1,000 pounds of biomass before winter killing. In 1994-95, however, which was considerably wetter, the hybrid produced almost 5,000 pounds of biomass by late November and captured more than 80 pounds of residual soil nitrogen. This strongly suggests that successful growth of this species is contingent upon adequate fall moisture, which can vary tremendously from year to year in this region.

The fava bean/Lana vetch mix produced the most total biomass (Table 1) in both years and provided a full cover in 1993-94 which was successful at controlling weeds. Good winter and spring growth of the fava bean provided a climbing structure for the vetch, allowing for further spring growth. This mix also contained the most nitrogen at incorporation (Table 1)in 1993-94. In 1994-95, incorporation was almost a month later due to late spring rains, and total N in this mix was considerably less than the previous year due to the reduced percentage of vetch in the mix. In 1993-94 the fava bean and vetch each consisted of about 50 percent of the mix, while the following year, the fava bean was more than 75 percent of the total biomass. A sampling from early March showed the fava bean/Lana vetch mix had almost 120 pounds of N and higher biomass than at the April sampling, indicating that there was probably some biomass reduction due to increased weed pressure, and that the mix had reached its peak N supplying capacity earlier in the spring.

In general, the Lana vetch mixes and the Lana vetch provided more effective weed control than the purple vetch in both years. In 1993-94, weeds were negligible in all Lana vetch treatments, while the weeds in the purple vetch treatment made up almost 35 percent of the total biomass (Table 1). We have observed in other experiments and other seasons that purple vetch is susceptible to early spring dieback, although the cause of this is still unknown. The death of the purple vetch clearly opened up the canopy and allowed for heavy weed growth. In 1994-95, the Lana vetch and fava/Lana vetch treatments were considerably more effective at choking out weeds than the purple vetch, although the cowpea/Lana vetch treatment was not more effective. In 1993-94, the vetch in the cowpea/Lana vetch treatment continued to grow after the cowpeas died, blocking the weeds until incorporation. In 1994, however, the vetch in this mix was unable to compete effectively with the heavy weed growth.

In 1994-95, all treatments with Lana vetch had reduced biomass production relative to 1993-94, while the purple vetch actually had slightly more. Although this did not translate to a significant difference in added nitrogen, data showed that the purple vetch biomass almost doubled between the March and April sample date. This strongly suggests that purple vetch put on the bulk of its growth later in the season and thus may be better suited as a cover crop for a later spring-planted crop.

There did not appear to be any benefit to planting cowpeas with vetch in either year, quite possibly because in both years planting date was beyond optimal for cowpeas. New lines of cowpea species bred for biomass production rather than seed, may prove to be more effective in this mix. The Lana vetch treatment was the most consistent producer over two very climatically distinct years and although it provided slightly less nitrogen than the fava/Lana vetch mix in 1993-94, total nitrogen production was sufficient to supply a subsequent safflower crop with nitrogen.

For more information contact: Steve Temple, Department of Agronomy and Range Science, University of California, Davis, CA 95616.

DEC.539

Contributed by Diana Friedman

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