Abstract
Summary
Prolactin and growth hormone in serum of 10 lactating cows was measured by radioimmunoassay at hourly intervals on 4 days. Prolactin in lactating cows was released in erratic pulses throughout the day. Serum prolactin concentration followed a 24-hr periodicity (p < .01), with the highest value occurring at 4:00 p.m. (58 ng/ml) and lowest values being recorded between 4 and 10 a.m. (28 ng/ml). However, circadian changes accounted for only 8% of the total variation in prolactin values, differences within cows accounted for 45% and differences among cows accounted for 47%. Serum growth hormone concentration was more stable than prolactin and did not exhibit circadian periodicity.
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