Livestock manure properties and pollution prevention: Part 2

By Harold Keener, Fuqing Xu, Mary Wicks

In our February article (https://ocj.com/2021/02/98547/), we discussed how manure storage conditions can affect water soluble phosphorus, a contributor to algal blooms. This month we look at levels of nitrogen (N), phosphorus (P), and potassium (K) in stored manure.

NPK levels in livestock manures

Researchers evaluated NPK levels in stored manure using 2018-2019 data provided by a commercial laboratory. The data consisted of analyses of 55 dairy (milking), 397 poultry (layers) and 501 swine manure samples. Farm names and manure management details were not provided. However, 219 of the swine manure samples, which were provided from 91 contract farms using the same feed formulations and deep pit storage, were evaluated separately for comparison. The data was evaluated on a dry basis for comparison purposes as moisture content affects nutrient levels, with high moisture manure being subject to large errors when converted to a dry basis. The moistures of manures in the study were 9.5% to 73.9% for poultry, 64.4% to 97.9% for dairy and 82.4% to 98.5% for swine.

Table 1 shows average NPK levels for the analyzed data as well as the book values of fresh livestock manure (OSU Bulletin 604, 2006). These results can be used as a reference by livestock producers to see if they are achieving low P/K or reasonable moisture levels compared to other livestock producers. Use of P/K is a way to see if feeding programs are reducing P levels in manure.  Findings based on data analysis were:  

1. NPK levels, on a dry basis (%db), range widely for manure, illustrating the importance of analysis to determine accurate application rates. 
2. P/K levels for swine manure showed two distinct groups, with the 91 contract farms having lower levels than the entire data set. Lower P/K values are likely due to use of phytase, an enzyme, and reduced P in feed. 
3. P/K levels in dairy and poultry manures were less than for fresh manure. Lower P/K levels for poultry are likely due to use of phytase and reduced P in feed. Lower P levels for liquid manures (dairy and swine) may be due to settling out of solids if the manure was not mixed well before sampling, as P levels are higher in solids than liquids.  
4. Dairy manure had the lowest P (%db) compared to poultry or swine, while swine manure had the highest. Thus, if using a fix application rate (#P per acre), you would need to apply more gallons per acre for dairy than swine, assuming the same moisture level.   
5. Poultry manure, with its lower moisture content, can be more economically trucked to farther fields for application for a given #P per acre.

Management implications to reduce application costs and water quality impacts

Land application of livestock manure provides nutrients for field crops and is the most accepted and economical use. Following the 4Rs of nutrient management — right nutrient, right place, right time, and right amount — will minimize runoff of manure nutrients. Practices to help achieve the 4Rs include:

1. Analyze manure for water and NPK before land applying. There is wide variability across farms due to differences in feeding programs and storage conditions. Accurate nutrient analysis is needed to calculate the appropriate application rate.      
2. Determine soil P levels and use along with cropping goals to determine manure application rates.  
3. Manage P in animal feed formulations to stay within recommended guidelines. The use of phytase in swine and poultry feeds lowers P levels in the manure, reducing land required for application and potentially lowering transportation costs. 
4. Manage feeding, housing and storage to minimize manure moisture levels, which will reduce the weight and volume to be transported and increase NPK levels. To reduce manure moisture, control salt levels in feed, use ventilation to enhance drying, maintain water lines to avoid spillage/waste, and cover manure storage to eliminate rainwater.
5. Inject or incorporate manure soon after land application to minimize N loss (as ammonia) and reduce potential P loss from runoff.

Thank you to the Ohio Higher Ed, Ohio Sea Grant, and University of Toledo for funding this research. Dr. Harold Keener is a Professor Emeritus, Fuqing Xu was a Research Scientist, and Mary Wicks is a Program Coordinator in the Department of Food, Agricultural and Biological Engineering of The Ohio State University.  E-mail:  keener.3@osu.edu; wicks.14@osu.edu. Phone: (330)202-3533.  This column is provided by the OSU Department of Food, Agricultural and Biological Engineering, OSU Extension, Ohio Agricultural Research & Development Center, and the College of Food, Agricultural and Environmental Sciences.