Fertility is an important component of herd production efficiency, as each additional oestrous cycle that does not result in a planned pregnancy adds to the cost of dairy farming. In addition to the negative impact on milk production, the high costs of veterinary intervention, ve-insemination and herd replacement, subfertility can affect the rate of genetic gain in traits of economic merit. In contrast with the steady increase in average milk yield per cow during the last 30 years, there has been a decline in conception rate to artificial insemination in both the USA and in the UK. The genetic correlation between yield and fertility has been equivocal. Attempts to improve the reproductive efficiency in dairy cattle through breeding and selection have been frustrated to date by the lack of heritable reproductive parameters conducive to high fertility. However, the traditional fertility parameters of interval to first service, services per conception, days open and calving intervals are highly influenced by managerial decisions and have, as expected, heritabilities too low to permit a meaningful genetic gain through selection. An alternative approach is to use the growing body of evidence that the majority of endocrine factors affecting reproduction are a result of gene expression at the hypothalamic, pituitary, ovarian or uterine level. Mechanisms such as commencement of post-partum cyclicity, follicle wave patterns, manifestation of oestrus, luteal competence and level of embryo mortality ave appropriate for study. Research is required to investigate the genetic component of the variation between animals in these parameters, their phenotypic and genetic correlations with fertility and their association with other production traits. In summary: (a) subfertility is a syndrome with multiple causes and only the symptom in common; (b) improvement in fertility will continue to be frustrated until recorded traits provide move accurate estimates of breeding values; (c) techniques are now available to estimate the genetic variation in physiological components conducive to improved reproductive efficiency; (d) once the heritable components of fertility are identified, these tools could be introduced into progeny testing and breeding nuclei, from which the genetic improvement can be widely disseminated. Selection for those components with sufficient genetic variation will result in the improvement of the integral endocrine and other physiological mechanisms favourably correlated with high fertility (e) these tools may also assist in detecting quantitative trait loci for faster genetic gain through markers-assisted selection.
