Cover crops are being increasingly implemented in United States cash cropping systems, and there are many unanswered questions regarding how cover crops may affect plant pathogens. The soybean cyst nematode (SCN) is the single most-important plant pathogen in soybean production. There are a handful of unsubstantiated claims that cover crops reduce SCN population densities. The goal of this research is to investigate these claims using rigorous scientific methods. Both small-plot and on-farm field experiments will be conducted to assess effects of cover crops on SCN population densities. On-farm experiments will be conducted in collaboration with the Iowa Soybean Association (ISA) On-Farm Network, five or more Iowa farmers, and one farmer from Illinois. SCN population densities will be monitored in field experiments by collecting soil samples at the time of cover crop seeding, before a killing freeze, and in the spring pre-cash crop planting. Field experiments will be conducted over three years and include corn-soybean rotations under typical agronomic management to best capture the effects of cover crops on SCN population densities in a farmer-managed setting. Greenhouse experiments will complement the field experiments and allow for a closer look at the impact of cover crops on SCN. Experiments will investigate if cover crops decrease SCN survival, and simulated field container experiments will permit testing effects of numerous species of cover crops on SCN population densities. By collaborating with the ISA On-Farm Network, this research will impact many farmers directly. Results will be presented at the ISA Research Conference, Iowa State University Integrated Crop Management Conference, and at professional scientific meetings, reaching farmers, consultants, and scientists alike. Additionally, results from these experiments will be published in extension newsletters and peer-reviewed scientific journals. This project will be evaluated biannually for progress, outputs, and outreach. The projected outcome of this proposal will be to know definitively whether cover crops can be used in an integrated pest management approach for suppressing SCN population densities in the soil. The answer to this question is much needed by soybean farmers throughout the Midwest.
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.
Initial soil samples were collected from all potential strip trial fields by the Iowa Soybean Association On-Farm Network® in the fall of 2016. A subset of soil samples from each of these fields were processed to estimate the SCN population densities within each field and to aid in selection process. After processing, two Iowa fields (northeastern and central Iowa) and one Illinois field were selected for inclusion in the cover crop SCN strip trial experiments.
On-farm collaborators drill cover crops in the fall following corn or soybean harvest as replicated strip trials. Fields have three strips with 8 to 20 rows per strip. Strips can be 305-460 m long, depending on the field. The farmer collaborators receive their cover crop mix (either a cereal rye and radish mix or a cereal rye, radish, crimson clover mix) in the fall from the Iowa Soybean Association and will drill the cover crop seed following the harvest of their cash crop. In addition to the initial soil samples, I collected soil samples from each field in the spring of 2017 and fall of 2017 to determine SCN population densities. There are two paired GPS-identified locations within each strip to collect the soil samples from the same area within a field.
Small-plot experiments were established at two Iowa State University (ISU) research farms: near Kanawha (north central Iowa) and Muscatine (southeast Iowa). There are two three-year experiments at each location: one initially planted with soybean, rotating to corn and then back to soy, and one initially planted with corn, rotating to soybean and then corn. Over the three years, the plots in these experiments will remain in the same locations to monitor effects of cover crops on SCN population densities over time, following a typical corn-soybean rotation. The cover crop species included in these studies were two annual ryegrass cultivars, two cereal rye cultivars, two radish cultivars (one oilseed and one daikon-type), two mustard cultivars, a bare soil control, and the same cover crop mix used in the strip trials.
Cover crops are hand-seeded using a fertilizer spreader at appropriate rates into standing corn/soybeans planted in 5.2 x 3 m plots, to simulate aerial seeding in the fall. ISU’s research farm in Muscatine has irrigation capabilities, which allows for watering after seeding, ensuring sufficient cover crop establishment.
These small-plot experiments have six replications of each treatment. Soil will be sampled for SCN at three times each year: in the fall on the same day as cover crop seeding, after crop harvest but before the soil freezes, and in the spring before planting of corn and soybean but after cover crop termination.
