Month: November 2020

Breeding Legume Cover Crops

Sandra Wayman1, Lisa Kissing Kucek2, Virginia Moore3, Lais Bastos Martins4, Matt Ryan1
1Soil and Crop Sciences Section, SIPS, Cornell University, 2USDA ARS Dairy Forage Research Center, 3currently: NC State University. Feb 2021: Plant Breeding and Genetics Section, Cornell University, 4Crop and Soil Sciences, NC State University.

Legume cover crops have room for improvement
Winter annual legume cover crops are essential management tools for organic farmers; they fix nitrogen, improve soil health, and suppress weeds. Winter annual cover crops are planted in the early fall, overwinter, then grow vigorously in the spring and complete their life cycle in the summer. However, many farmers struggle with these cover crops. Poor emergence, low vigor, and winter kill are basic challenges that could be addressed through plant breeding. Unlike cash crops, cover crops have received relatively little attention from plant breeders in the past. Thus, even modest investments in germplasm improvement could return large benefits. The Sustainable Cropping Systems Lab is taking advantage of this opportunity to improve legume cover crops for organic farmers by participating in the national Cover Crop Breeding Network (Fig. 1). Sites across the U.S. are developing cover crop lines best suited to each region. Our goal is to develop new varieties that boost the sustainability of organic farms, using classical plant breeding methods rather than genetic engineering. We are working with three species of winter annual legume cover crops: hairy vetch (Vicia villosa), crimson clover (Trifolium incarnatum), and winter pea (Pisum sativum) (Fig. 2).

US map with icons indicating locations
Figure 1. Sites participating in the legume cover crop breeding program.
photos of cover crops
Figure 2. Left, hairy vetch; top right, crimson clover; bottom right, winter pea.

The traits farmers want
To inform our breeding efforts, we conducted a national survey of organic and conventional farmers to learn which cover crop traits were important to them (Fig. 3, Wayman et al 2017). We received 417 responses to the survey, and 87% of the respondents reported they used cover crops. Organic farmers reported placing greater value on the ecosystem services from cover crops than did conventional farmers. The top four traits chosen by respondents as important for legume cover crops were nitrogen fixation, winter hardiness, early vigor and establishment, and biomass production (Fig. 3).

bar graph
Figure 3. Percentage of farmers (organic and conventional together) who rated the given traits for four focus cover crops as “important” or “very important” out of total of five rating levels (“not at all important” to “very important”). Numbers above bars indicate count of farmer respondents for each cover crop and trait. Stars on bars indicate significant differences between conventional and organic farmers for that particular trait and cover crop (chi-square test, * is P < 0.05, *** is P <0.001).

Genetic improvement
The steps in developing better cover crop varieties for farmers are 1) create better genotypes through breeding nurseries, and 2) select the best new varieties through advanced line trials. Researchers at different sites in the project are selecting for different legume traits based on their region. In the legume cover crop nurseries planted at Cornell University, we are selecting for winter-hardiness in addition to early-flowering.

We began the breeding program with seeds of hairy vetch, crimson clover, and winter pea from commercially available varieties, lines from worldwide breeding programs, landraces selected by farmers, and PI (plant introduction) lines from the U.S. National Plant Germplasm System Germplasm Resources Information Network (NPGS GRIN) seed bank.

For five seasons, we have planted breeding nurseries of the three legume cover crop species at our Cornell University site. We selected plants based on fall vigor, low winterkill, spring vigor, early maturity, and soft seed. We culled undesirable plants before flowering, and saved seeds from the best plants to replant in the following year. We selected between 2.8% and 4.6% of the hairy vetch individuals across the breeding seasons, and between < 0.01% and 2.8% of crimson clover individuals.

For winter pea, the first year of the breeding program evaluated the performance of accessions from the National Plant Germplasm System. The results informed what material to include in breeding nurseries. For the following three seasons, we planted and selected early generation breeding lines originating from the USDA-ARS Grain Legume Genetics Physiology Research Center in Pullman, WA. The best 0.5 to 1.4% of the winter pea plants were chosen as new breeding lines, based on winter survival and vigor. In 2019, the winter peas experienced severe winter conditions 900 feet above Cornell University’s campus, where almost all the winter peas died from winterkill.

