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Acta sci. vet. (Impr.); 41: Pub. 1130, 2013. tab

Artigo em Português | VETINDEX | ID: biblio-1372271

Resumo

Background: Efficient reproduction performance is important for optimal profitability on dairy operations. Unfortunately, most dairy farms do not attain optimal reproduction due to many factors including management, health, and physiology of high-producing dairy cows. The physiology limitations involved in reproduction of lactating dairy cows are complex and becoming worse as milk production increases. But many variables related to the interactions between nutrition, the complexity of some hormonal systems, and altered reproductive patterns in dairy cattle have been elucidated. This review will focus on the role of progesterone (P4) in reproduction of the lactating dairy cow. Review: Progesterone is a steroid hormone primarily secreted by the corpus luteum (CL) and placenta. Adequate circulating P4 concentrations are essential for establishment and maintenance of pregnancy. Thus, because of the central role of progesterone on fertility, this manuscript reviews the effect of progesterone (P4) during timed AI protocols in lactating dairy cows. Two sections summarize how P4 is produced and a model attempts to explain how P4 is metabolized in high producing dairy cows. Circulating P4 is determined by a balance between P4 production and P4 metabolism and stimulatory and inhibitory pathways regulating this balance are complex. However in dairy cattle, the volume of luteal tissue is a primary factor regulating P4 production; although, inadequate P4 is generally due to high metabolism of P4 resulting from extremely elevated liver blood flow. If P4 production is increased by an increase in luteal tissue without a change in liver blood flow, then circulating P4 will increase. Conversely, if there is an increase in liver blood fl ow then there will be a corresponding decrease in circulating P4 even though P4 production has not been altered. Thus, an understanding of regulation of circulating P4 must carefully consider the factors regulating P4 production and its metabolism. As will be discussed below, over 80% of P4 production during the estrous cycle of the cow is due to constitutive P4 production by the large luteal cell. Regulation of constitutive P4 production by the CL does not appear to require stimulatory pathways but can be dramatically decreased during luteolysis induced by exogenous or endogenous prostaglandin F2α (PGF2α). Conversely, although liver enzymes involved in P4 metabolism can be regulated, we speculate in this model that the primary determinant of P4 metabolism in the lactating cow is related to the rate of liver blood flow. These concepts are more fully discussed below. Three sections summarize the role of P4 concentrations prior to breeding, near the time of breeding, and after breeding. During timed AI protocols, elevations in P4 are achieved by increasing number of CL by ovulation of accessory CL or by supplementation with exogenous P4. Elevating P4 prior to the timed AI generally decreases double ovulation and can increase fertility to the timed AI. Near the time of AI, slight elevations in circulating P4 can dramatically reduce fertility with inadequate luteolysis to the PGF2α treatment prior to timed AI, being the underlying cause of this problem. After AI, circulating P4 is critical for embryo growth and establishment and maintenance of pregnancy. Many studies have attempted to improve fertility by elevating P4 after timed AI with primarily marginal elevations (<5%). Conclusion: Previous research has provided substantial insight into mechanisms regulating circulating P4 concentrations and actions. Understanding this prior research can focus future studies on P4 manipulation to improve timed AI protocols.

Biblioteca responsável: BR68.1