Ammonia and greenhouse gas emissions from housed Holstein steers fed different levels of diet crude protein.

The objective of this study was to determine the effect of diet CP levels on nitrous oxide (N2O), ammonia (NH3), and methane (CH4) emissions from 1) cattle housed in confined settings and 2) cattle manure following surface application to incubated soils. Twelve 500-kg Holstein steers were fed diets containing 10% CP (10CP) and 13% CP (13CP). The experimental design was a 2 × 2 Latin square conducted during two 20-d periods. Diets were fed for 14 d before each measurement period to allow for diet acclimation. Steers were housed in environmentally controlled rooms allowing for continuous emission measures of N2O, NH3, and CH4. At the end of the second period, manure was collected and surface applied to incubated soils to verify potential NH3 and N2O emissions. To assess emissions from incubated soils, 2 experiments were set up with 3 replicates each: Exp. 1, in which soil fertilization was based on manure mass (496 g of manure), and Exp. 2, in which soil fertilization was based on manure N content (targeted at 170 kg N/ha). Manure emissions were monitored for 7 d. Steers fed 13CP diets had increased daily NH3 emissions when compared to steers fed 10CP diets (32.36 vs. 11.82 ± 1.10 g NH3/d, respectively; P < 0.01). Daily N2O emissions from steers fed 13CP and 10CP diets were significantly different only during Period 1 (0.82 vs. 0.31 ± 0.24 g N2O/d; P = 0.04). Steers fed the 10CP diet had greater N2O emissions per unit of N consumed than steers fed the 13CP diet (9.73 vs. 4.26 ± 1.71 mg N2O/g N intake; P = 0.01). Diet CP levels did not affect enteric CH4 production from steers. In terms of soil emissions, different CP levels did not affect NH3, N2O, or CH4 emissions when soil fertilization was based on manure mass. However, NH3 emissions were reduced when manure from steers fed the 10CP diet was applied to soil based on N content. Ammonia emissions decreased during the 7-d incubation period. Conversely, N2O emissions increased over the period. Our results indicated that management of diet CP levels of confined finishing steers mitigates NH3 emissions from steers but does not affect enteric CH4. In addition, results suggested that soil characteristics might be as important as manure N content to generate NH3 and greenhouse gases from soils receiving manure fertilization.