Advanced line trials
In the 2018-2019 and 2019-2020 seasons, our breeding lines were tested against commercial varieties in multi-environment advanced line trials. Sites across the country (Fig. 1) grew replicated plots of breeding lines and commercial checks of each legume cover crop species. Each trial grew the legume cover crops alongside triticale to simulate grass-legume cover crop mixes typically grown by farmers. Breeding lines of each crop were compared with commercial check varieties to assess if our breeding program has produced something better than what is currently available to growers on the market. Lines were evaluated for emergence, winter survival, fall and spring vigor, flowering timing, disease, and biomass. The best lines of each species will be tested again in the 2020-2021 season, and performance of these lines will determine variety release and commercialization.

Nursery and advanced line trial performance
Testing variety performance is currently underway. An analysis of the advanced line trials will identify if any lines perform well across the U.S., or if certain lines excel in specific regions. Ideally, we would find a few breeding lines performing well across all sites. Such “broadly adapted” lines could be sold as varieties nationwide. If certain lines are excellent in specific regions, however, seed companies are interested in selling lines as “regionally adapted” varieties. In the meantime, data from the breeding nurseries indicated patterns in regional performance. The results suggest different trends among species, which are detailed below.

Hairy vetch
We found no hairy vetch line that performed best in both the fall and spring (Kissing Kucek et al. 2019). Instead a tradeoff between fall growth and spring growth was observed. As a result, the breeding program is screening and selecting for vigor at both times of the year, with the goal of finding ideal lines that have the best overall seasonal performance.

Over two seasons and a dozen U.S. sites (Fig. 1), we tested 16 hairy vetch breeding lines and six checks. Breeding lines developed by the Cover Crop Breeding Network beat the commercial check lines in both years. Winning lines, however, differed among sites. Colder northern environments had different winning breeding lines than warmer southern and western sites. Our Cornell University site proved to be an intermediate winter environment compared with the harsh upper Midwest and mild southeast and west. In cold winters like 2018-2019, Cornell University shared winning lines with MN, WI, and NE. In contrast, during warm winters like 2019-2020, NY was more similar to southern and western sites. These results suggest that the best performing lines in NY may vary depending on weather conditions, with warmer years in NY mimicking southern and Mid-Atlantic sites, and colder winters grouping NY with the northern Midwest. To select for resilient lines that can handle variable winter conditions, Cornell University breeding nurseries include material from warm and cold regions of the U.S.

The 2018 hairy vetch line from Cornell University was the second highest seed yielding in our OR trials, demonstrating 25% more seed yield than checks (Hayes and Azevedo, 2019). High seed yield is a very desirable trait for seed growers and seed companies.

Crimson clover
Two commercially available varieties of crimson clover, ‘Dixie’ and ‘Linkarus’ were included as checks in our trials. ‘Dixie’ is a variety developed in GA that exhibits high forage biomass production, ability to reseed, and high amounts of hard seed (Hollowell 1953). ‘Linkarus’ is a highly productive winter hardy crimson clover which was developed in Germany. In general, we have seen ‘Dixie’ perform well in the southern locations, while ‘Linkarus’ performs better at northern locations. In the harsh NY winter of 2018-2019, our breeding lines beat both ‘Dixie’ and ‘Linkarus’.

In two seasons, we also evaluated crimson clover breeding lines for biomass production at Cornell University. Biomass production is important for all farmers, who often use crimson clover as a green manure. The crimson clover lines with the highest biomass production were included in the next season’s nursery. At our Cornell University site in 2018, ‘Linkarus’ had the highest biomass production, with 1.5 to 2.9 times more biomass production than ‘Dixie.’ Additionally, to compare top-performing lines from nursery selections at a dozen sites across the country, we tested 13 crimson clover breeding lines and 2 checks over two seasons. In the first season, a soft-seeded MD breeding line produced the most biomass, followed by ‘Linkarus’ and a Cornell University breeding line. In the second season, ‘Dixie’ produced the most biomass, followed by a MD breeding line.

