Lowering dietary crude protein and cation-anion difference to reduce manure ammonia emissions in lactating cows

The objectives of the study were to determine the effects of reducing dietary CP, reducing DCAD, or the combination of reducing CP and DCAD on lactation performance and manure NH₃ emissions. A total of 16 mid-lactating Holstein cows were used in a replicated 4 × 4 Latin square design with a 21-d period consisting of 14-d diet adaptation and 7-d sample collection. Cows were randomly assigned to: CON, a control diet (17.0% CP and 221 mEq/kg DCAD on a DM basis); RCP, a reduced-CP diet (15.2% CP and 221.4 mEq/kg DCAD on a DM basis); RDCAD, a reduced-DCAD diet (16.8% CP and -11 mEq/kg DCAD on a DM basis) supplemented with fatty acids (FA) enriched with C16:0 and a Met analog; and RCP-RDCAD, a reduced-CP and DCAD diet (15.4% CP and -11.5 mEq/kg DCAD on a DM basis) supplemented with the FA enriched with C16:0 and Met analog. Samples were collected from individual cows to determine total-tract nutrient digestibility, N utilization, blood metabolites, and milk FA composition. Manure of individual cows was incubated over 6 d using a continuous air flux chamber system to determine NH₃ emissions. Reducing dietary CP (RCP + RCP-RDCAD vs. CON + RDCAD) did not affect DMI but decreased milk yield (40.9 vs. 41.8 kg/d) and milk protein yield (1.38 vs. 1.42 kg/d). Furthermore, reducing CP tended to decrease NDF digestibility (47.4 vs. 49.2%). Plasma concentration of total EAA decreased for cows fed the reduced-CP diets. Reducing dietary CP increased milk N secretion (33.2 vs. 30.9%) and decreased urinary N excretion (29.9 vs. 32.9%) as a proportion of N intake. Manures of cows fed the reduced-CP diets decreased cumulative NH₃ emissions by 35%. Reducing DCAD (RDCAD + RCP-RDCAD vs. CON + RCP) decreased DMI, milk yield (40.0 vs. 42.7 kg/d), milk fat yield (1.79 vs. 1.90 kg/d), and milk protein yield (1.34 vs. 1.46 kg/d). Reducing DCAD tended to increase total-tract digestibility of NDF (49.1 vs. 47.4%) and decreased the concentration of trans-10 C18:1 in milk FA, probably because of supplementation of palmitic acid and a Met analog. Increases in BUN and MUN were observed when the reduced-DCAD diets were fed, suggesting inefficient N utilization. Various blood metabolites and gases were affected and urine pH decreased (6.41 vs. 8.01) when cows were fed the reduced-DCAD diets, suggesting compensated metabolic acidosis. The cumulative NH₃ emissions from manure decreased by 21% for cows fed the reduced-DCAD diets. An interaction between CP and DCAD for NH₃ emissions from manure occurred because RCP or RDCAD decreased NH₃ emissions but the mitigation for RCP-RDCAD was lower than the sum of the reductions observed for RCP and RDCAD separately. In conclusion, reducing CP or DCAD was effective in lowering NH₃ emissions from manure, and the combination further decreased NH₃ emissions although the mitigation was not additive. Despite these potential environmental benefits, the reduced-CP diet and reduced DCAD diet supplemented with palmitic acid and a Met analog decreased production performance and N utilization efficiency, and therefore these are not practical.