Gonadotrophin-releasing hormone in postpartum dairy cattle: a meta-analysis of effects on reproductive efficiency

The efficacy of gonadotrophin-releasing hormone (GnRH) or synthetic analogues in improving the reproductive performance of dairy cows less than 40 days postpartum has not been established. It was postulated that disparity observed between the results of similar trials may have arisen from differences in study design, including the dosage of GnRH used; the number of days after calving at which GnRH was administered; the concurrent use of prostaglandins to induce oestrus; and the enrolment of cows with an abnormal puerperium. The results of 24 trials, extracted from 12 research papers were assessed using meta-analysis. When all trials were considered, treatment with GnRH did not significantly alter the number of days to first oestrus or first service or the relative risk of pregnancy at first service (P>0.05). While treatment did significantly reduce the number of days open by 2.75 days and the number of services per conception by 0.05 services, the results of these studies were heterogenous (P < 0.001) and the validity of pooled estimates questionable. The results of the subgroup of studies that enrolled only cows with a normal puerperium were homogenous for all outcomes examined (P>0.43), although none of the pooled estimates were significant. Sensitivity analyses, performed by excluding the consistently outlying results of one study, improved the homogeneity of all outcomes (P>0.03) and produced a significant reduction of 4.52 days to first oestrus in treated cows. Pooled estimates derived without the results of the outlying study were considered more valid indicators of the direction and likely magnitude of effect than those derived in the heterogenous overall analyses. The results of this meta-analysis showed that while blanket treatment of dairy cows in the postpartum period may reduce the number of days to first oestrus, subsequent reproductive performance is unaltered. The study also demonstrated a need to concentrate further research on the potential for treatment with GnRH during the postpartum period to improve the reproductive performance of cows with an abnormal puerperium.