Levels of Hormones in the Serum of Cycling Beef Cows

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Abstract
Concentrations of progestins, luteinizing hormone (LH), estrogens and adrenocorticosteroids were determined in peripheral blood serum during the estrous cycle of three nonlactating beef cows. Blood was drawn every 2 hr. for 60 days from indwelling jugular cannulas. Progestins and corticoids were measured by a competitive protein binding method in daily samples, LH by a double antibody radioimmunoassay in the samples from each 2-hr. bleeding, starting several days prior to the first estrus of each cow and estrogen levels by a solid phase radioimmunoassay in daily samples until 72 hr. prior to the LH peak, then in samples drawn at 4-hr. intervals. Progestin levels were lowest at estrus (1.31 ± .84 ng/ml), remained below 2 ng/ml until day four after estrus, then increased to a peak (6.15 ± 1.47 ng/ml) on day 15 of the cycle. LH levels showed a sharp peak 7.38 ± 5.15 hr. after the onset of estrus, which lasted 12 hours. Levels remained below 1.8 ng/ml until the next estrus, when another peak was observed. Increases seen at other times were not consistent. Progestins and LH were significantly (P<.05) and negatively correlated (r=.47). Estrogen levels did not differ significantly between days of the estrous cycle but individual estrogens were confounded by the nature of the assay employed. However, they rose prior to the LH peak to suggest involvement with LH release. Corticosteroid levels were not significantly different between days of the estrous cycle but were significant between cows, reflecting age, disposition or reaction to sample collection. Corticosteroid levels appear to be a good measure of the stress imposed by sampling. Copyright © 1974. American Society of Animal Science. Copyright 1974 by American Society of Animal Science

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