David Chaney

Adapted from UC SAREP Progress Report 1993-1995. UC Sustainable Agriculture Research and Education Program, Davis, CA. 1996

Editor's note: This research summary is based in part on a series of articles featured in California Agriculture, Volume 48, Number 5, 1994, University of California, Division of Agriculture and Natural Resources, Oakland, CA. The project is supported primarily by the USDA's Western Sustainable Agriculture Research and Education (SARE) program and UC SAREP. Additional funds have also come from the California Department of Food and Agriculture's Fertilizer Research and Education Program and the H.J. Heinz Foundation.

This review summarizes some of the latest information coming out of a long-term research project at the University of California, Davis. The project, now entering its ninth year of operation, has been set up to describe and quantify the environmental, agronomic and economic consequences of the transition from conventional farming systems to systems that are less dependent on synthetic fertilizers and pesticides. The focus is on cropping systems typical of the southern Sacramento Valley in California. The research team is multidisciplinary, and participating farmers and UC Cooperative Extension farm advisors play a key role in guiding the management decisions applied to the various production systems. By broadening and integrating the scope of investigation, researchers have been able to critically evaluate the success of different farming practices and their effects on the environment, as well as the special requirements for adapting alternative practices to farms in other locations.

Rotations and Experimental Design

The project was initiated in 1988, and is located on 28 acres at the UC Davis Agronomy Farm. The main experiment occupies about 20 acres and compares four cropping systems: 1) a conventional two-year rotation; 2) a conventional four-year rotation; 3) a low-input four-year rotation; and 4) an organic four-year rotation. The four systems are arranged in a split-plot design with four replicates of each system. All the cropping systems include processing tomatoes, a high-value commodity grown on approximately 310,000 acres in California (1990 data). Other cash crops grown include wheat, safflower, field corn, and beans. In addition, winter-spring cover crops are grown in the low-input and organic systems. The specific rotations used in the different management systems are shown in Table 1. Each replicate of the four systems started the rotation in 1989 with a different entry point in the sequence of crops.

Observations and Results

Researchers have been collecting data on: crop growth, yield and quality; soil biology; soil fertility; soil organic matter levels; soil water infiltration rates; weeds, pest and beneficial insect populations and disease levels; and economic performance. Some of the key findings and questions for future research are summarized below.

Crop growth and yield. Soil fertility and weed management have been identified as the most important factors limiting yields in the organic and low-input systems. Project managers have altered production practices to address these constraints. Organic and low-input tomatoes, for example, are now transplanted instead of direct seeded. This practice leaves more time for cover crop growth, gives tomatoes a head start in competing against weeds and allows for the use of efficient mechanical cultivation techniques. Manure fertilizers and foliar sprays have also improved yields in the organic systems. Nonetheless, it has been difficult to obtain the high quality transplants necessary for optimal growth and yield. In the 1994 growing season, for example, researchers found that transplants in the organic and low-input systems were infected with a virus. Symptoms became apparent four to six weeks after transplanting, and yields in the organic and low-input systems were lower than in the conventional systems.

Soil biology and fertility. Nitrogen availability appears to be an important factor determining tomato yields during the transition to organic production. Prior to 1992, tomato plants in the organic system were stunted and yellow early in the season, did not compete well with weeds, and had yields lower than tomatoes grown under conventional methods. These results were despite the fact that soil nitrate levels in the organic tomatoes in 1990 and 1991 were actually higher than or equivalent to levels in the conventional system. The reason for this phenomenon is probably related to the importance of microbial activity in an organic system. Unlike the conventional system where plants obtain nutrients from highly soluble chemical sources, the organic and low-input systems rely on microorganisms to make nutrients available for plant uptake. Through the first four years of the rotation, it has been determined that the low-input and organic systems derived 85 percent of their nitrogen from the vetch cover crop that was incorporated into the soil and broken down by microbes.

To measure the importance of soil microbial activity, researchers looked closely at changes in microbial biomass carbon over the growing season. Microbial biomass carbon is an estimate of both the size of the total microbial community and the mass of potential plant nutrients contained within the cells of the microorganisms. This variable was measured in tomato plots four times between March and September in both 1990 and 1992. Levels of microbial biomass carbon fluctuated similarly in all systems over the growing season. In 1990 the only significant differences among the four farming systems occurred following the early April incorporation of the cover crop, at which time the microbial biomass carbon was higher in the organic and low-input systems. In 1992 microbial biomass carbon was higher in the organic and low-input systems throughout the growing season.

Related studies have shown some interesting differences in nematode populations among the four farming systems. Nematodes can be classified by what they feed on: bacteria, fungi, plants, or other nematodes. In the two conventional systems, the total numbers of all nematodes in the soil (to 30 cm depth) did not change significantly between 1988 and 1992. In contrast, during the same period, there were significant decreases in the total number of nematodes in the low-input and organic systems. Bacterial feeding nematodes are of particular interest because of their role in mineralizing nitrogen. Thirteen of these species have been identified in this research site. The proportion of all nematodes that are bacterial feeders declined over the four years in the two conventional systems, increased in the low-input system and declined in the organic system. The decline in the organic system is surprising given the high levels of microbial biomass measured in those plots. Researchers suggest that the late sampling date for nematodes may have been responsible for this discrepancy.

