EFFECT OF GONADOTROPIN-RELEASING-HORMONE AT ESTRUS ON SUBSEQUENT LUTEAL FUNCTION AND FERTILITY IN LACTATING HOLSTEINS DURING HEAT-STRESS

The experiment used lactating Holstein cows (n = 94) from three herds in Mississippi. The experiment was conducted during late summer when t emperatures were hot enough to cause a measurable stress response in c ows in order to determine the effect of GnRH administration during est rus on fertility and subsequent luteal function of cows under these co nditions. The mean ranges for ambient temperature, relative humidity, and temperature-humidity index during the study were 21.4 to 32.8 degr ees C, 67 to 95%, and 21.6 to 29.6, respectively. After injection of P GF(2 alpha) for synchronization of estrus, cows were alternately injec ted with 100 mu g of either GnRH or saline (control) at detection of e strus, followed by AI 10 to 12 h later. From 14 treated cows and 14 co ntrol cows, blood samples were drawn by venipuncture just prior to tre atment and at 5-d intervals until 30 d after treatment. Serum progeste rone concentrations were determined by radioimmunoassay. Mean progeste rone concentrations were higher for the cows treated with GnRH than fo r the controls. The pregnancy rate from first AI was 28.6% for all tre ated cows and 17.7% for control cows. On d 20, 42.8% of the treated co ws and 57.1% of the control cows exhibited progesterone concentrations that were typical of pregnancy. When pregnancy was diagnosed in these cows after d 45, pregnancy rates had decreased significantly for cont rols but not for cows given GnRH at estrus, suggesting greater embryon ic survival in treated cows. We concluded that GnRH treatment enhanced secretion of luteal progesterone and embryo survival.