Purpose
Composting is one way that livestock operations can effectively dispose of animal carcasses on their farm site during many times of the year, even in cold climates. Various carbon sources such as wood chips or chopped corn stalks can be used and tend to be coarsely chopped so that airflow is improved through the pile to increase degradation. This makes finished carcass compost different from typical manure or garden compost. Land application is recommended for the final product, but there is little information on nutrient availability of carcass compost for crop production. Since state regulations often require this information to determine appropriate agronomic land application rates, livestock producers are left unable to utilize this material efficiently in a way that meets the law. The goal of this project is to evaluate the fertilizer equivalent value of carcass compost that uses different carbon sources (wood chips versus corn stalks) for corn production.
What Did We Do
The Minnesota Department of Agriculture conducted winter swine carcass composting trials at the University of Minnesota Southwestern Research and Outreach Center in Lamberton, MN during the winter of 2020. In these trials, swine carcasses were ground with the carbon source prior to initiating the compost process. Carbon sources included wood chips or corn stalks. The compost went through the active composting cycles (thermophilic phase) and curing (mesophilic) phase over approximately one year. Samples were collected from each pile in the spring of 2021 and sent to Penn State Analytical Laboratory for standard compost tests.
In 2021, a field experiment was carried out at the University of Minnesota Southwestern Research and Outreach Center in Lamberton, MN to test the different carcass composts as a nutrient source. The field trial was set up as a randomized complete block design with four replications as the blocks. The 10 treatments included:
- 0 N (full P, K, S) fertilizer (control)
- 3 urea fertilizer N rates (50%, 100%, 150% of corn nitrogen [N] needs)
- 3 woodchip compost and 3 corn stalk compost rates (50%, 100%, 150% of corn N needs)
The compost rates were determined assuming 50% of the total N from lab analysis is plant available. The overall N rates were determined using the University of Minnesota fertilizer guidelines for non-irrigated corn. Fertilizers and compost were applied by hand in spring at their appropriate rates and incorporated within 12 hours of application. Approximately 7, 14, and 21 tons per acre of compost were applied to achieve 50%, 100%, and 150% of corn N needs (this corresponds to 80, 160, and 240 pounds of N per acre). Corn was planted and managed for pests according to typical production practices in the region. The middle two rows of each plot were harvested in the fall with a plot combine and yield and grain moisture were recorded.
What Have We Learned
The lab analysis results of each compost are in Table 1. Total N content was similar between sources and primarily made up of organically bound N. The carbon:N ratio of the wood chip compost was higher at 20.8:1 than the corn stalk compost at 14.2:1, though both were below 20:1 and should not theoretically tie up N from the soil after land application. Respirometry tests suggested that the corn stalk compost was not yet mature (respiration was greater than 11 mg CO2-C/g organic matter/day) while the wood chip compost was considered in the “curing” phase (2-5 mg CO2-C/g organic matter/day). The bioassay results suggested that neither compost had any phytotoxins (both had emergence greater than 90%).
Table 1. Swine carcass compost test results from Penn State Analytical Laboratory. Samples were collected and sent to the lab in spring 2021 prior to land application.
| Carcass Compost Carbon Source | ||
| Tests | Wood Chips | Corn Stalks |
| pH | 7.3 | 8.0 |
| Soluble salts (1:5 w:w), mmhos/cm | 1.2 | 2.5 |
| Moisture content, % | 29.1 | 41.2 |
| Organic matter, % | 49.1 | 33.4 |
| Total nitrogen (N), lb ton-1 | 22.0 | 24.0 |
| Organic N, lb ton-1 | 22.0 | 22.0 |
| Ammonium-N, lb ton-1 | 0.32 | 0.66 |
| Nitrate-N, lb ton-1 | 0.04 | 0.00 |
| Carbon: N ratio | 20.8 | 14.2 |
| Phosphorus (as P205) lb ton-1 | 5.4 | 4.6 |
| Potassium (as K20) lb ton-1 | 6.6 | 9.4 |
| Particle Size (<9.5 mm), % | 82.6 | 68.1 |
| Respirometry, mg CO2C/g organic matter/day | 4.8 | 17.4 |
| Bioassay (cucumber seedling emergence), % | 100.0 | 96.0 |
| Bioassay (cucumber seedling vigor), % | 100.0 | 100.0 |
As for the field trial, the 2021 growing season endured a sustained drought and yield was lower than expected (ranging from 64 – 117 bushels per acre; Figure 1). Yield increased with each incremental increase in fertilizer N with the highest yield at 240 pounds of N per acre. The carcass composts differed in their effect on yield. Corn yield responded to the corn stalk compost and fertilizer up to 150 pounds of available N per acre (or about 14 tons per acre), but the carcass compost reduced yield at 240 pounds of N per acre. Yield only responded to the wood chip compost at the lowest rate (80 pounds of N per acre, or about 7 tons per acre) but at higher rates yield was similar to the control. This is likely due to the high carbon content of both composts. As more carbon was applied, the soil microbes needed a higher amount of N to degrade the carbon, thus taking it from the soil. Overall, we suggest that less than 15 tons per acre of carcass compost should be used for land application. If wood chips were used as the carbon source, do not expect a significant nitrogen credit.
Future Plans
We will repeat this study in 2022 at the same site but with compost generated during the winter of 2021. Besides wood chips and corn stalks, a new carbon source will be introduced to the project, wheat straw. Along with yield, we are also evaluating soil and plant samples for nitrogen and phosphorus changes.
Authors
Melissa L. Wilson, Assistant Professor and Extension Specialist, University of Minnesota
mlw@umn.edu
Additional Authors
-Erin L. Cortus, Associate Professor and Extension Engineer, University of Minnesota;
-Paulo H. Pagliari, Associate Professor, University of Minnesota
Acknowledgements
This project is funded by USDA’s Animal and Plant Health Inspection Service through the National Animal Disease Preparedness and Response Program. Thanks to our partners at the Minnesota Department of Agriculture for supplying the carcass compost.
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