Designing and Evaluating Complex Cover Crop Mixtures

Final report for GS16-162

Project Type: Graduate Student
Funds awarded in 2016: $10,994.00
Projected End Date: 08/31/2018
Grant Recipient: Virginia Tech
Region: Southern
State: Virginia
Graduate Student:
Major Professor:
Dr. Mark Reiter
Virginia Polytechnic Institute and State University
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Project Information

Summary:

Cover crops are used by farmers to provide a range of ecosystem services including nitrogen scavenging and fixation, weed suppression, reducing soil compaction and increasing water use efficiency that can result in increased crop yield. Success with simple, two-species cover crop mixes has led to interest in more complex cover crop mixtures, with three or more species. But research on complex cover crop mixtures is inconsistent because it fails to capture all of the potential ecosystem services provided by complex mixtures. This research hoped to show the cumulative effect of the ecosystem services provided by cover crop mixes and to describe the best cover crop mixture. Rye, vetch, clover and radish cover crops were grown in 1, 3 and 9 species mixes. Corn yield, cover crop biomass and C:N ratio, soil compaction, weed pressure and soil moisture status were measured. A method of classifying ecosystem service interactions and ranking develop by Jopke et. al. (2015) was used to evaluate the cumulative ecosystem service contributions of cover crop mixtures. Multiple linear regression was used to estimate the individual species contribution of rye, vetch, oats and radish to each ecosystem service; which can be used in designing new complex cover crop mixtures. The expected results of this project were to develop a tool that can be used to evaluate a wider range of ecosystem services and ranking system for new cover crop mixtures as well as provide guidelines for mixing species in new cover crop mixtures.

Project Objectives:
  1. Measure ecosystem services provided by monocultures and cover crop mixtures, including corn yield.
    1. Provisioning services of corn yield and cover crop biomass.
    2. Weed suppression.
    3. Reduced soil compaction.
    4. Nitrogen accumulation by cover crops.
    5. Increased soil water conservation to reduce irrigation needs.
  2. Evaluate the range of ecosystem services provided by cover crop mixtures to select cover crop mixtures that provide the desired ecosystem services and the lowest amount of trade-offs.
  3. Estimate the contribution of individual cover crop species or plant functional groups (legumes, small grains, brassicas and tap-roots) toward the desired ecosystem service to design new cover crop mixtures.

Research

Materials and methods:

Objective 1

This research compliments and builds upon a three-year experiment established in fall 2014 at the Eastern Shore Agricultural Research and Extension Center (Painter, VA) evaluating cover crop mixtures in no-till corn rotations. The cover crop experiment is a randomized complete block design with twelve different treatments with cover crop mixtures and is replicated four times. Cover crop treatments contain varying amounts of species mixes, from a 9-species “kitchen sink” mix to a “purposeful diversity” 3-species mixes. In rotation after the cover crops, no-till corn was planted during summer of year 3. Four key species—rye, vetch, crimson clover and tillage radish were selected due to their popularity as cover crop species and their contributions to different supporting services (Clark, 2007). There are eight treatments evaluated: one no-till control without cover crops, four monocultures (rye, vetch, radish and clover), two three-species mixtures and one high diversity, nine-species mixture.  A full description of the cover crop mixtures, including species and seeding rates, is shown in table 1.

 

Cover Crop Mixes

Function Group

Species

Seed Rate

Total Seeding Rate

 

#

#

kg ha-1

kg ha-1

Rye

Grass

Cereal Rye

128.8

128.8

Vetch

Legume

Hairy Vetch

25.8

25.8

Radish

Brassica

Forage Radish

8.9

8.9

Clover

Legume

Crimson Clover

28.0

28.0

Diverse Mix 1

Grass

Rye

22

44

 

Brassica

Forage Radish

2

 

 

Legume

Austrian Winter Pea

30

 

 

Legume

Hairy Vetch

12

 

SARE Mix

Grass

Rye

24.6

38

 

Legume

Hairy Vetch

2.2

 
 

Brassica

Forage Radish

11.2

 

Kitchen Sink Mix

Grass

Spring Oat

11.2

56

 

Grass

Triticale

11.2

 
 

Grass

Rye

11.2

 
 

Brassica

Tillage Radish

1.1

 
 

Brassica

Rapeseed

1.1

 
 

Forb

Phacelia

1.1

 
 

Legume

Crimson Clover

3.4

 
 

Legume

Austrian Winter Pea

11.2

 
 

Legume

Hairy Vetch

4.5

 

Table 1. Description of cover crop treatments and mixtures.

