Changing dietary electrolyte balance for dairy cows in cool and hot environments.

Two Latin square studies, each containing eight primiparous cows (four Holstein, four Jersey), were conducted to determine the effect of changing dietary electrolyte balance during cool and hot environmental conditions on performance of lactating dairy cows. Electrolyte balance, expressed as Na + K - Cl in milliequivalents per kilogram of diet, was altered by changing K and Cl content in the diet using potassium bicarbonate or calcium chloride. Maximum and minimum temperatures averaged 26.7 and 15.0 degrees C during the cool phase and 32.3 and 22.5 degrees C during the hot phase of the study. Milk yield improved linearly with increasing electrolyte balance with nonsignificant treatment by phase interaction, whereas DMI of cows improved quadratically with increasing dietary electrolyte balance. A treatment by phase interaction for DMI was detected, although intake of DM reached a plateau at a similar dietary electrolyte balance during the cool and hot phases. Body (milk) temperature of cows appeared to be related to the level of feed consumed and varied by treatment within phase. Body (milk) temperature was higher during the hot phase of the experiment. Blood bicarbonate and pH were lowest in cows offered the low electrolyte balance (high Cl) diet, and blood and urinary Na + K-Cl increased linearly with increasing dietary electrolyte balance. The response to dietary electrolyte balance appeared to be mediated through blood buffering and the impact on physiologic systems of the cow.