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

ABSTRACT Animal manure is the largest source of ammonia (NH 3 ) 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 NH 3 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/m 2 per hour; P  3 emissions were lower for flush versus gutter-scrape and gravity-flow manure-management systems (167 vs. 352 and 426 mg/m 2 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 NH 3 -emitting potential of manure can be reduced by moderately decreasing dietary CP content without affecting milk yield and composition in dairy cows.