Gonadotrophin-releasing hormone treatment in cattle: a meta-analysis 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 data from numerous trials into a single analysis) to evaluate the effects of injecting gonadotrophin-releasing hormone (GnRH) in cattle at the time of insemination on the risk of pregnancy. 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 GnRH and insemination number (first, second or third and greater). Mantel-Haenszel analysis revealed that use of GnRH at insemination significantly increased the overall risk of pregnancy by 12.5% in treated cows (P < 0.05). However, increases in risk of pregnancy were greatest (22.5%) when repeat breeders were tested. Results for use of GnRH or analogue at first service were similar, with increased risks of pregnancy 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 > or = 250 micrograms and when used at second insemination (9.9%). The analyses demonstrated that while treatment at insemination with GnRH and GnRH analogues increased conception rates in dairy cattle, some variation in study results was attributable to the number of inseminations after calving at which GnRH is administered. Further efforts should be made to determine characteristics of populations of cows that have good fertility responses to GnRH and to determine the mode of action of GnRH in increasing fertility.(ABSTRACT TRUNCATED AT 250 WORDS)