Progesterone Concentration in Blood of Mares during the Estrous Cycle, Pregnancy and after Hysterectomy

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Abstract
Serum progesterone concentration was determined by radioimmunoassay on the day of necropsy for mares which were killed on day 7 of estrus and day 2, 6, 10, 14 or 18 of diestrus (three or four mares/day). Progesterone concentrations on day 7 of estrus and day 18 of diestrus were less (P< .05) than on day 6, 10 or 14 of diestrus. Mean progesterone concentrations for days 6, 10 and 14 did not differ significantly. Progesterone concentration was determined in plasma of 17 pregnant, six nonpregnant and 12 hysterectomized mares. Plasma was collected serially on days 7, 10, 13, 16 and 19 after the end of estrus. Additional determinations for progesterone were made in six of the pregnant mares on days 32, 44, 56, 68, 80,92, 104, 116, 128, 140, 154, 168, 182, 196 and 210 and in six of the hysterectomized mares on days 32, 44, 56, 68, 80, 92, 104, 116, 128 and 140. There were no significant differences between pregnant and hysterectomized mares on days 7 to 19. On day 19 progesterone concentration was greater (P< .05) for pregnant mares and hysterectomized mares than for nonpregnant mares. Regression analyses indicated that over days 7 to 19 there was a linear decrease in progesterone concentration in both pregnant and hysterectomized mares, but there was a sharp decline (quadratic regression line) after approximately day 13 in nonpregnant mares. In hysterectomized mares, progesterone concentration continued to decrease in a linear fashion after day 19. However, progesterone concentration increased significantly between days 32 and 44 in pregnant mares. The mean progesterone concentration in pregnant mares after day 44 continued to increase to approximately day 90 and decreased between days 150 and 180; during this time the concentrations were quite variable among mares and the differences among mares were significant. Copyright © 1974. American Society of Animal Science . Copyright 1974 by American Society of Animal Science.