According to recent studies, one of the largest contributors to atmospheric ammonia is dairy cattle excreta. In this study, seven lactating Holstein cows were subjected to four feeding trials with diets containing 14.5, 15.5, 17, and 18% crude protein (CP). The first objective was to determine the effect of protein content/intake on ammonia fluxes to the atmosphere from each stage of manure handling at a flush dairy. The second objective was to examine the effect of manure handling itself on ammonia fluxes. We examined four different stages of manure handling: 1. Fresh mixture of urine and feces to represent what may be scraped from a barn floor 2. Diluted mixture of urine and feces to represent what is flushed from a barn floor 3. Solids portion of a diluted mixture of urine and feces to represent what is retained by a solids separator at a dairy 4. Liquid portion of a diluted mixture of urine and feces to represent the permeate from a solids separator at a dairy
Ammonia fluxes from scraped manure began low and rose to higher fluxes and peaked at an average of 25 hours after mixing urine and feces. Fluxes from flushed manure showed the same behavior. The average pH values for the scraped and flushed manure were relatively low at 6.87±0.17 and 7.05±0.13, respectively, and unfavorable toward ammonia versus ammonium. Fluxes from the separated solids and liquid manure, whose pH values were higher at 8.70±0.30 and 8.55±0.19, respectively, peaked almost immediately. For a time period of 18 hours after beginning flux measurements from the separated liquid manure, fluxes stayed relatively constant. Compared to hours 0-9, fluxes from the separated solids manure decreased 11% in hours 9-18. These results indicate that ammonia fluxes from the scraped and flushed manure were initially limited by the relatively low pH of the slurries, until they aged and released carbon dioxide, after which fluxes became controlled by temperature. Fluxes from the separated solids and liquid manure, however, were temperature-controlled from the start, as this manure had already undergone some aging and the initial pH was more favorable to ammonia loss.
Multivariable regressions predicting the mass of ammonia lost to the atmosphere as a function of total ammoniacal nitrogen (TAN) and temperature (T) show that fluxes from the scraped and flushed manure are very sensitive to temperature compared to the separated solids and liquid manure. The same regressions show that ammonia fluxes from the separated solids and liquid manure are predicted well by TAN and T (R2 = 0.906 and 0.812, respectively), indicating that dietary protein manipulation, which affects TAN content of the manure, may have a greater effect in this stage of handling. Regressions predicting the mass of ammonia lost to the atmosphere as a function of crude protein intake show crude protein intake is a poor predictor of the overall ammonia flux from a flush dairy. Low correlation coefficients from these regressions are likely a result of cow-to-cow variability in the effect of crude protein intake on ammonia emissions.
Significant reductions in ammonia fluxes are possible; however, they will require new regulations on manure handling at flush dairies or incentives for flush dairies to pursue alternate means of manure handling with low emissions. Dairies can begin attacking the problem of excessive emissions by lowering the dietary protein content for older cows to reduce the mass of TAN excreted in their manure, thereby reducing ammonia fluxes in accordance with Henry’s law. Once manure is excreted on the barn floor, dairies must encourage mixing between urine and feces and use the natural acidity of the feces in delaying emissions by converting aqueous ammonia in the urine to ammonium. Dairies can achieve well-mixed slurries by manually mixing the manure or with a sloped floor, channeling urine and feces to a common area. Covering holding tanks containing flushed manure to minimize CO2 stripping and a rise in pH is one measure that may reduce ammonia fluxes. Finally, requiring nitrification or nitrification and denitrification in on-site storage tanks will biologically convert ammonia to nitrate or nitrogen gas, respectively, reducing fluxes from the separated liquid manure. Although burdensome to dairies, these measures would drastically reduce ammonia emissions from flush dairies. Regulatory agencies may choose to ignore the separated solids manure when considering strategies to reduce ammonia emissions, since ammonia volatilization becomes limited by aqueous ammonia in the first 18 hours following separation.