Breeding lines have also been tested for seed yield in OR, where most crimson clover seed is produced. The two checks beat all breeding lines for seed yield (Hayes and Azevedo 2019). As a result, we have increased our focus on selection for seed yield in the crimson clover breeding program.

Winter pea
In NY, winter peas have often been challenging for farmers due to poor winter survival. In the 2017-2018 season, 0.5% of plants were selected based on winter survival and vigor. Their seed is currently being increased so they can be included in future advanced line trials. In the 2019-2020 season, our Cornell University site experienced optimal weather to discriminate cold tolerance. Data from 39 new and different genotypes helped us choose the entries with the best potential to be increased for the advanced line trial.

Over two seasons, we tested 21 winter pea lines and five checks in our advanced line trial. In the 2018-2019 season, the winter pea advanced line trials did not survive at Cornell University and in MN due to harsh winter conditions. Southern locations (CA, GA, NC, OR) of the advanced line trials had overall higher biomass production than did the northern locations (MD, MO, NE, WI) in 2018-2019. Across all sites, our breeding lines performed better than the checks. Indeed, one of our breeding lines was in the top five performers across five different locations, showing good potential for release as a variety. Many of our breeding lines performed better than the two commercially available cultivars in the trial.

An additional observation for winter pea is that lines with the highest vigor in fall may have poor biomass production in the spring. This is not uncommon in peas; if the plants grow too much in the fall their exposed above-ground biomass is susceptible to frost damage and winter kill.

Next steps
As part of this project, we will release varieties of legume cover crops adapted to specific regions. Our next steps include selecting for high-vigor and improved material in our nurseries, continuing advanced line trials with this new material, planting seed increases, and inviting farmers and seed company representatives to the breeding sites to evaluate the lines. We planted a third year of advanced line trials in 2020, after which we will determine if any lines are consistently high performers and good candidates for variety release.

Cover Crop Breeding Network team member coming to Cornell
In February 2021, a Postdoctoral Scholar with the team, Virginia Moore, will join Cornell as an Assistant Professor with SIPS in the Plant Breeding and Genetics Section. Virginia’s program will focus on breeding for sustainable cropping systems. Virginia has been involved in the national Cover Crop Breeding Network as a project manager since 2019. She is currently based at USDA-ARS in Beltsville, MD, and completed her graduate work at the University of Wisconsin, with a MS in Agroecology and Agricultural & Applied Economics and a PhD in Plant Breeding & Plant Genetics. She sees plant breeding as a powerful tool to increase sustainability of cropping systems, with goals like a) reducing pesticide inputs through breeding for pest resistance, b) increasing cover crop adoption by developing regionally adapted cultivars, c) selecting crops for organic systems, and d) diversifying cropping systems through rotations and intercropping. She is excited to continue working in cover crop breeding and to take on new crops including alfalfa and other forages, hemp, and switchgrass.

Acknowledgements:
Thanks to Gerald Smith for sharing data and resources. Thanks to Chris Pelzer, Katherine Muller, Dylan Rodgers, James Cagle, Nina Sannes, and Matt Spoth for help planting the nurseries.

References
Hayes, R and M. Azevedo. 2019. Seed yield of hairy vetch and crimson clover breeding lines. Raw data available upon request.

Hollowell, E. A. 1953.  Registration of varieties and strains of crimson clover (Dixie crimson, Reg. No. 1). Agron. J. 45:318-320

Kissing Kucek, L.; H. Riday; et al. 2019. Environmental influences on the relationship between fall and spring vigor in hairy vetch (Vicia villosa Roth). Crop Science. 59:1-9

NordGen. Accession Number: NGB8658. Accessible at: https://sesto.nordgen.org/sesto/index.php?scp=ngb&thm=sesto&lst=&accnumtxt=NGB8658. Accessed April 26 2019.