Five of the seven cover crop treatments were grown in the current experiment, but three treatments (two monocultures and one mixture) were added to the experiment so that the four cover crop species being examined appeared in a range of proportions. Ecosystem services measurements during the cash crop year of the rotation include corn yield, weed suppression and soil water conservation to reduce irrigation needs. The amount of weed suppression was assessed with total weed counts in the corn crop. Volumetric soil moisture was monitored continuously for the two years of the study in two replications in all 6 plots. The sensors were placed at 15, 30, 45 and 60 cm to capture changes in soil moisture above and below a hard pan located between 30 and 45 cm. Monitoring the hard pan allowed monitoring of deep rooted radish on hard pans. Soil samples were taken to measure inorganic soil nitrogen and carbon concentrations.

Ecosystem services being measured in cover crops included cover crop biomass, plant nitrogen uptake and biological fixation, and improved soil quality through decreased compaction. Above ground biomass was sampled immediately before cover crop termination from four one foot quads randomly placed across the plots. The biomass was separated by plant species and dried individually and weighed. The individual dried species weights were totaled for treatment for dry biomass weight. The nitrogen content of cover crop biomass and soil inorganic N concentration at cover crop termination was measured. Nitrogen accumulation in the cover crop biomass was assumed to be from either biological nitrogen fixation by legumes or nitrogen scavenging by grains and brassicas. The soil nitrogen concentration was measured again before sidedress to assess how much N was added by the cover crop. Compaction was measured with a digital penetrometer after corn planting, when the soil was at field capacity.

Objective 2

A method similar to the one used Jopke et al (2015) was used to look at ecosystem interactions. All possible pairs of ecosystem services measured in objective 1 were correlational and graphed with bagplots using the alpack package in R. Bagplots are similar to box plots as they show the median of the data and where 50% of the data lies, but uses polygons instead of squares. The visualization of the data using the polygon shows if the data follows a non-linear trend, resulting in a weak correlation (Jopke et al., 2015). From bagplots, interactions can be categorized as synergistic, a trade-off or neutral interactions. The total number of trade-offs and synergies for each cover crop mixture and monoculture was tallied to create a ranking to find the cover crop mixture with the greatest number of ecosystem services. This investigative approach highlights potential interactions between ecosystem services, but cannot quantify the level of ecosystem service being provided or lost.

Objective 3

Multiple linear regression was used to estimate significance of cover crop species to ecosystem services. Cover crop data sets were compiled and contained levels of ecosystem services for each monoculture and mixture as the various responses and the proportion of the four cover crop species. Cover crop proportions were expressed as weights (biomass dry weight) or percentage of mixture (species counts prior to termination) because some ecosystem services, not all ecosystem services provided by cover crops are well correlated with above ground biomass (Smith et al., 2014). Model building will be used to select the best model from the full model containing all possible species and species interaction terms. The analysis was repeated using cover crop functional groups—legumes, small grains, and tap-roots—instead of individual plant species to see if the ecosystem service provided is species specific.

The interpretation of the regression analysis can be used to design new cover crop mixtures. Species that have a significant and positive slope should be kept in the mixture because they provide benefits to the mixture, but negative or insignificant regressions could be excluded. In addition, some interactions may be significant and indicate either synergies (mutualistic interaction) or trade-offs (competition) between plant species.

Research results and discussion:

Figures_1-4

Results

There is a significant difference (p<0.001) in the amount of biomass produced by cover crop treatments (Figure 1). The 9 species kitchen sink mixture produced the most biomass (6591 kg ha-1) and radish monoculture produced the least (865 kg ha-1), less than the no-till, no cover crop control (917 kg ha-1). The three-species mixtures, Diverse Mix 1 and SARE Mix, both outperformed all of the monoculture cover crop treatments.

There was a significant difference in the number of weeds per square foot at sidedress (p<0.01). The average number of weeds per square foot at sidedress was highest in the clover and radish monoculture plots while the lowest weed numbers occurred in the rye and vetch monoculture plots (Figure 2).

Soil moisture at the time of corn planting (Figure 3) was significantly different between the cover crop treatments when the no-till, no cover crop control was included (p<0.05). But there was no difference in moisture content between the cover crop treatments when the control was excluded (p=0.80).

Soil compaction measurements showed subsurface compaction in all treatments between 15 and 30 cm depths (Figure 4). Subsurface compaction was highest in the SARE mixture (473 psi) and radish monoculture (424 psi). The 9-species kitchen sink mix cover crop had the lowest subsurface compaction (277 psi).

There was a significant difference in the number of weeds per square foot at sidedress (p<0.01). The average number of weeds per square foot at sidedress was highest in the clover and radish monoculture plots while the lowest weed numbers occurred in the rye and vetch monoculture plots (Figure 2).

Figure 1. Cover Crop Biomass in monoculture and mixture cover crop plots at termination, grouped by the number of species in each mixture.

