Reducing dietary cation-anion difference to mitigate ammonia emissions from manure without compromising production of...

Previously, we observed potential decreases in ammonia emissions from manure when a diet with reduced DCAD was fed to cows. However, DMI, milk yield, and milk fat yield were negatively affected. The objective of this experiment was to evaluate the effects of a reduced DCAD diet supplemented with palmitic acid or a Met analog to alleviate the risk of the negative production responses to feeding a low DCAD diet but still decrease ammonia emissions from manure. Sixteen mid-lactation Holstein cows were assigned to a replicated balanced 4 × 4 Latin square design with 4 dietary treatments: a control diet (CON; 190.6 mEq/kg DM of DCAD), reduced DCAD (rDCAD; 16.1 mEq/kg DM), rDCAD supplemented with palmitic acid (rDCAD-PA), and rDCAD supplemented with a Met analog (2-hydroxy-4-(methylthio)butanoic acid; rDCAD-MA). Each period lasted 21 d, including 14 d of adaptation and 7 d of sampling. Feces and urine collected from 2 squares of cows (n = 8 per treatment) were used for manure incubation to measure ammonia emissions for 7 d using a continuous air flux multichamber system. Dry matter intake and milk yield were lower (25.9 vs. 26.9 kg/d and 36.8 vs. 38.6 kg/d, respectively) for the low DCAD diets (i.e., rDCAD, rDCAD-PA, and rDCAD-MA) compared with CON. Milk fat concentration and yield did not differ between the low DCAD diets and CON, but they were greater (4.71% vs. 4.31% and 1.73 vs. 1.57 kg/d, respectively) for rDCAD-PA and rDCAD-MA compared with rDCAD. The increase in milk fat yield for rDCAD-PA occurred with increasing the proportion of mixed FA in milk FA. Although milk protein concentration was not affected by treatments, protein yield was lower (1.22 vs. 1.27 kg/d) for the low DCAD diets compared with CON. Milk and BUN concentrations were greater for the low DCAD diets compared with CON and tended to be and was greater, respectively, for rDCAD-PA and rDCAD-MA compared with rDCAD. Feeding the low DCAD diets increased blood Cl^- concentration and decreased blood pH, HCO₃^-, and base excess and urine pH compared with CON, reflecting a physiological shift toward compensated metabolic acidosis. Because of the decreases in urine pH, feeding the low DCAD diets lowered cumulative ammonia emissions from manure (73.2 vs. 89.1 g/cow) compared with CON, and rDCAD-PA and rDCAD-MA had no effect on ammonia emissions from manure compared with rDCAD. In conclusion, reducing DCAD from 190 to 16 mEq/kg DM lowered urine pH and resulted in decreasing ammonia emissions from manure by 18% compared with CON. Feeding the low DCAD diets decreased DMI, milk yield, and milk fat yield. However, the supplementation with palmitic acid or the Met analog restored milk fat yield and increased ECM to the level of CON.