Soil samples consist of 10 soil cores collected from each of the center two rows in each plot. These soil samples are dried and crushed before processing to extract SCN cysts from a 100-cc subsample of homogenized soil from each plot. Cysts are extracted using an automated wet-sieving extraction machine. Eggs are extracted from cysts for quantification by crushing the cysts and catching eggs on a 25-µm-pore sieve.
As of February of 2019, two complete years of soil samples have been processed. Percent change factor (PCF) values are calculated to determine the change in SCN population densities between two successive soil sampling times, resulting in two PCF values for each year: cover crop seeding/ pre-winter and pre-winter/ spring. The PCF values are analyzed for each field using analysis of variance (ANOVA) followed by a multiple means comparison test.
Short-term effects of cover crops on SCN population densities
The first run of this experiment was conducted as a 2 x 14 factorial with two lengths of growth and 14 different cover crop treatments. The two growth period lengths were 30 days and 60 days. This was achieved by staggering the plantings 30 days apart.
The soil used in this experiment was a mix of naturally SCN-infested soil, construction sand, and field soil with the target starting population density of 5,000 SCN eggs/100cc of soil. An initial sample of at least 100cc of soil was collected while filling each experimental unit (600cc) and processed to determine the initial population density (Pi) of that experimental unit. Seeds were sown into the soil except for the fallow control. The other control treatments were susceptible soybean and a non-cover crop, non-host tomato. The plants were grown in a controlled environment for their corresponding 30 and 60 days, after which another soil sample was collected from each experimental unit to determine the final population density (Pf).
After a soil sample was collected at the end of the experiment from each experimental unit, 125cc of soil remaining in each experimental unit was transferred to a 125cc capacity cone-tainer.
Seeds of the susceptible soybean cultivar Williams 82 were sown in these cone-tainers and grown for 30 days to determine ability of SCN to reproduce on soybean grown in soil following the cover crop treatments and controls.
At the end of this experiment, soybean roots were removed from the soil and SCN females were removed from the roots using a high-pressure water spray and counted. Additionally, the eggs were extracted and counted as described above to determine the number of eggs per female, or fecundity.
The results from the first run of this experiment showed that there was no significant difference in the reduction of SCN population densities by cover crops grown for 30 or 60 days, so the second run of this experiment was conducted only for 60 days.
Long-term effects of cover crops on SCN population densities
To assess the effects of cover crops on SCN population densities in a more “natural” scenario with the use of a larger volume of soil and using similar environmental conditions to what cover crops would experience in the fall and winter in Iowa, this experiment used a similar soil mix as described above to fill 4,725cc capacity square pots (15 cm x 15 cm x 20 cm). An initial soil sample from each experimental unit of at least 100cc was used to determine the Pi of the soil in each pot. In the fall, the cover crops were sown into the soil and placed in a growth chamber with a 12-hour photoperiod, 27ºC day 18ºC night temperature regime to simulate the environmental conditions at the time of cover crop seeding. Pots were watered every two to three days as needed but care was taken not to saturate the soil. The plants grew for eight weeks, after which the pots were moved outside to endure the same winter conditions that cover crops would encounter in agriculture fields. After four weeks of sitting outside, the pots were moved into the greenhouse for final processing, including collecting a 100cc sample of soil to determine Pf for each pot. An additional 125cc of soil was transferred to a 125cc-capacity cone-tainer and the susceptible soybean cultivar was sown into each pot and grown for a 30-day bioassay as described above. SCN females were collected from soybean roots and counted as previously described.
The first run of this experiment was conducted from December 2016 through March 2017. A second run was planted in November 2017 and completed in March 2018. The data from the second run showed that there had been some processing error while counting the initial SCN population density. This rendered the data from the second run unusable. A third run of this experiment was planted in November of 2018 and will be completed in April 2019.
Short-term effects of cover crops on SCN population densities
The analysis of the Pf/Pi ratio can provide an insight into the potential for cover crops to directly decrease SCN population densities in the soil. There was no significant direct effect of cover crops on PCF ratio when soybean was removed from the analysis. While we included soybean in the experiment as a positive control, we did remove it from the data prior to analyses.
