CASE STUDY: Reducing dietary protein decreased the ammonia emitting potential of manure from commercial dairy farms

Animal manure is the largest source of ammonia (NH3) and is the third-largest source (after soil management and enteric fermentation) of greenhouse gas emissions from animal agriculture. The objective of this study was to decrease manure NH3 emissions through reducing dietary CP concentration in field conditions on commercial dairy farms. Eleven free- and tie-stall Pennsylvania dairies with gutter-scrape, gravity-flow, or flush manure-management systems participated in the project. Fecal and urine samples were collected from randomly selected cows, and barn-floor and laboratory manure ammonia and greenhouse gas emissions from manure were measured during 8 sampling periods (2 in each: fall 2009, spring 2010, fall 2010, and spring 2011). Crude protein concentration of the high-producing-cow rations was decreased from an average across all farms of 16.5% during the background period (fall 2009-spring 2010; HCP period) to 15.4% during the experimental period (fall 2010-spring 2011; LCP period). Laboratory ammonia emission of reconstituted manure was on average 23% lower for LCP versus HCP manures (291 vs. 378 mg/m2 per hour; P < 0.001). Barn-floor NH3 emissions were lower for flush versus gutter-scrape and gravity-flow manure-management systems (167 vs. 352 and 426 mg/m2 per hour, respectively; P = 0.02). Milk yield (32.2 vs. 32.5 kg/d) and milk composition were not different between the LCP and HCP periods (P > 0.12). Milk urea N concentration tended to be lower (P = 0.06) and milk N efficiency was higher (P = 0.02) during the LCP versus HCP periods. This on-farm study demonstrated that the NH3 -emitting potential of manure can be reduced by moderately decreasing dietary CP content without affecting milk yield and composition in dairy cows.