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Influence of the Stage of Lactation and Environmental Temperatures on the Salt Balance of Milk¹

Department of Dairy Husbandry, University of Missouri, Columbia

ABSTRACT

Salt balance in milk was determined under controlled climatic laboratory conditions with two groups of six nonpregnant Holstein cows at comfort temperature (65° F.) and heat (80 and 90° F.). Each group consisted of two early-, mid-, and late-lactation cows. The two groups were alternately exposed (switch-back design) at 2-wk. intervals to comfort temperature (65° F.) and heat (80 or 90° F.) in the following successive temperatures: 65, 80, 65, 90, 65, and 90° F.

At 65° F. the average values for salt balance, i.e. (citric acid + phosphorus) – (calcium + magnesium), phosphorus, magnesium, and milk yield were, respectively, 0.55 Mol. x 10^–3/100 ml., 88 mg/100 ml., 10.9 mg/100 ml., and 36 lb/day, and at 80° F. they were, respectively, 0.29 Mol. x 10^–3/100 ml., 82 mg/100 ml., 10.7 mg/100 ml., and 33 lb/day.

High temperatures depressed citric acid and calcium contents during early lactation. The values at 65, 80, 65, and 90° 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 sodium and sodium/potassium ratio, but it decreased potassium in all lactation periods.

With the progress in the stage of lactations at comfort temperature (65° 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 three 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 potassium; and 4.2, 4.6, and 6.0 for milk yield. Sodium and sodium/potassium ratios consistently increased at all lactation periods, with average rates of 2.2 and 2.9% per week, respectively.

Phosphorus and magnesium declined during early- and mid-, and increased during late-lactation periods. Calcium decreased during early lactation and increased at later stages of lactation.

¹ Contribution from the Missouri Agricultural Experiment Station, Journal Series No. 2250. Approved by Director.

² Present address: UT-AEC Agricultural Research Laboratory, Box 1067, Oak Ridge, Tennessee.

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