- Agronomic: soybeans
- Crop Production: cover crops
- Education and Training: on-farm/ranch research
- Pest Management: allelopathy, biological control, cultural control, trap crops
- Production Systems: holistic management
The soybean cyst nematode (SCN), Heterodera glycines, is consistently the top yield suppressing pathogen in soybean (Glycine max) production throughout the United States. Management of SCN has been largely dependent on the use of host resistance. Although there are at least seven sources of SCN resistance available through plant introductions (PI) in the soybean germplasm, there is one source of resistance that dominates the commercially available soybean cultivars with SCN resistance. In 2018, 95% of the commercially available soybean cultivars with SCN-resistance contain genes derived from PI88788 in Iowa (Tylka and Mullaney, 2018). As a result of the prolonged a single source of SCN resistance in commercial soybean cultivars, an increase in SCN reproduction has been observed on cultivars with resistance derived from PI88788 (McCarville et al. 2017; Howland et al. 2018).
With SCN management becoming more challenging, there is a growing interest in alternative methods through which to mitigate the pest problem. There have been a few published and unpublished studies that have piqued interest among farmers, agronomists, and seed dealers about whether cover crops have an additional benefit of reducing SCN population densities. However, the results from these studies have been inconsistent or have yielded data that were less than robust. It has been difficult to draw broad conclusions on cover crop effects on SCN population densities based on this past research. With the use of cover crops on the rise, and an interest from seed dealers in marketing their cover crop seed for nematode control, a new series of scientifically rigorous studies could help farmers know whether cover crops can be used to help with the growing SCN problem.
The focus of this project was to conduct experiments on a wide range-of-scale using cover crop cultivars that are marketed for nematode suppression or reduction with the goal of providing reliable information to farmers seeking alternative management methods for SCN. The first series of experiments was conducted in highly controlled greenhouse and growth chamber settings. This was the best way to see if, when barring any additional external factors and keeping the sampling error to a minimum, cover crops can impact SCN population densities or have any residual effects on SCN reproduction on soybean roots. The second experiment was conducted on a slightly larger scale, with larger pot sizes and accounting for the effect of winter-killed cover crops on SCN population densities and residual effects on SCN reproduction. Additionally, small-plot cover crop experiments were established in north central Iowa at the Iowa State University (ISU) Northern Research Farm (NRF) in Kanawha, Iowa and in southeastern Iowa at the ISU Muscatine Island Research farm in Fruitland, Iowa. There were two fields per location, designated by their grain crop rotation starting in 2016, including soybean-corn and corn-soybean. The cover crop treatments were applied to the same plots each year as way to observe if multiple years of the same cover crop could have additional effects on SCN population densities.
In the first greenhouse experiment, the SCN population densities decreased for every cover crop treatment as well as the non-planted soil and tomato (non-cover crop, nonhost) controls over the 60-day growing period. The variable used to estimate the change in population density (population change factor, PCF = initial population density ÷ final population density) ranged from 0.41 to 0.81. (Any number less than 1.00 indicates a decrease in SCN population density over the sampling interval). When the mean SCN population density decreases for the cover crop treatments were compared to the non-planted soil control, there were no significant differences. Although there were no significant decreases in SCN population densities as a result of cover crop growth when compared to the non-planted soil control, it is important to note that there were no cover crop treatments that increased the SCN population density. When the leftover soil from the 60-day experiment was used to grow SCN-susceptible soybean cultivar Williams 82 for 30 days, there were significant differences in the number of SCN females that developed on the soybean roots following different cover crop treatments. There were significantly fewer SCN females found on soybean plants grown following six cover crop treatments and the tomato control compared to the number of SCN females that developed on soybean roots grown following the non-panted soil control. This could be indicative of a negative residual effect of cover crops on SCN reproduction.
