Levels of Luteinizing Hormone and Progesterone During the Estrous Cycle and Early Pregnancy in Mares

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Abstract
Jugular venous blood was collected from 16 mares at daily intervals throughout the estrous cycle. Twelve of these mares were bred to fertile stallions. Of the twelve, four were palpated daily throughout the estrous cycle, four were palpated daily during estrus and four were not palpated. The remaining four mares were not palpated or bred and blood samples were collected at 6-h intervals during estrus. Serum was harvested from all blood samples by centrifugation. The serum samples were analyzed for luteinizing hormone (LH) and/or progesterone by radioimmunoassay. Levels of LH in serum increased gradually beginning at the onset of estrus and reached a peak (85 ± 18 ng/ml) approximately 24 h prior to the end of estrus. The levels of LH was 10-15 ng/ml (baseline) throughout diestrus. Levels of progesterone were low (<1 ng/ml) prior to the LH peak, began to rise just prior to the onset of diestrus and reached a plateau (4-10 ng/ml) between Days 5 and 14 of diestrus. Levels of progesterone during the 12-day period after estrus were not affected (P<0.05) by palpation or breeding.

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