Chronological profiles of prolactin and growth hormone in lactating cows.

Prolactin and growth hormone were measured in blood plasma at 30-min intervals for three consecutive 24-h periods in 6 cows during midlactation. Concentrations of prolactin in milk and yields of milk also were quantified. Prolactin in blood plasma appeared to have a 24-h rhythm as well asd a 6-h rhythm by time series spectral analysis. Lowest prolactin occurred between 0600 and 0900 h each day, whereas maximums occurred between 1030 and 1600 h. Neither milk prolactin concentration nor yield of milk differed between milkings at 0530 and 1730 h. Concentration of prolactin in milk was not significantly correlated with prolactin concentration of plasma. However, prolactin of plasma averaged over the 12-h prior to milking was associated significantly with yield of milk and with total quantity of milk prolactin removed at that milking.