Effects of feeding a diet with reduced dietary cation and anion difference to lactating cows on production, nutrient digestibility, and ammonia emissions from manure

Abstract

Lowering dietary cation and anion difference (DCAD; Na + K - Cl - S) can reduce urine pH and this may lower manure pH, potentially mitigating ammonia emissions from manure. However, previous studies reported reducing DCAD can negatively affect DMI, production, and fiber digestibility. Therefore, the objective was to examine effects of a diet with reduced DCAD on urine pH and ammonia emitting potential of dairy manure. Also, production and nutrient digestibility were determined. Twenty Holstein cows (mean ± SD; 140 ± 15 days in milk and 40 ± 6 kg/d milk yield) were used in a randomized block design and fed a diet with DCAD of 220, 150, or 75 mEq/kg (n = 6, 7, and 7, respectively). A commercial acidogenic product (SoyChlor®) was included to decrease DCAD by mainly replacing soybean meal. The diets were formulated to be balanced for all nutrients. The experiment was conducted with a 10-d covariate period followed by 5 week for production. Spot fecal and urine samples were collected in week 5 to estimate fecal and urine output for nutrient digestibility and nitrogen excretion. The estimated fecal and urine outputs were used to reconstitute manure for individual cows and the manures were incubated for 5 d to monitor ammonia emissions using a continuous air-flux chamber system. Data were analyzed using the MIXED procedure of SAS. Reducing DCAD lowered (25.3–23.6 kg/d; P = 0.02) dry matter intake (DMI) in a linear manner. Milk yield, milk composition, energy-corrected milk, and feed efficiency were not different among treatments. Total tract apparent digestibility of dry matter and neutral detergent fiber did not differ among treatments. Urine N and urea-N excretion were quadratically affected with 150 DCAD being the lowest, which occurred due to slight differences in dietary crude protein content among diets. Estimated urine volume increased (21.7–24.1 kg/d; P = 0.04) and urine pH decreased (8.25 to 7.86; P < 0.01) in a linear manner with decreasing DCAD. However, this decrease of urine pH failed to lower manure pH significantly and ammonia emissions from manure. In conclusion, lowering DCAD of a lactation diet from 220 to 75 mEq/kg decreased DMI but did not affect milk yield and composition and total tract nutrient digestibility. In addition, reducing DCAD to 75 mEq/kg did not affect ammonia emissions from manure, suggesting that further reducing DCAD below 75 mEq/kg is worth studying.