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Plasma sodium, potassium and aldosterone levels, daily milk production and milk sodium and potassium were measured in 10 Holstein X Friesian cows during a whole lactation period beginning in November and ending in November the following year. The milk production (4 p. 100 fat content) of these animals was 6 170 +/- 66 kg (mean +/- SEM). During the whole experimental period, the cows had free access to salt blocks and were thus always sodium-replete. Plasma sodium and potassium levels showed no significant variations during lactation. The daily excretion of sodium and potassium through milk paralleled that of daily milk production. Plasma aldosterone levels decreased sharply from 77.4 +/- 4.0 pg.ml-1 at calving to 13.2 +/- 3.6 pg.ml-1 (P less than 0.01) on day 7 of lactation, then remained stable until day 50 (16.4 +/- 4 pg.ml-1). They increased slightly on day 155 (60 days after mating: 36 +/- 5 pg.ml-1; P less than 0.05) and abruptly after spring grazing (54.9 +/- 11 pg.ml-1; P less than 0.01), then remained high until the end of lactation (48.5 +/- 12 pg.ml-1). Plasma aldosterone levels were 11.9 +/- 2.4 pg.ml-1 in seven 24-month old, non-pregnant heifers fed the same winter ration as the cows. No relationship could be demonstrated between sodium and potassium concentrations in blood and milk or between those parameters and plasma aldosterone levels. Thus, in high-yield dairy cows, aldosterone does not seem to play a major role in the regulation of sodium and potassium excretion through milk.

Sodium and potassium in blood and milk and plasma aldosterone levels in high-yield dairy cows