However, the results from the soybean bioassay following the eight weeks of cover crop growth yielded some significant results. After analyzing the number of SCN females that developed on soybean roots grown in soil after 60 days of cover crop growth, the main effect of species was significant. However, there was no significant decrease in the number of SCN females per root gram compared to the no-plant control or the non-cover crop non-host control, tomato (Fig. 1).
To assess the reproduction efficiency of SCN populations in the soil following cover crop growth, the ratio of the number of SCN females that formed on soybean roots per 100 eggs from Pf was calculated and analyzed. In such analysis, a lower number might indicate that the SCN population was adversely affected by some residual effects of cover crops. In my experiments, the reproductive efficiencies following all cover crop species were not significantly different from the no plant control (Fig. 2). The lowest reproductive efficiency was soybean, which can be attributed to the large number of SCN eggs that were present at the end of 60 days of growth in the soil (Fig. 2). The non-cover crop, non-host tomato had the next smallest reproductive efficiency and was significantly less than the value for radish, mustard, and cover crop mixes (Fig. 2).
Long-term effects of cover crops on SCN population densities
After one run of this experiment, the main effect of treatment was significant (P < 0.0001) data not shown. However, PCF value was not significantly different between any of the cover crop treatments compared to the non-cover crop control or the fallow control. The soybean control was significantly higher than all other treatments. When removed from the analysis, the main effect of treatment was no longer significant. The analysis of variance for the soybean bioassay response variable of number of females per gram of soybean root was not significant after the soybean control was removed.
These preliminary results suggest that there may not be any added benefit of using cover crops to help reduce SCN population densities compared to the use of a non-host crop. The complementary field studies with multiple years of data collection will provide a more comprehensive view of whether there is any added benefit in cover crops decreasing SCN population densities beyond that of a non-host control rotation. Regardless, we can still confidently say that cover crops have many benefits agronomically.
Small plot experiments. After two years of data collection, there have been no observed significant differences among treatments for change in population density between any two sampling periods for either crop rotation at Kanawha, IA (Table 1) or Fruitland, IA (Table 2).
Educational & Outreach Activities
Field day presentation
This was a field day presentation for the Muscatine Island Research and Demonstration Farm in Fruitland, Iowa in August of 2018 entitled “Interaction between soybean cyst nematodes and cover crops.” In this presentation, I shared preliminary data from one year of small plot experiments generalized to the species level. There was a total of 10 people in attendance for this field day presentation.
Iowa State University Research Farm Reports 2018
I co-authored research farm reports for the Muscatine Island Research and Demonstration Farm (Fruitland, IA) and Northern Research and Demonstration Farm (Kanawha, IA). These reports are intended as useful information for farmers in the area that have similar soil or growing conditions.
ICM Blog Post:
This blog post was written to fulfill requirements for a student travel scholarship to the national Integrated Pest Management Symposium in March of 2018. It was an overview of how cover crops can impact SCN that was geared towards letting farmers know where we currently stand on the topic. Thus, while not a part of my grant proposal, it was directly related to the work in my grant and some of the work was briefly mentioned in the post.
I was asked to give a webinar on “Cover crops and pest management: the good and the bad” by Dr. Daren Mueller for the Agronomy Society of America. This webinar was not directly related to my grant, but I mentioned the work that I am doing for my grant and encouraged attendees to follow me on social media to see what I am up to in my research. A total of 428 people registered for the webinar. I am unaware of what number of those were farmers/ranchers versus agricultural professionals.
My project may contribute to future sustainability if I find that some cover crops can reduce or suppress SCN population densities in the soil beyond that of a non-host crop. If such an effect occurs reliably, we can recommend that specific cover crops be included as part of an integrated pest management tool for SCN, which would mean that cover crops have additional benefits in agricultural systems besides the known agronomic benefits. If my project shows that there is no impact of cover crops on SCN population densities, this will encourage the use of cover crops for the conservation benefits without the fear of increasing SCN population densities in the soil.
My knowledge has improved having read more about cover crops as a sustainable agriculture approach; however, my attitude has remained the same. I am and have always been in favor of implementing practices that can improve the sustainability of agricultural systems, especially cover crops. My advisor’s awareness and knowledge of sustainable agricultural practices continues to increase by learning about the benefits of cover crops as being involved in my project.