GONADOTROPIN-RELEASING-HORMONE TREATMENT IN CATTLE - A METAANALYSIS OF THE EFFECTS ON CONCEPTION AT THE TIME OF INSEMINATION

Data from 40 trials described in 27 published papers were analysed by meta-analysis (pooling of date from numerous trials into a single anal ysis) to evaluate the effects of injecting gonadotrophin-releasing hor mone (GnRH) in cattle at the time of insemination on the risk of pregn ancy. A total of 19 019 cows were involved in these studies. Analyses were stratified by trial and by the effects of dose of GnRH, type of G nRH and insemination number (first, second or third and greater). Mant el-Haenszel analysis revealed that use of GnRH at insemination signifi cantly increased the overall risk of pregnancy by 12.5% in treated cow s (P < 0.05). However, increases in risk of pregnancy were greatest (2 2.5%) when repeat breeders were treated. Results for use of GnRH or an alogue at first service were similar, with increased risks of pregnanc y in treated cows of 5.2 and 8.0%, respectively. The risk of pregnancy tended to be higher (11.1%) when the dose of GnRH was greater-than-or -equal-to 250 mug and when used at second Insemination (9.9%). The ana lyses demonstrated that while treatment at insemination with GnRH and GnRH analogues increased conception rates in dairy cattle, some variat ion in study results was attributable to the number of Inseminations a fter calving at which GnRH is administered. Further efforts should be made to determine characteristics of populations of cows that have goo d fertility responses to GnRH and to determine the mode of action of G nRH in increasing fertility. Meta-analysis proved to be a useful techn ique for evaluating the apparently conflicting results from trials and in evaluating the effect of factors such as insemination number dose and type of GnRH on responses to treatment.