Water, sodium, potassium, and chlorine metabolism of dairy cows at the onset of lactation in hot weather.

Water, Na, K, and Cl balances, blood plasma composition, and urine and fecal outputs were studied in 5 high yielding cows (greater than 30 kg/d milk) at 2 wk prepartum and at 2 and 7 wk postpartum during the summer in Israel. Cows were fed complete diets with electrolyte content exceeding dietary recommendations. Plasma volume, as assessed by hematocrit changes, was greater postpartum, probably due to increased heat load and water turnover. Milk secretion markedly increased electrolyte output, which was compensated for only partially by increased intake. This was associated with marked reduction of electrolyte losses in excreta, particularly that of Na and Cl. On the basis of urea excreted in the urine, it seems that the current practice of abruptly increasing protein content of the diet at the onset of lactation might reduce the efficiency of dietary protein utilization, compared with efficiency of protein utilization at a later stage of lactation. The need to excrete excessive N also adversely affected the water and electrolyte balances. At initiation of lactation, when DMI is still limited and hot weather obstructs its rapid increase, the current recommendations for electrolytes as a percentage of the ration do not meet the needs.