In the larger-scale greenhouse experiment, the decrease in SCN population density over time was not as evident. The PCF in this study varied from 0.76 to 1.05. Like the last experiment, there were no population density decreases because of cover crop growth that were significantly greater than the decrease observed in the non-planted soil control. In the subsequent soybean bioassay to observe how SCN develops on soybean roots grown in soil following cover crops, there were significantly fewer SCN females that developed on soybeans grown following six cover crop treatments and the tomato control compared to on soybeans grown following the non-planted control. However, not all six cover crop treatments that had reduced SCN development on soybean were the same as the six cover crop treatments observed in the previous study.
In the small plot experiments, there were two PCF for each year, consisting of PCF1 (population density at cover crop seeding ÷ pre-winter population density) and PCF2 (pre-winter population density ÷ spring population density). The PCF1 for both experiments, locations, and years ranged from 0.42 to 4.11 and the PCF2 ranged from 0.41 to 6.30. There were no significant differences detected in t-tests that were conducted to determine if PCF in cover crop plots were larger or smaller than the PCF from the non-planted soil plots. It is possible that the spatial variability of SCN population densities was too large despite efforts to sample from the same area (the center two grain crop rows) in each plot. The high spatial variability likely affected our sampling error and ability to detect differences or observe consistent PCF changes in these experiments.
Throughout all of our experiments, there were no significant impacts on SCN population density as a result of cover crop treatment when compared to the non-planted control. Despite this, it is worth noting that the SCN population densities did decrease in cover crop treatments overall, specifically in greenhouse experiments. We know that the cover crop treatments used in these experiments do not support SCN reproduction, and it is encouraging to see that there was no increase in SCN population density due to cover crop treatments. It is fair to say that although cover crops may not significantly reduce SCN population densities, there could be an adverse residual effect on SCN reproduction following some cover crop cultivars. This effect needs to be examined further and in a more-applied field experimental setting. Cover crops should continually be used for their known agronomic benefits; however, they should not be used for SCN management based on the results from these studies.
With increasing implementation of cover crops in the Midwest accompanied by the ever-persistent problem of the soybean cyst nematode (SCN), there is much interest in, speculation about, and even some unsubstantiated claims made about cover crops affecting SCN population densities. The goal of this project is to determine if and how cover crops can affect SCN population densities and if cover crops have potential as an IPM tool for SCN management.
Results from this research will answer the question to which many soybean farmers, crop consultants, and seed companies do not have an answer: Can cover crops reduce the population densities of the soybean cyst nematode (SCN)? There is increasing interest about cover crops among soybean farmers, including whether they can reduce SCN population densities in the soil, yet there are almost no scientific data on the topic. With this research, I intend to not only answer the baseline question of “do cover crops affect SCN populations in the soil?”, I plan to dig deeper into the potential relationship of cover crops and SCN. In addition to field experiments, greenhouse experiments will be conducted to investigate multiple possible modes of SCN suppression that can occur as a result of interactions with cover crops: potential as a biofumigant, trap crop, or hatch-inducing non-host cover crops. Results will be shared in presentations and written publications for both the scientific community and the soybean-producer and crop consultant communities. Ultimately, this research will determine if cover crops can be utilized as an integrated pest management (IPM) approach for SCN, adding to the other benefits of using cover crops in corn-soybean rotations.
The expected outputs of this research will be new research-based information on effects of cover crops on SCN population densities in the Midwest. Results will be published in various outlets including, but not limited to, extension newsletters and refereed scientific journals. The results will also be shared with farmers, extension agents, industry personnel, scientists and whomever else may attend or be interested in local, regional, or nationwide extension or scientific society meetings. NCR-SARE will be acknowledged for its support anytime results are presented orally or in print. Results from this study will help shape the way farmers perceive the utility of cover crops beyond their soil-saving and nutrient-scavenging abilities because: (1) methods for managing SCN are losing efficacy; (2) there is lack of scientific data on the effect of various cover crop species on SCN in the soil; and (3) determination of the effects of cover crops on SCN will help farmers decide if they can use specific species of cover crops as another method of SCN management.