Cover Crop Variety Tests in Tennessee 2020
Virginia R. Sykes, Assistant Professor, Variety Testing Coordinator and Agroecology Specialist
Aleksandra Wilson, Research Associate I, Variety Testing and Agroecology
Gary Bates, Professor, Interim Department Head Plant Sciences, UT Beef and Forage Center Director
David McIntosh, Coordinator III, UT Beef and Forage Center Director
Angela Thompson McClure, Professor, Corn & Soybean Specialist
Tyson Raper, Associate Professor, Pettigrew Cotton Specialist
Ryan Blair, Extension Area Specialist, Grain Crops & Cotton
Forbes Walker, Professor, Environmental Soils Specialist
Agronomic Crop Variety Testing and Demonstrations
Department of Plant Sciences
Institute of Agriculture
University of Tennessee Knoxville
phone: (865) 974-7285
This research was funded by the Tennessee Agricultural Experiment Station, UT Extension, and the Tennessee Soybean Promotion Board.
We gratefully acknowledge the assistance of the following individuals in conducting these experiments:
Francisco Palacios, Dana Landry, Hannah McClellan, Savana Denton, Cheyenne Williams, Dalton McCurley, Matt Davis, Freeman Brown, Caden Johnson, and Wyatt Raines
AgResearch and Education Centers:
East Tennessee AgResearch and Education Center (Knoxville, TN)
Robert Simpson, Center Director
BJ DeLozier, Farm Manager
Cody Fust, Farm Crew Leader
Charles Summey, Senior Field Worker
Nicholas Tissot, Vehicle Operator II
Middle Tennessee AgResearch and Education Center (Spring Hill, TN)
Kevin Thompson, Director
Joe David Plunk, Research Associate
AgResearch and Education Center at Milan (Milan, TN)
Blake Brown, Center Director
Chris Bridges, Research Associate
Weston Bracey, Research Associate
Jason Williams, Research Associate
Table 1. Variety Characteristics
Table 2. Seed Company Contact Information
Table 3. REC location information
Table 4. Across location mean biomass, cover, and height
Table 5. Milan mean biomass, cover, and height
Table 6. Spring Hill mean biomass, cover, and height
Table 7. Knoxville mean biomass, cover, and height
Table 8. Across location mean biomass, nitrogen content, and nitrogen release
Table 9. Milan mean biomass, nitrogen content, and nitrogen release
Table 10. Spring Hill mean biomass, nitrogen content, and nitrogen release
Table 11. Knoxville mean biomass, nitrogen content, and nitrogen release
Table 12. Location comparison mean biomass
Table 13. Location comparison mean percent cover
Table 14. Location comparison mean height (November and February)
Table 15. Location comparison mean height (April and May)
Table 16. Location comparison mean percent total nitrogen content
Table 17. Location comparison mean estimated nitrogen release
COVER CROP VARIETY TESTS IN TENNESSEE 2020
Cover crop variety tests were conducted at the East Tennessee (Knoxville; ETREC), Middle Tennessee (Spring Hill; MTREC), and Milan (Milan; RECM) AgResearch & Education Centers (REC). All locations were planted with a drill to a length of 30 ft. Plot widths varied slightly by location based on equipment. Plots were planted at ETREC in 8 rows on 7.5 in. spacing, at MTREC in 7 rows on 7 in. spacing, and at RECM in 10 rows on 7.5 in. spacing. Plots were planted in a randomized complete block design and replicated three times at each location.
Varieties were planted at the appropriate seeding depth for each species (Table 1). The trial included varieties within the broader groups of brassicas, cereals, and legumes; however, all varieties were evaluated in a single trial in order to provide a better head-to-head comparison of the many cover crop varieties available. Contact information and websites for seed suppliers are summarized in Table 2.
A September and October planting date were planned for this study; however, due to extreme drought in the fall of 2019, only the earlier planting date was cut. All plots were planted in early to mid-October (Table 3).
Assessment of Ground Cover
Two 15 in. x 15 in. PVC square were randomly placed in each plot and photographed. These photographs were then analyzed for percent green cover using Canopeo software (Oklahoma State University Department of Plant and Soil Sciences, Stillwater, OK). Plots were photographed one month after planting (mid-Nov), in mid-Feb, early April, and early May; however, the height of many of the cereal and brassica species made this method ineffective and these data are not presented.
