GONADOTROPIN-RELEASING-HORMONE IN POSTPARTUM DAIRY-CATTLE - A METAANALYSIS OF EFFECTS ON REPRODUCTIVE EFFICIENCY

The efficacy of gonadotrophin-releasing hormone (GnRH) or synthetic an alogues in improving the reproductive performance of dairy cows less t han 40 days postpartum has not been established. It was postulated tha t disparity observed between the results of similar trials may have ar isen from differences in study design, including the dosage of GnRH us ed; the number of days after calving at which GnRH was administered; t he concurrent use of prostaglandins to induce oestrus; and the enrolme nt of cows with an abnormal puerperium. The results of 24 trials, extr acted 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 rela tive risk of pregnancy at first service (P > 0.05). While treatment di d significantly reduce the number of days open by 2.75 days and the nu mber of services per conception by 0.05 services, the results of these studies were heterogenous (P < 0.001) and the validity of pooled esti mates questionable. The results of the subgroup of studies that enroll ed only cows with a normal puerperium were homogenous for all outcomes examined(P > 0.43), although none of the pooled estimates were signif icant. Sensitivity analyses, performed by excluding the consistently o utlying 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 di rection and likely magnitude of effect than those derived in the heter ogenous overall analyses. The results of this meta-analysis showed tha t while blanket treatment of dairy cows in the postpartum period may r educe the number of days to first oestrus, subsequent reproductive per formance is unaltered. The study also demonstrated a need to concentra te further research on the potential for treatment with GnRH during th e postpartum period to improve the reproductive performance of cows wi th an abnormal puerperium. (C) 1997 Elsevier Science B.V.