| A meta-analysis was conducted to examine potential empirical relationships between dietary cation-anion difference (DCAD3) (Na + K - Cl) and the performance and acid-base balance of lactating dairy cows. The database was developed from 12 studies that included a total of 17 trials, 69 dietary treatments, and 230 cows. The results suggested that milk yield and feed intake increased quadratically with DCAD3 up to 34 and 40 meq/100 g of dry matter (DM), respectively. Blood pH and HCO3 - increased with DCAD3, indicating improved acid-base status of the lactating cows. The effect of DCAD3 was likely mediated via modification of acid-base physiology in the cows.; The objectives of two subsequent experiments were to examine the effect of DCAD4 (Na + K - Cl - S) on performance and acid-base status of cows in early lactation, to evaluate the interaction of DCAD4 and dietary protein, and to study the potential role of dietary protein in systemic acid-base regulation.; The first experiment utilized six lactating Holstein cows, averaging 44 days in milk, in a 6 x 6 Latin square design with a 2 x 3 factorial arrangement of treatments: DCAD4 of -3, 22, or 47 meq/100 g of DM and 16 or 19% crude protein (CP) on a DM basis. Results indicated that DM intake, milk fat percentage, and 4% fat-corrected milk production increased linearly with increasing DCAD4; whereas, milk production itself was unaffected by DCAD4. Blood pH, jugular venous HCO3 - concentration, and urine pH increased, but blood Cl concentration, and urine titratable acidity and net acid excretion decreased linearly with increasing DCAD4. Elevated ratio of plasma essential amino acids (AA) to nonessential AA with increasing DCAD4 indicated that N metabolism in the rumen was affected, probably a result of more microbial protein flowing to the small intestine. The DM intake, milk production, milk composition, and acid-base status did not differ between 16 and 19% CP treatments. Feeding 16% dietary CP to cows in early lactation, compared to 19% CP, maintained lactation performance while lowering milk and urine urea N excretion.; In the second experiment, 16 Holstein and 8 Jersey multiparous cows were used immediately after calving to compare two DCAD4 (22 and 47 meq/100 g of DM) in a completely randomized design. An additional 2.3 kg of alfalfa hay was fed during the first 5 days postpartum, and then milk, blood, and urine samples were collected weekly for 6 weeks. Repeated-measures, with an extra between-cow effect, mixed model analysis indicated that DCAD 4 did not affect DM intake, milk production, milk composition, and jugular venous pH and HCO3- concentration. Elevated concentration of plasma branched chain AA and ratio of essential AA to total AA in cows with DCAD4 of 47 versus 22 meq/100 g of DM indicated N metabolism in the rumen was affected, probably a result of more microbial protein flowing to the small intestine. Higher net acid excretion in cows with DCAD4 of 22 versus 47 meq/100 g of DM suggested that net acid excretion was a much more sensitive indicator of acid load than blood acid-base parameters in cows postpartum. |
