Some aspects of dystocia in sheep with particular reference to Romney stud ewes : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Veterinary Science at Massey University
Three studies were undertaken to investigate dystocia in Romney stud ewes. In the initial study an analysis was made of 4,400 individual performance records collected over a period of nine years from a Romney stud flock. On the basis of the records, dystocia in the flock was attributed to a physical disproportion between the lamb and the maternal pelvis, and its incidence was shown to be highly correlated with the mean birth weight of single lambs (r=0.84). Dystocia appeared to be unrelated to the effects of parity, weight at mating, or litter size of ewes. A high repeatability of the condition was demonstrated. Differences between sires with respect to dystocia in their progeny at birth were demonstrated in some years and were shown to be related to lamb weight, such that those sire groups with higher average lamb weight experienced the greatest incidence of dystocia. The decline in the incidence of dystocia, apparent from the records, was attributed to a reduction in mean birth weight of lambs and a possible increase in pelvic dimensions of the ewes, brought about by selective breeding. Secondly, a tocometric study of 18 Romney ewes was made using intra-amniotic open ended catheters and intra-abdominal balloons implanted at laparotomy. Dystocia occurred in 12 ewes and, with the exception of one ewe that developed uterine inertia, was associated with abnormalities in presentation, position or posture (maldisposition) of the lamb at birth. By comparison, the presentation, position and posture (disposition) of the lambs born to the eutocous ewes was normal. A significant difference in birth weight of lambs was demonstrated between ewes that gave birth to lambs in normal disposition as opposed to those that gave birth to maldisposed lambs, and between ewes that experienced eutocia and dystocia. Uterine activity during late pregnancy, characterised by alternate periods of activity and quiescence, was not related to the disposition of the lamb at birth. During first stage labour uterine activity was higher in those ewes that experienced dystocia. However, this was considered to be a reflection of the higher lamb birth weight in this group as birth weight was correlated with uterine activity during the final three hours of first stage labour. Low abdominal bearing down effort was thought to contribute to the lack of progress during parturition in three ewes with posteriorly presented foetuses. Except for one case of uterine inertia, low uterine pressures or abdominal bearing down effort was not implicated as a primary cause of dystocia in these ewes. In the final study a method of radiographic pelvimetry was developed which was used to study the relationship between pelvic size and lamb size at birth in ewes with histories of eutocia and dystocia. In addition, three different age groups of ewes were radiographed to provide information on changes in pelvic dimensions with age. Large differences in respect to the relationship between estimated area of the pelvic inlet and the size of the lamb were demonstrated between ewes with histories of eutocia and dystocia. The relatively large pelvic area of the eutocous ewes was considered to be largely responsible for differences in the recorded incidence of dystocia between the groups. Estimates of correlations between internal and external pelvic measurements taken from the radiographs were thought to be too low to be of practical use in selection for larger internal pelvic area. Furthermore the relationship between internal and external pelvic measurements was found to alter with age in some cases, indicating that the effect of age on these relationships would have to be examined further if selection by means of external pelvic measurements were to be attempted. As a result of these investigations, the hypothesis was advanced that dystocia in Romney stud ewes is commonly caused either by the relatively small size of the pelvic inlet of the ewe, or the relatively large size of the lamb, or both. There were indications from the first and third studies that the flock incidence of dystocia can be reduced by genetic selection.