Evaluating the potential of dietary crude protein manipulation in reducing ammonia emissions from cattle and pig manure: A meta-analysis

Dietary manipulation of animal diets by reducing crude protein (CP) intake is a strategic NH3 abatement option as it reduces the overall nitrogen input at the very beginning of the manure management chain. This study presents a comprehensive meta-analysis of scientific literature on NH3 reductions following a reduction of CP in cattle and pig diets. Results indicate higher mean NH3 reductions of 17 ± 6% per %-point CP reduction for cattle as compared to 11 ± 6% for pigs. Variability in NH3 emission reduction estimates reported for different manure management stages and pig categories did not indicate a significant influence. Statistically significant relationships exist between CP reduction, NH3 emissions and total ammoniacal nitrogen content in manure for both pigs and cattle, with cattle revealing higher NH3 reductions and a clearer trend in relationships. This is attributed to the greater attention given to feed optimization in pigs relative to cattle and also due to the specific physiology of ruminants to efficiently recycle nitrogen in situations of low protein intake. The higher NH3 reductions in cattle highlights the opportunity to extend concepts of feed optimization from pigs and poultry to cattle production systems to further reduce NH3 emissions from livestock manure. The results presented help to accurately quantify the effects of NH3 abatement following reduced CP levels in animal diets distinguishing between animal types and other physiological factors. This is useful in the development of emission factors associated with reduced CP as an NH3 abatement option.