Assessment of Height
Height of cover crop varieties was measured in November, February, April, and May for species taller than 4 in. Species shorter than 4 in. were not measured but recorded as 1 in. for statistical purposes.
Assessment of Biomass
Cover biomass was measured in a single, randomly selected, 15 in. x 15 in. square area within each plot. Biomass within that square was cut to a height of 1 in. above the soil surface. Biomass was dried to a constant weight and dry matter biomass was calculated on a tons per acre basis.
Assessment of Nitrogen Content and Nitrogen Release
Dried biomass was ground to pass a 1 mm mesh and run through near infrared spectroscopy to determine quality constituents. NIRS estimated CP, ADF, NDF, lignin, and ash were used to derive the following values, according to Woodruff et al. (2008): percent nitrogen (CP / 6.25), carbohydrates (NFC + CP + fat), cellulose (ADF – (Lignin + Ash)), and hemicellulose (NDF – 4 ADF). Mean values were calculated for each species by location and termination month. Mean values for lignin, carbohydrates, and cellulose + hemicellulose were normalized to 100% and inputted into the UGA cover crop nitrogen calculator (http://aesl.ces.uga.edu/mineralization/, Gaskin, 2016), along with mean percent nitrogen and biomass, to estimate nitrogen release. The Walker County, Georgia location (bordering Hamilton County, Tennessee) was used since no Tennessee location was available for temperature and precipitation values. For background options, “no” for high organic matter soil, and cover crop residue will be “left on surface” were selected.
Interpretation of Data
The tables on the following pages have been prepared with the entries sorted by group (brassica, cereal, legume), common name, and variety. Biomass, cover, height data, total nitrogen, and NIRS quality constituents were analyzed using the MIXED procedure in SAS v. 9.4 (Cary, NC) with mean separation performed using the Fisher’s Protected LSD (Least Significant Difference) test. All analyses used a mixed model with treatment as a fixed effect and replicate and location as random effects with an alpha level of 0.05 to determine significance. The model for cover also included sample as a random effect. Mean separation letters have been listed next to mean values for each trait. Across all entries, varieties that have any letter in common within a column are not significantly different at the 5% level of probability. Varieties with performance statistically equivalent to the top performing variety will have an “A” included in the list of mean separation letters next to that entry. Mean separation letters of “A-group” varieties are highlighted in dark orange. Additionally, within functional group (brassicas, cereals, legumes) mean values between the 50th and 75th percentile are highlighted in light orange and above the 75th percentile are highlighted in dark orange.
Sixty varieties were evaluated in the 2019 – 2020 cover crop variety trial (Table 2). Treatments fell into three groups, brassicas (11 varieties), cereals (20 varieties), and legumes (29 varieties).
Species with the greatest representation included radish (6 varieties), barley (5 varieties), cereal rye (9 varieties), crimson clover (6 varieties), hairy vetch (5 varieties), and winter pea (7 varieties).
Variety performance is given across locations (Tables 4 and 8) and for each individual location, Milan (RECM; Tables 5 and 9), Spring Hill (MTREC; Table 6 and 10), and Knoxville (ETREC; Table 7 and 11). Results are also presented in a side by side comparison of locations for each evaluated trait (Tables 12 to 17).
Variety performance differed significantly among locations (P < 0.001). While most species performed similarly across locations, the varieties of radish generally exhibited lower February canopy cover and spring biomass at MTREC compared with RECM and ETREC (Tables 12 and 13). Crimson clover varieties also exhibited location differences, with all varieties exhibiting above average February canopy cover and spring biomass at MTREC, while at the RECM and ETREC, fewer varieties were above average (Tables 12 and 13). These differences may have 5 been due to environmental differences among the locations. Both temperature and precipitation immediately prior to planting and during early establishment can have a significant impact on successful cover crop establishment. Among the three locations, MTREC had the largest rain event immediately prior to planting which may account for the better performance of the clovers at this location. However, it is unclear why the brassica species did not perform well at MTREC.
