Effect of nitrogen content and additional straw on changes in chemical composition, volatile losses, and ammonia emissions from dairy manure during long-term storage

Twelve 200-L barrels were used to determine the effects of N content and straw addition on changes in chemical composition and volatile losses measured by mass balance of dairy manure during a 136-d storage period. In addition, on d 0, 3, 6, 12, 28, 56, and 136, rate of NH3-N emission was measured; and core samples were collected to characterize fermentation pattern. High N (3.06% N, HN) and low N (2.75% N, LN) manures were obtained from cows fed diets with 17.2 and 15.2% crude protein (dry matter basis), respectively. On d 0, manure scraped from a freestall barn floor was diluted with water to 10% dry matter and loaded in barrels with (+S) or without (-S) mixing 22 g of chopped wheat straw per kilogram of undiluted manure. Data were analyzed as a randomized complete block with a 2 x 2 factorial arrangement of treatments and 3 replications. We observed no interaction between treatments for the reported measurements, but several day-of-storage by treatment interactions were found. Throughout storage, total NH3-N (TAN, NH3-N + NH4+-N; 71.9 vs. 104.3 mg/dL), pH (6.40 vs. 6.74), and total volatile fatty acids (TVFA, starting on d 12) were lower for LN relative to HN manure. In the presence of straw, crust formation occurred between d 12 and 28, and pH became lower and TVFA became higher starting on d 56, compared with no straw. Treatments did not influence loss of organic matter, organic N, organic C, or N, which averaged 31, 29, 26, and 20%, respectively However; neutral detergent fiber loss was 44% higher for +S relative to -S manure. Consistent reductions in the C:N ratio indicated proportionally higher volatile C loss than volatile N loss during storage. Overall rate of NH3-N emission was 36% lower for LN than for HN manure. In the presence of straw, rate of NH3-N emission did not differ until after crust formation, but was 67% lower on d 56 and 95% lower on d 136, when it was barely detectable, compared with manure with no straw. Manure pH was highly correlated with TVFA:TAN ratio (r = -0.78), and rate of NH3-N emission was correlated with pH, TVFA:TAN, TVFA, and TAN (r = 0.47, -0.44, -0.23, and 0.28, respectively). In this trial, both microbial fermentation and crust formation influenced NH3-N emission rate and other measured responses, highlighting the importance of long-term sampling to evaluate treatment effects in manure storage studies.