Abstract
Bovine plasma progesterone concentrations were determined in beef heifers before and after mating. In 20 heifers (group I), plasma was collected every other day from day 16 of an estrous cycle until mating and on the third day after mating, when the ova were recovered. Another 20 heifers (group II) were bled at 3-day intervals for 39 days beginning on the third day after mating. Fertilization was determined on the basis of cleavage and presence of spermatozoa in the zona pellucida, and pregnancy was based on palpation per rectum. A yield of 16 cleaved ova was obtained from group I. In group II, 12 of 20 heifers were pregnant 42 days after mating. Progesterone was measured by the competitive protein binding method. Progesterone levels declined to below 1.0 ng/ml, 0 to 2 days before estrus. There was an inverse relation between progesterone concentrations during the 2-day period before estrus and the number of blastomeres in the ovum 3 days after mating. As early as the 9th day after mating, progesterone concentrations in heifers that returned to estrus 18 to 20 days after mating were significantly lower than in pregnant heifers. Over the first 15 days after mating, pregnant heifers had about 1.7 times more progesterone present in peripheral plasma than those that returned to estrus. Copyright © 1971. American Society of Animal Science . Copyright 1971 by American Society of Animal Science.