Effects of Acidifying Pig Diets on Emissions of Ammonia, Methane, and Sulfur from Slurry during Storage

Ammonia (NH3) volatilization from intensive livestock production is a threat to natural ecosystems. This study investigated pig diet manipulation by 1% (w/w) benzoic acid (BA) amendment and lowering of dietary electrolyte balance through substituting 1.4% (w/w) CaCO3 with 2.0% (w/w) CaCl2. Urine and feces were collected separately from 24 pigs fed one of four diets (Control, +BA, +CaCl2, +BA+CaCl2) in metabolic cages and mixed as slurry. During 103 d of storage, all acidifying diets consistently reduced pH in the slurry by 0.4 to 0.6 units. There was a strong relationship between slurry pH and NH3 emissions, which were considerably reduced by the three acidifying diets. The +BA diet decreased NH3 emission by 28%, the +CaCl2 diet by 37%, and the combined +BA and +CaCl2 diet by 40%. Acidifying diets had no effect on S cycling or emission of volatile S compounds under the prevailing conditions of restricted S feeding. Methane (CH4) emissions were increased by 73% in diets with CaCl2. An initial delay in CH4 emissions was investigated in a separate experiment with manipulation of pH (5.4, 6.7, or 8.8) and inoculation with adapted pig slurry (0, 4, 11, or 19%), which showed that methanogenic potential, rather than inhibitory effects of the chemical environment, caused the delay. In conclusion, NH3 emissions from slurry could be reduced by addition of BA to pig diets or by controlling the dietary electrolyte balance, but there was no additive effect of combining the two strategies. However, CH4 emissions from slurry may increase with acidifying diets.