Inbreeding and the inheritance of fertility in the thoroughbred mare

The reproductive efficiency of the thoroughbred mare in Britain and Ireland is low Weatherby's General Stud Book shows an average annual foaling rate of 67%. The primary aim of this study was to measure the extent to which the poor foaling rate was attributable to inbreeding. Coefficients of inbreeding were calculated and lifetime reproductive histories were assembled for 6550 mares at stud in Britain and Ireland in the early 1960s. The average degree of inbreeding for the five most recent generations was found to be only 0.01, there was little variation among animals in amount of inbreeding, and the mating of sibs or parents and offspring was rare. Coefficients of total inbreeding were calculated for a group of 60 mares. For the 21.5 recorded generations from the foundation of the breed in the seventeenth century to 1964 the average degree of inbreeding was 0.125. There was considerable variation among estimates for different animals, but this was mainly attributable to sampling within pedigrees.Each mare's lifetime reproductive performance was summarised as the proportion of her successful years at stud, adjusted for the decline in fertility with age, scaled to have an average of 1.0, and transformed to stabilise variance. An analysis of variance of the resulting fertility measures was carried out. Recent inbreeding was not an important source of variation in fertility since the mating of close relatives was rare.Although lower fertility was associated with inbreeding, the effect was not statistically significant. The largest source of variation in fertility was the number of years at stud and there were significant differences among year of birth groups. In addition, there were significant differences among groups of paternal half sibs, and the heritability of fertility was estimated as 0.077 ± 0.030. Over the recorded history of the breed it is probable that selection, both natural and artificial, has counteracted any effect of inbreeding on fertility.