15 THE ADMINISTRATION OF GONADOTROPIN-RELEASING HORMONE AROUND THE FIRST POSTPARTUM MONTH INCREASES SUBSEQUENT REPRODUCTIVE PERFORMANCE IN DAIRY COWS

Abstract

Early postpartum cyclicity is related to subsequent fertility. This field study evaluated the effect of gonadotropin-releasing hormone (GnRH) administration around the first postpartum month on subsequent reproductive performance in dairy cows. Holstein dairy cows underwent ultrasonography at 30.0 (±6) days postpartum to determine the existence of ovarian structures including corpus lutea (CL), follicles, or cysts. The cows without CL were divided randomly into 2 treatment groups: GnRH group and control group. Cows in the GnRH group received an injection of 100 µg of GnRH (gonadorelin, Godorel, Uni-Biotech Co. Ltd., Korea; n = 151). Cows in the control group received no GnRH (n = 171). Cows diagnosed with endometritis at the time of ultrasonography were excluded from this study. Cows received the normal herd reproductive management program including estrous detection or synchronization, or synchronization of ovulation in the breeding period. The voluntary waiting period from calving to first AI in this study was 40 days. Pregnancy was determined per rectum 60 days after AI by both ultrasonography and palpation. Reproductive performance data were collected for a minimum of 210 days postpartum. Reproduction data between the GnRH and control groups were evaluated by chi-square test or the Cox proportional hazard model with the PHREG procedure using the SAS program (version 9.1, SAS Institute Inc., Cary, NC, USA). The number of days to first insemination after calving did not differ (P > 0.05) between the control group (80.9 ± 2.2 days) and the GnRH group (82.1 ± 2.3 days), while the pregnancy rate to first insemination tended to be higher (P = 0.1) in the GnRH group (42.3%) than in the control group (33.3%). Furthermore, the pregnancy hazard was higher by 210 days in milk for the GnRH group compared with the control group (hazard ratio = 1.33; P < 0.05), which resulted in a 26-day reduction in the median interval to pregnancy. In conclusion, the administration of GnRH around the first postpartum month improved subsequent reproductive performance in dairy cows.

This work was carried out with the support of the Cooperative Research Program for Agriculture Science & Technology Development (project no. PJ008464) Rural Development Administration, Republic of Korea.