Statistical models predicting embryo survival to term in cattle after embryo transfer.

Embryo survival to term in recipient cattle is highly variable. We examined calving data in the published literature to determine whether a model of binomial independence or a model which includes an embryo (e) and recipient term (r), adequately explain observed embryo survival rates following attempts to induce twin calving using transfer of two embryos. To achieve this we examined 32 published papers which provided us with 47 sets of data concerning 4560 recipients with either 0, 1 or 2 calves born. In each set of data, the observed embryo survival rate to term (p) (number of calves born/number of embryos) was calculated and the expected number of recipients with either 0, 1 or 2 calves born was determined, assuming a binomial distribution. Parameters for the second model were estimated using maximum-likelihood procedures. The model of embryo independence was rejected in 85% of the sets of data, suggesting that factors other than the embryo are important sources of variation in embryo survival or loss. The proposed e and r model of embryo survival adequately describes the published data in recipients receiving either single or twin embryos. In general, only 50-70% of embryos and recipients are sufficiently competent to result in a calving. Variation among laboratories producing either in vitro or in vivo derived embryos was due to variation in recipient and not embryo competence. It is argued that e rather than observed embryo survival rate, and r rather than observed pregnancy rate, should be used to compare differences among embryo treatments and groups of recipients, respectively. Acceptance of this proposition should permit faster progress in identifying the biology of superior embryos and recipients, which is a prerequisite to improving embryo survival rate in cattle. Collectively, the published data are not consistent with a model of embryo independence, and that a model of embryo survival to term which recognises recipient as well as embryo contributions to embryo survival may be more appropriate in cattle.