Effect of variable water intake as mediated by dietary potassium carbonate supplementation on rumen dynamics in lactating dairy cows.

Water is a critical nutrient for dairy cows, with intake varying with environment, production, and diet. However, little work has evaluated the effects of water intake on rumen parameters. Using dietary potassium carbonate (K2CO3) as a K supplement to increase water intake, the objective of this study was to evaluate the effect of K2CO3 supplementation on water intake and on rumen parameters of lactating dairy cows. Nine ruminally cannulated, late-lactation Holstein cows (207±12d in milk) were randomly assigned to 1 of 3 treatments in a replicated 3×3 Latin square design with 18-d periods. Dietary treatments (on a dry matter basis) were no added K2CO3 (baseline dietary K levels of 1.67% dietary K), 0.75% added dietary K, and 1.5% added dietary K. Cows were offered treatment diets for a 14-d adaption period followed by a 4-d collection period. Ruminal total, liquid, and dry matter digesta weights were determined by total rumen evacuations conducted 2h after feeding on d 4 of the collection period. Rumen fluid samples were collected to determine pH, volatile fatty acids, and NH3 concentrations, and Co-EDTA was used to determine fractional liquid passage rate. Milk samples were collected twice daily during the collection period. Milk, milk fat, and protein yields showed quadratic responses with greatest yields for the 0.75% added dietary K treatment. Dry matter intake showed a quadratic response with 21.8kg/d for the 0.75% added dietary K treatment and 20.4 and 20.5kg/d for control and the 1.5% added dietary K treatment, respectively. Water intake increased linearly with increasing K2CO3 supplementation (102.4, 118.4, and 129.3L/d) as did ruminal fractional liquid passage rate in the earlier hours after feeding (0.118, 0.135, and 0.141 per hour). Total and wet weights of rumen contents declined linearly and dry weight tended to decline linearly as dietary K2CO3 increased, suggesting that the increasing water intake and fractional liquid passage rate with increasing K2CO3 increased the overall ruminal turnover rate. Ruminal ammonia concentrations declined linearly and pH increased linearly as K supplementation increased. As a molar percentage of total volatile fatty acids, acetate increased linearly as dietary K increased, though propionate declined. Increasing dietary K2CO3 and total K in the diets of lactating dairy cows increased water consumption and modified ruminal measures in ways suggesting that both liquid and total ruminal turnover were increased as both water and K intake increased.