Figure 1. Whole Farm Profits per Acre

Insects, Weeds and Diseases. The shift from conventional to low-input or organic pest control did not result in large increases in relative abundance of most pest species over the period of this study. However, there were some significant short-term problems in individual farming systems. Significantly greater damage occurred in organic and low-input plots due to tomato fruitworm in 1989 and stink bugs in 1992, while insecticides prevented damage to conventional plots. Similarly, the cover crop residue appeared to increase damage by seed corn maggot to safflower and corn in two consecutive years. Verticillium increased in soils on the conventional two-year plots, probably because of the increased frequency of tomato plantings in this system. The presence of the disease organism, however, does not seem to have affected yields in those plots. Bacterial spot of processing tomato was severe in the spring of 1993, due to rain and a hailstorm. Aerial treatments were not practical for the small plots in this study, but by the time the fields had dried sufficiently for ground application, the weather was hot and dry, stopping the epidemic. Rust occurred every year on all safflower plots. However, it was not observed to cause yield loss. Corn smut was observed in all plots, but the level of its incidence remained below the treatment threshold.

Weeds were a problem in all systems, but the different control methods employed in each system favored the growth of several key species. In the low-input and organic systems, barnyardgrass has become a significant problem. In the conventional systems, field bindweed and nightshade have been more problematic. The shift to these weed species has resulted in modifications to the control strategies, including herbicide changes or increased cultivation frequency, but there has been little or no change in total weed cover. The vagaries of the weather presented an added challenge in low-input and organic systems. For example, the wet spring in 1993 prevented timely cultivation and resulted in increased weed competition in the organic corn where herbicides were not used.

Economics. Results to date suggest that similar crop yields may be obtained when "best farmer" management practices are used in each of the different systems. Similar yields, however, do not necessarily translate into similar profits. The yields and organic price premiums of tomatoes, and year-to-year variation in production costs for each system were the most important factors determining relative whole-farm profit (Figure 1). During the 1991 growing season, for example, gross returns, per acre operating costs, and net returns above total costs were all highest in the conventional two-year rotation (all crops combined). The 1992 season, by contrast, showed that the organic systems had the highest figures in each of these same categories. Reduced whole-farm profits per acre in the 1993 and 1994 seasons can be attributed to problems with tomato transplants as described earlier.

Results of this study support what other studies have found: The transition period (as evidenced by the performance of the low-input system) carries significant risk. There are no price premiums for "transitional commodities," costs of production may be high, growers are generally on the steep part of the learning curve, and the new production system can be ecologically unstable for a time.

Conclusions

From the standpoint of crop performance and yield, it appears that a rotation of processing tomatoes, safflower, field corn and wheat or winter legume, followed by double-cropped dry beans, is a good crop rotation on which to make systems comparisons. The use of nitrogen-fixing winter cover crops for green manure and as seed crops has merit, but also resulted in crop management challenges that required "best farmer" experience and flexibility to work within the constraints imposed by time and weather. The late winter, early spring management of cover crops, including residue management, seedbed preparation, supplemental manuring and the retention of sufficient soil moisture to germinate tomatoes, corn and safflower has become a central research theme for continuing studies in the large companion plots adjacent to the main experiment. The interdisciplinary group is focusing on several key issues as the project continues its second rotation cycle. These include identifying the best cover crops for each system/season combination and observing phenomena that have an impact on soil fertility and plant nutrition, particularly the season-long monitoring of cover crop nitrogen, crop growth and yield. The long-term implications of weed control, as well as the related demand for creative management and appropriate equipment, are critical.

Conclusions about the preferable crop with which to enter the rotation are still premature. The attractive premiums offered for organically grown tomatoes, and regulations that specify a minimum of three years without pesticides prior to certification, suggest that field corn would be the best starting point of the rotation, but pest control (and especially weed management) implications of this choice must be considered. Choices will also depend on the grower's economic situation and a consideration of the wide range of costs and returns for the five cash crops in the rotation. The challenges of managing winter cover and grain legume cash crops without herbicides (organic), or with short-lived, post-emergence herbicides (low-input), are at least partially offset by the opportunities to plant or replant catch crops, such as spring barley after lupine and pink beans after safflower in this study.

For more information contact the SAFS Project: Department of Agronomy and Range Science, Davis, CA 95616. (916) 752-8940.

The UC SAREP Progress Report 1993-1995 may be ordered from SAREP, University of California, Davis, CA 95616, Tel. (916) 752-7556, and is available through the SAREP homepage on the World Wide Web.

(DEC.537)

Contributed by David Chaney