Pregnancy rate in peripuberal beef heifers following treatment with syncro-mate-B and gonadotropin releasing-hormone.

ABSTRACT

A study was conducted to evaluate the effectiveness of treatment with Syncro-Mate-B (SMB) and gonadotropin releasing-hormone (GnRH) on inducing a fertile estrus and/or ovulation in peripuberal beef heifers. Two hundred and twenty-three, 13 to 15 month peripuberal beef heifers, primarily of Angus or Simmental breeding, were alternately allotted by weight and age within breed to a non-treated control group (C) and three groups which received the SMB treatment regime. Treated groups differed in that one group was inseminated approximately 12 hr after detected in estrus (T) while two other groups were inseminated at a fixed time after implant removal (T-G-I42, T-I48). Heifers in group T-G-I42 received 125 mug GnRH at 30 hr, and were inseminated 42 hr after implant removal. The remaining group of SMB-treated heifers received no additional hormone therapy but were inseminated 48 hr after implant removal (T-I48). Age had no significant effect on estrous response or pregnancy rate nor were there significant age by weight interactions on these parameters. Weight was a significant source of variation on interval from start of breeding to pregnancy (P<.01) and the effect on interval to estrus approached statistical significance (P<.0.9). In control heifers, increased weight had a positive effect on estrous response during the first 21 days of breeding (P<.05). Conversely, weight had no effect on estrous response during the 5-day synchronized period in heifers treated with SMB and observed for estrus (group T). Cumulative estrous response in control heifers was 10, 47, 48 and 55% in 5, 21, 27 and 45 days of breeding, respectively. Cumulative estrous response in SMB-treated heifers obsrved for estrus (group T) was 88, 91, 91 and 91% in 5, 21, 27 and 45 days of breeding, respectively. Pregnancy rate after 5 days of breeding was 9, 27, 18 and 30% for heifers in groups C, T, T-G-I42 and T-I48, respectively. Pregnancy rate after 21 days of breeding for heirfers in groups C, T, T-G-I42 and T-I48, respectively, was 33, 38, 21 and 41%; after 27 days of breeding was 36, 43, 40 and 48%; after 45 days of breeding was 48, 61, 51 and 69% and after the total 88-day breeding season was 59, 68, 71 and 72%. While a large percentage of peripuberal beef heifers with adequate age but insufficient weight were induced into estrus with the SMB treatment, pregnancy rates were low. Timed insemination programs with or without GnRH do not appear to be effective in improving pregnancy response in non-cyclic beef heifers.