Getting Cows Pregnant and Keeping Them Pregnant Requires Progesterone

Abstract

Early embryonic mortality in sheep and cattle averages 38%, with the majority of losses occurring prior to 30 days post-conception. This loss is primarily due to two factors: 1) genetic abnormalities in the embryo (7 to 10%), and 2) inadequate secretion of progesterone at critical times (approximately 30%). Supplementation of progesterone in cattle increased pregnancy rates by 13 to 30%. Thus, the requirement for continued secretion of progesterone has been clearly documented. Therefore, it is important to understand the complex hormonal interactions between the developing embryo, uterus, pituitary gland and ovary that regulate the secretion of progesterone from the corpus luteum. The mechanisms whereby the pregnant uterus prevents regression of the corpus luteum, which occurs at the end of a normal reproductive cycle, are complex and sensitive to a number of environmental and management factors. These factors include heat stress, level of nutrition, genetics and general herd health. Any biological process which retards the growth of the embryo may result in early embryonic mortality due to a lack of synchrony between the embryo and the uterus. The basic physiological mechanisms involved in establishing and maintaining pregnancy are presented and discussed.