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A novel procedure using a gonadotropin-releasing hormone agonist to increase pregnancy rates in lactating dairy cattle
An earlier postovulatory increase in circulating progesterone (P4) concentration is associated with greater pregnancy success, likely through P4 alteration of the uterine environment conducive for embryo survival. We have shown that chronic administration of a gonadotropin-releasing hormone (GnRH) agonist (Deslorelin; 1μg/kg BW-1 Day-1) beginning d 3 of the estrous cycle increases mean and basal secretion of luteinizing hormone, size of the corpus luteum (CL), and circulating P4 concentrations in beef heifers. The objectives of the present studies were to increase circulating P4 concentrations and pregnancy rates in lactating dairy cows with chronic administration of Deslorelin. Luteal volumes and P4 concentrations were analyzed using a mixed model procedure with repeated measures. Pregnancy rates were analyzed using a Chi Square Analysis. Data were considered different at P < 0.05. Deslorelin treatment for 12 d (d 3 – 15; d 0 = estrus) increased CL volumes in primiparous lactating dairy cows (n = 4) compared to untreated primiparous lactating dairy cows (n = 6). Circulating P4 concentrations were increased in cows treated with Deslorelin as compared to controls. In a small pilot pregnancy study, cows of various parity received Deslorelin treatment for 7 d (d 3-10) or 12 d (d 3 – 15) following estrous synchronization and timed artificial insemination (d = 0). Pregnancy rates were 50% greater in cows treated with Deslorelin 7 d (n=10; P < 0.05) compared to controls (n=10). No difference was observed in pregnancy rates for cows treated 12 d with Deslorelin (n=10) compared to controls. In summary, chronic administration of Deslorelin altered luteal volume and increased circulating P4 concentrations in primiparous lactating cows only, albeit not during the early postovulatory critical period. Chronic administration of Deslorelin for 7 d increased pregnancy rates in cows although no differences in circulating P4 concentrations were observed. None of the animals in these experiments exhibited insufficient luteal function as defined by circulating P4 concentrations less than 1 ng/ml; therefore, it is unlikely that the induced P4 concentrations by Deslorelin treatment resulted in increased pregnancy rates. These findings provide evidence that chronic administration of a GnRH agonist increases pregnancy rates in dairy cattle. The mechanism by which chronic administration of a GnRH agonist increases pregnancy rates is unknown, but could potentially be a direct effect of Deslorelin acting on the oviduct or uterine horn since GnRH receptors have been identified in bovine uterine tissue.
Keywords: dairy cow, GnRH-agonist, pregnancy rates