Brassica species tend to be more prone to winterkill; however, the MTREC location had similar average and minimum monthly temperatures to ETREC.
Varieties that had high biomass in April, generally also had high biomass in May (Table 4).
Across all entries, top-performers (“A-group” varieties – not significantly different from the highest value) for biomass were dominated by cereal rye and hairy vetch. These included Bates RS4, NF95319B, and NF7325 cereal rye and AU Merit and Patagonia Inta hairy vetch. Within top-performing species, cereal rye exhibited the greatest difference in biomass between top and bottom performing varieties, with a difference of 0.9 DM tons/ac in April and 1 DM ton/ac in May. This was also true for hairy vetch varieties, with a difference in top and bottom performing varieties of 0.5 DM tons/ac in April and 0.7 DM tons/ac in May.
Top-performers for canopy cover varied by evaluation month (Table 4). One month after planting, all top-performers for canopy cover were cereal species, with the cereals averaging 22% cover, compared with only 9% for brassicas and 4% for legumes. However, by February, legumes were dominating the top, in particular varieties of hairy vetch. In February, top-varieties of legumes, including AU Merit and Patagonia Inta hairy vetch, provided 71 to 75 % canopy cover, while cereals maxed out at 27% and brassicas at 29%.
Height may be important for producers interested in grazing cover crops. Cereal rye varieties were the tallest in all four evaluation months (Table 4). Wintergrazer 70 was a top-performer for height during all four evaluation months, while Bates RS4 and NF97325 also stood out, appearing in the “A-group” in three out of the four evaluation months. These three topperformers averaged 6 inches in November, 9 inches in February, 34 inches in April, and 58 inches in May.
Variation in total nitrogen content (as a percentage of dry biomass) and estimated nitrogen release was observed both among and within functional groups (Table 8). Cereals had the lowest nitrogen content, averaging 1.5% in April and decreasing to 0.9% in May as biomass increased.
In April, most varieties provided a slight nitrogen credit over a 12-week period (1.9 lbs/ac); however, by May, this became a nitrogen deficit (-1.6 lbs/ac). Varieties of cereal rye exhibited the greatest nitrogen deficits (-2.3 to -6 lbs/ac).
Brassicas had slightly higher nitrogen content, averaging around 2% in April and dropping slightly to 1.7% in May. Estimated nitrogen release over 12 weeks, in both April and May, was positive, but slight, averaging 4.9 lbs/ac in April and 8 lbs/ac in May. Very little variation was observed within species for nitrogen content or estimated nitrogen release within both the brassicas and cereals.
As expected, legumes exhibited the highest nitrogen content and nitrogen release. Within the legumes, varieties of common vetch, hairy vetch, woolypod vetch, and winter pea stood out as 6 top-performers (>75th percentile) for estimated nitrogen release. This was primarily due to a combination of both high biomass and high total nitrogen content. AU Merit, Patagonia Inta, Villana, and WinterKing hairy vetch were top-performers in both April and May, averaging 40 lbs/ac in April and 77 lbs/ac in May released over a 12-week period.
Overall, results from this trial illustrate the variation both among species and among varieties within species as well as highlight top-performing varieties for East, Middle, and West Tennessee. While top-performing varieties were generally the same across locations and termination timings, some variation did exist. It is important to consider the specifics of a production system and select varieties that will excel under those conditions. Selecting a mix of top-performing varieties that offer complementary benefits, such as early season cover, biomass at termination, and nitrogen release after termination, can help maximum the benefits of cover crops to a succeeding cash crop system.
Woodruff, L.K., R. Hitchcock, L. Sonon, U. Saha, D.E. Kissel, J. Gaskin, N. Romano, M.L.
Cabrera, M.Y. Habteselassie, M. Vigil, J. Rema. 2018. A web-based model of N mineralization from cover crop residue decomposition. Soil Sci. Soc. Am. J. 82:983-993. Doi: 10.2136/ssaj2017.05.0144.
Gaskin, J., M. Cabrera, D. Kissel. 2016. Predicting nitrogen release from cover crops: The cover crop nitrogen availability calculator. UGA Extension Bulletin 1466. 7