Maltz, E., Agricultural Research Organization, Volcani Center, PO Box 6, Bet Dagan, 50250, Israel
Silanikove, N., Agricultural Research Organization, Volcani Center, PO Box 6, Bet Dagan, 50250, Israel
Shalit, U., Agricultural Research Organization, Volcani Center, PO Box 6, Bet Dagan, 50250, Israel
Berman, A., Hebrew University of Jerusalem, Faculty of Agriculture, Rehovot, 76100, Israel

The presumption that water, Na, K, and Cl homeostasis is affected by the initiation of lactation, especially in hot weather, was examined. The study was conducted using five Israeli Holstein cows at 1 to 2 wk before parturition (period 1) and at 2 wk (initiation of lactation) and 7 wk (established lactation) postpartum (periods 2 and 3, respectively). In period 1, most water intake was related to feed (in the absence of overt Na or osmotic stimuli). In contrast, during lactation, drinking was probably a combined response to feed-related and hypertonicity stimuli. Diurnal fluctuations in plasma ions, frequency of drinking, and volume ingested at each drinking increased postpartum. In period 2, plasma ion concentrations still were regulated around mean values, which was not the case in period 3, during which large diurnal variation in mean values occurred. The lowest values and highest variabilities were recorded in period 3 at midday. With the transition from the dry period to lactation, plasma concentrations of Na and Cl were reduced and became more labile, but the relationships between them remained stable. Plasma K seemed to vary independently of the two other ions. The reduced homeostatic efficiency of plasma ions during early and established lactation in hot weather may have consisted of an accumulation of their deficiencies, enhanced excretion of K in sweat, and sequestration of Na and Cl in the rumen. © 1994, American Dairy Science Association. All rights reserved.