A meta-analysis of studies of the effect of GnRH 11-14 days after insemination on pregnancy rates in cattle.

A number of studies have used GnRH between 11 and 14 days after insemination to improve pregnancy rates in cows, however published results have not been consistent. We wished to investigate whether a consolidated estimate of the response could be quantified. Therefore we conducted a meta-analysis of the available data, 19 studies from 14 published papers, on the effect of GnRH analogs on pregnancy rate when given between Days 11 and 14 after first insemination. Odds ratios (the relative probability of pregnancy between treated and control cows) were compared for each study and found to vary significantly among studies, ranging from 0 to 22%. We showed by logistic regression analysis that response to GnRH treatment varied with cow type (beef or dairy), age (cow or heifer), estrus synchronization (synchronized or natural), pregnancy diagnosis (method and time) and effect of individual study. When all these variables were included only 6 of 14 publications could be analyzed, representing 2,541 instead of 10,945 cows. In this limited subgroup of data, a significant improvement in pregnancy rate was detected among treated cattle (odds ratio = 1.33; P<0.01).