METABOLISM OF WATER, SODIUM, POTASSIUM, AND CHLORINE BY HIGH-YIELDINGDAIRY-COWS AT THE ONSET OF LACTATION

We studied the balance of Na+, K+, Cl-, and water in six high yielding (>39 kg/d of milk) cows between wk 2 to 1 prepartum and at 2 and 7 wk postpartum during winter in Israel. Cows were fed complete diets; Na and Cl- contents exceeded dietary recommendations, and K+ content was equal to dietary recommendations. Milk yield was related positively a nd significantly to retention of Cl- and K+, indicating that ions that are the main constituents of sweat can limit the ability of cows to e xpress full genetic potential. The highest ion retention was recorded for cows that had the highest dry matter intake and, hence, the highes t ion intake. Retention of Cl- was highest for cows that were most eff icient in retaining Cl- in the kidney. In hot climates, increasing the concentrations of ions in the diet of early lactation cows according to the actual dry matter intake could prevent or reduce the severity o f ion deficiencies. Water turnover rate of the cows was dependent on d ry matter intake, milk yield, and respiratory-cutaneous water loss. Th e milk-free water balance (water turnover rate minus water secreted in milk) could be very efficiently predicted for lactating and nonlactat ing cows by the following equation: milk-free water balance (kilograms per day) = digestible energy intake (megacalories per day) x 0.58 + r espiratory-cutaneous loss (kilograms per day) x 0.97 (n = 18; R-2 = 0. 97). This formula provides a tool to assess the evaporative-cutaneous water loss from feed and water intake measurements to evaluate the sev erity of heat stress.