Prolongation of the bovine estrous cycle with a gonadotropin-releasing hormone analog

A series of in vitro and in vivo experiments was conducted to determine the effects of gonadotropin-releasing hormone (GnRH) on bovine luteal function. Biosynthesis of progesterone by bovine luteal cells during a 2-h incubation was determined following addition of 0, 10, 20, 40, 80 and 100 ng GnRH. Synthesis of progesterone was significantly depressed only by the 100-ng dose of GnRH. Luteal cells were incubated with 0.1 and 1.0 microgram GnRH in the presence and absence of 0, 1, 2 and 5 ng bovine luteinizing hormone (LH). Again, only the highest dose of GnRH significantly depressed LH-stimulated production of progesterone. Three experiments were conducted to assess the effects of repetitively administered GnRH. In the first, twice daily intrauterine infusions of 100 micrograms GnRH on Days 12, 13 and 14 of the bovine estrous cycle was without effect on plasma concentrations of progesterone and the functional life span of the corpus luteum (CL). In the second experiment, 10 micrograms of a highly potent GnRH analog (GnRH-A) was injected subcutaneously four times daily on Days 9-12 of the estrous cycle. GnRH-A-treated heifers had longer (P less than 0.05) mean estrous cycle lengths (26.2 +/- 0.72 days) when compared to control heifers (20.25 +/- 0.25 days). Plasma concentrations of progesterone were higher in the GnRH-A-treated group on Days 9-13 and Days 15-22 of the estrous cycle.(ABSTRACT TRUNCATED AT 250 WORDS)