METABOLIC AND PRODUCTIVE RESPONSES OF DAIRY-COWS TO INCREASED ION SUPPLEMENTATION AT EARLY LACTATION IN WARM WEATHER

We found previously that the current recommendations for Na+, K+, and Cl- contents in the diet do not meet the needs of lactating cows. The responses of cows receiving a ration with increased amounts of Na+, K, and Cl- (E cows) were compared with those of cows consuming the same ration with a fir;ed concentration of these ions (C cows) between wee ks 2 and 8 post partum (PP).,Milk. protein: fat and lactose yields, an d dry matter intake between weeks 2 and 4 PP were higher in E than in C cows. These differences did not occur between weeks 4 and 8 PP, main ly because of a higher incidence of PP complications in E cows. A grea ter increase in plasma insulin-like growth factor-1 concentration in E than in C animals during weeks 2 and 3 PP was consistent with the mil k responses. A reduction in aldosterone concentration in E cows in wee ks 2 and 3 PP was a consequence of their Na+ requirements being: satis fied as a result of their enhanced Na+ intake. A subsequent elevation in aldosterone concentration in E animals was probably related to a mo derate excess in K+ intake. This increase in aldosterone explains the urinary potassium loss that was detected at week 6 PP. The absence of differences between E and C cows in plasma renin activity was consiste nt with an absence of differences in urine volume and with the apparen t utilization of the enhanced ion intake for body functions.