PHYSIOLOGY AND MANAGEMENT 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 temperatures were hot enough to cause a measurable stress response in cows in order to determine the effect of GnRH administration during estrus on fertility and subsequent luteal function of cows under these conditions. The mean ranges for ambient temperature, relative humidity, and temperature-humidity index during the study were 21.4 to 32.8"C, 67 to 95%, and 21.6 to 29.6, respectively. After injection of PGF2,y for synchronization of estrus, cows were alternately injected with 100 pg of either GnRH or saline (control) at detection of estrus, followed by AI 10 to 12 h later. From 14 treated cows and 14 control cows, blood samples were drawn by venipuncture just prior to treatment and at 5-d intervals until 30 d after treatment. Serum progesterone concentrations were determined by radioimmunoassay. Mean progesterone concentrations were higher for the cows treated with GnRH than for the controls. The pregnancy rate from first AI was 28.6% for all treated cows and 17.7% for control cows. On d 20, 42.8% of the treated cows and 57.1% of the control cows exhibited progesterone concentrations that were typical of pregnancy. When