Effect of repeated injections of GnRH on reproductive parameters in postpartum anestrous dairy cows

To induce cyclicity in dairy cattle with prolonged postpartum anestrous, repeated dosages of gonadotrophin releasing hormone (GnRH) were administered. Twenty-one (21) Holstein dairy cows and heifers calving between October 1, 1989, and January 1, 1990, at the Louisiana State University Dairy were used in the study. The animals were defined as anestrous if their plasma progesterone remained < 1.0 ng/ml until 32 to 36 days post partum. They were randomly assigned to one of two treatment groups. Group 1 (n=6) received two injections 1 hour apart of a GnRH analogue (50 mug) (i.m.). The treatment was repeated twice weekly at 3- to 4-day intervals. Group 2 controls (n=6) received saline (1 ml, i.m.) on the same schedule as Group 1. A maximum of 12 to 13 treatments were given. Cattle that had plasma progesterone >1.0 ng/ml by 32 to 36 days post partum were identified as Group 3, or cyclic contemporaries (n=9). Postpartum anestrous in the herd was 46.2% (18 39 ). Cows in Group 1 had significantly fewer days to first plasma progesterone > 1.0 ng/ml than those in Group 2 (P < 0.05), but more days than Group 3. Cows in Group 1 also had significantly fewer treatments to induce plasma progesterone > 1.0 ng/ml than those in Group 2 (P < 0.05). There were no significant differences among treatment groups in the number of days from calving to first observed estrus or the number of days open (P > 0.05).