Gonadorelin hydrochloride

Virgin Angus heifers across two different years (year 1, n = 38; year 2, n = 28; total n = 66) during the month of April and located at a University of Tennessee AgResearch and Education Center were grazed on mixed grass pastures and had ad libitum access to hay. Average heifer age was 1.2 ± 0.1 yr. Estrus was synchronized according to Figure 1A and defined as the time when a heifer first stood to be mounted by another. Depending on the year, prostaglandin F_2α (PGF_2α) administered was dinoprost tromethamine (25 mg i.m.; Lutalyse, Zoetis, Parsippany, NJ, USA) or cloprostenol (500 μg i.m., Estrumate, Merck, Rahway, NJ, USA). Eleven days later, gonadotropin-releasing hormone (GnRH) was administered (100 μg i.m., gonadorelin hydrochloride, Factrel, Zoetis or 100 μg i.m., gonadorelin diacetate tetrahydrate, Cystorelin, Boehringer Ingelheim, Duluth, GA, USA). A controlled internal drug release (CIDR) device was placed intravaginally (1.38 g progesterone, Eazi-Breed CIDR, Zoetis). Seven days later, the CIDR was removed, PGF_2α was administered, and an iButton that had been previously affixed to a blank CIDR (containing no progesterone) was placed intravaginally. Beginning ~24 h after PGF_2α, estrus activity was visually assessed hourly until the first heifer displayed signs of estrus at which point monitoring was continuous by a team of individuals on a rotating schedule.