A study of fertility in two-tooth ewes

Abstract

In general farming practice in New Zealand maiden awes are not mated at puberty but are carried forward unmated under conditions more or less favourable to full growth and development until the second breeding season after birth. They thus bear their first lamb at an age of 2 years. Puberty may occur at 9 or 10 months of age and although it is the experience of many farmers that fertile matings can and do occur at this age, their occurrence has been found too unreliable, even if desirable, for adoption into general practice. These observations result from unintentional matings often occurring as late as June or July well after the normal mating period. No accurate data exists in this country as to the frequency of oestrus, level of fertility and size and vitality of lambs born to immature sheep or of their productivity in subsequent years. It is generally held that the unreliability of conception and the penalty which early pregnancy places on the growth and development of the young animal in its first critical winter period make the practice most unsound. Doubt is also expressed as to the ability of the immature animal to rear the lamb adequately and provide the milk necessary for its rapid growth. Because full growth and development at the hogget stage is considered necessary to lifetime productivity it was decided to compare the performance in their first productive year of normal well grown hoggets with those penalised from weaning by a poor inadequate level of feeding. These comparisons were expected to yield information on the following points: (1) The effect of nutritional treatment on the nature and time of onset of the breeding season. (2) The influence of other environmental factors common to both groups in determining the characteristics of the breeding season. (3) The regularity and pattern of oestrus after initiation. (4) The effects of nutritional regime on stock health and mortality. (5) The effects of treatment on the number of viable lambs born and the incidence of multiple births within the groups. Although not specifically designed to study influences other than nutrition, the data gained has been used to support considerations of a more fundamental nature in relation to the initiation of sexual activity in seasonal breeding animals. Two such important aspects have been given consideration. It is recognised that without careful design reliable analyses of environmental conditions are difficult. Under field conditions they interact freely and it is seldom easy to gain critical information on one or the other without creating some degree of artificiality or by imposing extremes which are seldom found in the natural environment. This defect can be noted in the nutritional regimes imposed. Although the well grown hoggets received conditions comparable with normal good management of the district, the poorly fed group, were subjected to severe conditions seldom if ever pertaining on this class of land. This was done to accentuate the possible results of the treatment and to throw into greater relief their response to subsequent management after the conclusion of this part of the investigation. Studies are being continued by Lincoln College with these animals on the lifetime productivity of ewes harshly reared in this apparently critical stage of development to detect the degree of permanence, if any, of the initial handicap and the resilience they display in responding to normal conditions.