Figure 2. Average number of weeds per square foot in cover crop plots at sidedress. Number of species refers to the number of cover crop species in the mixture.

Figure 3. Volumetric soil moisture (%) at 15 cm depth, at the time of corn planting. Species refers to the number of cover crop species in the cover crop mixtures.

Figure 4. Soil Compaction (psi) by depth under cover crop treatments with divided by number of species in the mixture (0, 1, 3 or 9).

Discussion

Results from cover crop biomass, weed counts, moisture and compaction showed differences between the cover crop mixtures and monocultures. The results for monoculture radishes were unexpected; monoculture radish had the lowest amount of biomass at termination and experienced the most soil compaction. Radishes were expected to produce large amounts of biomass, equal or higher than rye (Clark, 2007) and they did produce large amounts of biomass in the early spring. But at termination, the radishes were going to seed and the above ground leaf biomass had ceased. Because of the lower biomass production, monoculture radish also performed poorly in suppressing weeds. Lastly, monoculture radish plots had the highest amount of soil compaction. The large tuber roots of tillage radishes were expected to relieve soil compaction, but that was not observed in these plots. The higher degree of compaction under the monoculture radishes, as well as the SARE mix, were likely because of prior soil management. The three additional treatments added to the existing cover crop experiment, monoculture radish and clover and the SARE mixture, were planted on adjoining plots and are in the first year of no-till, cover crop management. While the previously existing cover crop plots were managed with no-till and intensive cover cropping for three years. Prior management may also be contributing to different weed seed banks and compaction in the soil. The observations made on tillage radishes, as well as other cover crop mixtures and species, will help farmers to manage radish cover crops to ensure the most biomass is available at termination to ensure they receive all of the cover crop benefits.

Sources
Clark, A. (Ed). 2007. Managing cover crops profitably. 3rd ed. Sustainable Agriculture Research Extension, College Park, MD.

Jopke, C., J. Kreyling, J. Maes, and T. Koellner. 2015. Interactions among ecosystem services across Europe: Bagplots and cumulative correlation coefficients reveal synergies, trade-offs, and regional patterns. Ecol. Indic. 49: 46–52.

Smith, R.G., L.W. Atwood, and N.D. Warren. 2014. Increased Productivity of a Cover Crop Mixture Is Not Associated with Enhanced Agroecosystem Services (W-X Lin, Ed.). PLoS ONE 9(5): e97351.

 

Participation Summary
25 Farmers participating in research

Educational & Outreach Activities

100 Consultations
2 Curricula, factsheets or educational tools
1 On-farm demonstrations
2 Tours
5 Webinars / talks / presentations
2 Workshop field days

Participation Summary

400 Farmers
200 Ag professionals participated
Education/outreach description:

A two-day training for farmers, regulators and extension agents about managing cover crops was held at the Eastern Shore Agriculture Extension Center in late April 2017, in Melfa, VA in January 2018, and in Petersburg, VA in March 2018. The training included research presentations by Virginia Tech faculty, presentations by local farmers using cover crops and a tour of the cover crop experiments being conducted at the ESAREC (by person when on the ESAREC and by photo when in Melfa and Petersburg, VA). Topics that were covered during the field tour include:

  • Nitrogen accumulation and biomass production of different cover crop mixtures
  • Identification of cover crop species and how to incorporate them into mixtures
  • Demonstration of improved soil quality under no-till and cover crop management
  • Observing soil pits to see changes in soil quality and cover crop root depths

Project Outcomes

100 Farmers reporting change in knowledge, attitudes, skills and/or awareness
10 Farmers changed or adopted a practice
2 Grants received that built upon this project
5 New working collaborations
Project outcomes:

This project has contributed to greater sustainability in agriculture production by learning about possible challenges farmers might face growing these cover crop mixtures and by educating people about the many options for cover crop use. One example of potential challenges that we have observed this year that will impact farmers are that tillage radish biomass decreases significantly if it is terminated after flowering. This project has facilitated greater education about cover crop varieties available for use on Coastal Plain soils and ideas of how to combine different varieties into mixture to meet specific needs, when the plots were visited during a field day. Another major concern we realized was how different seed size can drastically impact germination of one species versus another. Also, we noticed how some species are difficult to desiccate and can add to a weed bank of seeds. We now have a weed scientist working on these issue due to this project. 

Knowledge Gained:

First, the results of the experiment have demonstrated the benefit of diverse cover crop mixtures over monocultures because they produce more biomass. Second, it does not take very long to see changes in soil quality that result from improved soil management, such as cover crops and no-till farming. We were surprised how quickly soil parameters can change and how adding diverse mixes allows the cover crops to thrive under varying environmental conditions as compared to monoculture systems. 

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.