Wayman, Sandra & Kissing Kucek, Lisa & B. Mirsky, Steven & Ackroyd, Victoria & Cordeau, Stéphane & Ryan, Matthew. (2017). Organic and conventional farmers differ in their perspectives on cover crop use and breeding. Renewable Agriculture and Food Systems. 32. 376-385. 10.1017/S1742170516000338.

 

Managing Corn Rootworm in NY to delay Bt resistance (& save seed costs)

Elson Shields, Entomology, Cornell University

Across the US and within NY, corn rootworm (CRW) is developing resistance to the Bt-RW traits in our GE corn varieties, causing increased root damage and decreasing yields.  Yield losses from CRW root feeding can surpass 10% without any above ground symptoms, making this type of losses difficult to detect.  In addition, corn grown for silage is more sensitive to yield losses from CRW feeding than corn grown for grain.  As CRW resistance increases to Bt-RW, the damage becomes more apparent and easier to detect, but losses have been occurring in the field in prior years, going undetected.  Increased damage has been reported in NY for all of the Bt-RW traits regardless of company.

Important points about CRW biology:  There are two important points about CRW biology which need to be remembered when managing this pest and reducing its potential for developing resistance to any of our management tools.  1)  In NY, all eggs are laid in existing corn fields during August, and 2) if the newly hatch CRW larvae in the spring do not find a corn root, they die.  Since CRW eggs are laid in existing corn fields in August of prior year, crop rotation is our best resistance management tool.  Since the majority of the corn grown in NY is in rotation with alfalfa for our dairy farms, NY trails the rest of the nation in the development of CRW resistance to Bt-RW.

For our dairy farmers, that grow corn in rotation with alfalfa, corn is typically grown in a field for 3-5 years.  The longer corn is grown continuously in a field, the higher risk the field has for economically damaging CRW root feeding and yield losses.  After rotating out of a non-corn crop, first year corn does not need any CRW management (or expensive Bt-RW trait costs).  A non-Bt-RW corn variety should be planted with a seed corn maggot/wireworm effective seed treatment.  This choice in year 1 saves $15-$20 per acre in seed costs.  In year 2, the risk of CRW loss increases to 25-30% in NY.  To offset this risk, a farmer has several options.   Many farmers will assume the risk and plant a non-Bt-RW corn variety without any additional protection such as a soil insecticide.  A second option in year 2 is to use either a 50% rate of soil insecticide (if insecticide boxes are available), high rate of neonic seed treatment or an insecticide added to the liquid popup fertilizer.  The CRW pressure in year 2 is not high enough to recommend the use of Bt-RW in most cases and the option of an insecticide is often a less expensive route to reduce production costs.   The deployment of different modes of toxicity in year 2 from Bt-RW significantly reduces the selection for Bt-RW resistance by CRW.  In continuous corn years 3-5, the risk of economic loss from CRW is high enough to merit the use of Bt-RW corn varieties.  A second option in years 3-5 of continuous corn is the use of a full rate of soil insecticide, if insecticide boxes are available.  Adding insecticide to the popup fertilizer during years 3-5 is not recommended due to unreliable efficacy with the higher CRW populations and increased risk for economic damage.

Strategy 2 for our dairy farmers:  Incorporating biocontrol nematodes into their rotation and crop production.   

By using the biocontrol nematode technology developed to combat alfalfa snout beetle in NNY, our dairy farmers can reduce their corn seed costs by eliminating the purchase of the Bt-RW traits in their corn varieties.  A single inoculation of each field with native persistent NY biocontrol nematodes provides protection from corn rootworm larval feeding by attacking these insects before they damage the corn roots.  NY research data indicates a single soil inoculation ($50-$60/acre) establishes these NY adapted biocontrol nematodes in the soil profile for many years, where they attack a wide range of pest soil insects across a wide variety of crops.  During the corn years, these biocontrol nematodes attack rootworm larvae and during the alfalfa years, attack wireworms, white grubs, clover root curculio feeding on the alfalfa and grass in the field.

