The genetic basis of growth, oestrous behaviour and fertility in Romney Marsh ewes : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Animal Science at Massey University
The rate of reproduction or fertility of the ewe in a breeding flock determines not only the number of surplus animals available for sale as a source of income but in addition, affects the scope of selection for improvement in all productive characters. The ewe's fertility therefore is a character whose economic significance is more nearly matched by its biological ramifications than other attributes of the sheep. The extensive usage of the Romney Marsh breed in New Zealand both in breeding flocks and as the basic ewe breed for meat production only adds importance to the need for higher fertility among the ewes of this breed. Although it is known that under New Zealand conditions, Romney ewes can be mated as hoggets (between four and 14 months in age) for lamb production (Apps 1953; Lewis 1959), the common practice is to delay first mating until they are on average about 18 months old. This system of flock management has a two-fold effect on improvement of fertility by mass selection. First, selection for fertility can not take place until after the first lambing and second, being a corollary of the first, the annual rate of genetic gain in fertility is reduced by the increase in generation interval. Therefore, so long as the current system of flock management remains in practice, there is an obvious need to investigate hogget characters which may be used as criteria of potential fertility at the time of selection for replacements when the ewes are less than 18 months old.