Pre-natal growth in Romney X Southdown cross and Austrailan Merino sheep. II. Carcass conformation in the two breed groups

Abstract

The carcass conformation of the sheep foetus has been studied during the period from 50 to 140 days after conception by taking a series of linear measurements over the surface of the body. The mean lengths and the rates of increase in the length of such measurements, with increases in foetal age, are compared between the Romney southdown cross and the Australian Merino breed groups, and also between sexes within each breed group. Although these differences have been analysed on an age basis, the relationship between age and total body length is similar in the two breeds and therefore the same breed effects should also be found if total body length is taken as the primary growth measurement. Further, the rates of increase in length of the measurements of the parts of the body, relative to the rate of increase in total body length measured as curved crown-rump, have been estimated, foetal age being used as the independent variable rather than total body length. The results show that ram foetuses have larger head and neck measurements in both breed groups during early foetal life. The major difference between breeds is the greater width and circumference of the neck and trunk in the Romney x Southdown cross. In addition, the width of the head and the length of the nose are higher in the Romney x Southdown cross, while the humerus head is displaced forward and downward in this breed group, giving a relatively longer and deeper trunk and a shorter neck than the Merino. The allometric growth gradients are broadly similar to those for other mammals, and also to measurements obtained by other workers for the sheep during pre-natal life. No conclusive evidence was obtained to show whether or to what extent the breed differences resulted from alterations to the allometric growth rates or to changes in the dimensions of the parts during morphogenesis. The results do indicate, however, that the alterations to early-developing parts such as the head may result partly or wholly from changes in the initial dimensions, whereas changed in the allometric growth rates appear to produce the differences in the width and circumference of the neck and trunk. The latter effect may also be determined to a relatively greater extent by maternal influences.