Effects of an agonist of gonadotrophin releasing hormone (Buserelin) in cattle. III. Pregnancy rates after a post-insemination injection during metoestrus or dioestrus

Abstract

Groups of lactating dairy cows in each of four trials were injected once intramuscularly with 5 μg Buserelin (Bus) from 1 to 13 days after (post) first insemination (days 1–13 PFI). Two trials also included groups injected with 10 μg Bus from days 7 to 13 PFI. Uninjected cows inseminated on the same days as injected herdmates were used as control groups in pregnancy rate (PR) comparisons based on results of uterine palpation from 6 to 9 weeks after insemination. The average first insemination PR for all cows in the control groups ( n = 1013) was 65.7% compared to 61.9% among cows injected with 5 μg Bus ( n = 988). The PR difference (Bus vs controls) was greatest among cows injected during the early luteal phase of the cycle (days 1–6 PFI; 61.8 vs 71.9%) compared with later stages of dioestrus (days 7–13 PFI; 62.0 vs 63.6%). The 10 μg Bus treatment did not affect PR when injected during days 7–10 PFI (64.9 vs 65.4%), but increased PR by 11.5% during days 11–13 PFI (72.4 vs 60.9%). The average second insemination PR among cows previously injected with 10 μg Bus during days 11–13 PFI was increased by 15.6% (85.1 vs 69.5%). A Bus injection after day 6 PFI reduced the proportional incidence of return to service intervals of less than 20 days, particularly among cows treated on days 11–13 PFI when the percentage of these return intervals among cows injected with 5 or 10 μg Bus was 8.7% and 1.6% respectively compared with 20.1% in the control group. The increase in PR and the altered distribution in return to service intervals produced by an injection of 10 μg Bus during days 11–13 PFI appeared to be dose related and was probably associated with delayed luteolysis previously reported in unmated cycling dairy cows treated with this GnRH analogue.