Kidney function and nitrogen balance of high yielding dairy cows at the onset of lactation.

The balance of water, Na+, K+, Cl-, and total solute in the kidney and the N balance of the whole body were measured for six high yielding cows at 2 wk prepartum (period 1) and at 2 wk (period 2) and 7 wk (period 3) postpartum during winter and autumn in Israel. The onset of lactation was not followed by an increase in glomerular filtration rate and urine volume. To meet the deficiency in Na+, K+, and Cl- ions, the kidney conserves them during early lactation, mainly by increasing tubular reabsorption. An interdependent relationship exists between the excreted urea and nonurea solutes. Thus, reduction of ion concentrations in urine during early lactation reflects both the need to excrete a large load of urea and the effort to conserve Na+, K+, and Cl-. The high load of urea during early lactation was a result of a negative N balance that was most likely related to a negative energy balance. We concluded, through a comparison of our results with those of a similar study during summer, that the role of the kidney in ion conservation is more pronounced during summer than during winter and that the recovery of cows from a negative balance of energy and N is faster in summer than in winter.