If the biocontrol nematodes are inoculated into the field during the alfalfa portion of the crop rotation, the farmer can use corn varieties without Bt-RW for the entire corn rotation.  Biocontrol nematodes take until the second growing season after application to become fully established in the soil profile and when applied to the alfalfa crop, become fully established before corn is planted.  If the field is inoculated with biocontrol nematodes during the first year of the corn rotation, the corn variety planted in year 1 can be without the Bt-RW trait because rootworm is never a problem in 1st year corn in NY.  By the second year, the biocontrol nematodes are fully established and corn varieties can be planted without Bt-RW for the remaining years of the corn portion of the rotation.

However, if the corn field is inoculated with biocontrol nematodes during the 2nd-4th year when rootworm damage risk is higher, the corn variety planted during the year of inoculation needs to have the Bt-RW trait to provide some additional protection while the biocontrol nematodes become fully established in the field.  If the cost of establishing biocontrol nematodes in a field is a one-time cost of $50-60/acre and the Bt-RW trait adds $20/acre/year to the seed costs, the breakeven point for the nematode application is year 3 when the Bt-RW trait is not purchased or used.  In the years beyond 3-years after application, the seed cost savings will continue to be the cost of the Bt-RW which is an unnecessary expense.

For our cash grain farmers, an annual rotation of corn and a non-host crop like soybeans completely eliminates the need for any CRW management tools.  During the corn years, non Bt-RW corn varieties can be safely planted without risk of losses from CRW.  The elimination of the Bt-RW trait in the corn planted reduces the seed cost $15-$20 per acre and the use of a Bt-RW trait is completely unnecessary.  However, a seed treatment for seed corn maggot to protect plant emergence is recommended due to our typically wet cold soils.  The enhanced adoption of cover crops to protect our soil from erosion and any history of animal manure application significantly increases the risk of plant stand losses from seed corn maggot.

Long-term continuous corn fields:  The culture of corn continuously in the same field for multiple years using only Bt-RW to control CRW places tremendous selection pressure for the insect to develop resistance to the Bt-RW toxins.  This widespread practice across the corn belt has resulted in the documented CRW resistance to all Bt-RW traits and the insect is causing economic losses for farmers adopting these continuous corn practices.  Closer to home, Bt-RW failures have been reported in Central NY corn fields, multiple corn growing areas of Ontario, Canada and to the south in Pennsylvania.  With no new technology against CRW available for the next few years, these growers have a real challenge on their hands to minimize losses from this adaptable insect, if these farmers continue with long-term continuous corn production without breaking the CRW cycle with crop rotation.  Farmers with fields producing corn continuously for multiple years need to seriously consider working a crop rotation into their farming practices.  There are well documented agronomic yield advantages/responses from crop rotation over continuous corn, even without considering the reduction in CRW root feeding damage.

However, if farmers insist on growing continuous corn in field without interruption, there are several issues to consider.  The continued use of Bt-RW accelerates CRW resistance and the single field failure becomes the source of highly resistant beetles moving into neighboring fields, causing significant yield losses even in neighboring fields where farmers are utilizing crop rotation to minimize CRW-Bt-RW resistance development and yield losses.  The farmer growing continuous corn and producing highly resistant beetles becomes “a neighborhood social problem” for his neighbors.  Some farmers add a soil insecticide over the top of the Bt-RW trait, think this is a solution to the resistance issue.  While the corn stands better with less damage at the plant base, selection for CRW Bt-RW resistance continues to accelerate within the root system in areas outside of the soil insecticide treated zone.

The addition of biocontrol nematodes to the continuous corn culture is a way of introducing an independent mortality factor to help the Bt-RW trait control rootworm larval populations.  However in these high CRW pressure systems, biocontrol nematodes should not be used alone.  CRW has developed resistance to every other management strategy used to manage its damage, biocontrol nematodes used alone will also select for CRW resistance.  If farmers are interested in incorporating biocontrol nematodes into their continuous corn production, farmers should continue to use varieties with the Bt-RW trait to continue to kill the susceptible CRW larvae or match the use of biocontrol nematodes with a full rate of soil insecticide.