Embryonic and early foetal losses in cattle and other ruminants.

Embryo survival is a major factor affecting production and economic efficiency in all systems of ruminant milk and meat production. For heifers, beef and moderate yielding dairy cows, does and camelids it appears that fertilization generally lies between 90% and 100%. In high-producing dairy cows there is a less substantive body of literature, but it would appear that it is somewhat lower and perhaps more variable. In cattle, the major component of embryo loss occurs before day 16 following breeding with some evidence of greater losses before day 8 in high-producing dairy cows. In cattle late embryo loss, while numerically much smaller than early embryo mortality loss, nevertheless, causes serious economic losses to producers because it is often too late to rebreed females when they repeat. In multiple ovulating small ruminants, the loss rate is positively related to ovulation rate. Systemic concentrations of progesterone, during both the cycle preceding and following insemination, affect embryo survival rate with evidence that too high or indeed too low a concentration being negatively associated with survival rate. Uterine expression of mRNA for progesterone receptor, oestradiol receptor and retinol-binding protein appears to be sensitive to changes in peripheral concentrations of progesterone during the first week after artificial insemination. Energy balance and dry matter intake during 4 weeks after calving are critically important in determining conception rate when cows are inseminated at 70-100 days post-calving. Concentrate supplementation of cows at pasture during the breeding period has minimal effects on conception rates though sudden reductions in dietary intake should be avoided. For all systems of milk production, more balanced breeding strategies with greater emphasis on fertility and feed intake and/or energy balance must be developed. There is sufficient genetic variability within the Holstein breed for fertility traits. Alternative dairy breeds such as the Jersey or Norwegian Red could also be utilized. Genomic technology will not only provide scientists with an improved understanding of the underlying biological processes involved in fertilization and the establishment of pregnancy, but also, in the future, identify genes responsible for improved embryo survival. Its incorporation into breeding objectives would increase the rate of genetic progress for embryo survival.