Influence of the Stage of Lactation and Environmental Temperatures on the Salt Balance of Milk

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Abstract
Salt balance in milk was determined under controlled climatic laboratory conditions with 2 groups of 6 non-pregnant Holstein cows 65[degree]F and heat (80[degree] and 90[degree]F). The two groups were alternately exposed (switch-back design) at 2 week intervals to 65[degree]F. and heat (80[degree] or 90[degree]F.) in the following successive temperatures 65, 80, 65, 90, 65, and 90[degree]F. At 65[degree]F the average values for salt balance, i.e. (citric acid + P) minus (Ca + Mg), P, Mg, and milk yield were, respectively, 0.55 Mx 10-3/100 ml, 88 mg/100 ml, 10.9 mg/ 100 ml and 36 lb/day, and at 80[degree]F. they were, respectively, 0.29 M, x 10-3/100 ml, 82 mg/100 ml, 10.7 mg/100 ml, and 33 lb/day. High temperatures depressed citric acid and Ca contents during early lactation. The values at 65, 80, 65, and 90[degree]F. were 181, 159, 150, and 148 mg/100 ml, respectively, for citric acid, and they were 107, 105, 104, and 103 mg/100 ml., respectively for calcium. Heat caused no marked change in Na and Na/K ratio, but it decreased K in all lactation periods. With advancing lactation at 65[degree]F., the early-, mid-, and late-, lactation groups decreased exponentially in salt balance, citric acid, potassium and milk yield. The percentage decline per week for the 3 lactation groups in sequence were 1.3, 5.8, and 12.2 for salt balance; 1.4, 1.1, and 2.1 for citric acid; 0.5, 0.6, and 1.9 for K; and 4.2, 4.6, and 6.0 for milk yield. Sodium and Na/K ratios consistently increased at all lactation periods, with average rates of 2.2 and 2.9% per week, respectively. P and Mg declined during early-, mid-, and increased during late-lactation periods. Ca decreased during early lactation and increased at later stages of lactation.