Growth and development of Scottish Blackface and Icelandic sheep

Abstract

Two studies of growth and development in sheep were undertaken, one at Edinburgh, the other in Iceland, between 1977 and 1979. The main objectives were twofold: (1) to examine the 'normal' pattern of growth and development, in relation to presently acknowledged growth principles; and (2) to evaluate genetic and sexual influences, with special emphasis on the effects of conformation on growth and carcass characteristics.

Two breeds of sheep were involved; at Edinburgh, the Scottish Blackface, and in Iceland, the Iceland sheep. Common to both experi­ments was the comparison of different genotypes of the same breed, distinct in external body form. These had been created by continuous selection, over 20 - 25 years, the sole criterion in Edinburgh being live weight corrected cannon bone length, while in Iceland, other criteria of conformation were included in addition to cannon bone length. Both experiments involved the serial slaughter and full anatomical dissection of lambs from the time of birth to weights approaching maturity. At Edinburgh, controlled individual feeding was involved after weaning, whereas in Iceland, most lambs were slaugh­tered off pasture.

The data have been analysed in several different ways, including, for relative growth, the use of Huxley's allometric equation, the computation of relative weight increases, based on the weight at birth, and the comparison of percentage proportions at different weights or ages.

Significant differential growth patterns were demonstrated at all levels of the anatomy. Frequent changes in these made the application of Huxley's formula unsafe over extended periods of growth. The devel­opmental orders of the various body organs, parts or tissues were, in the main, consistent with present ideas. However, some questions were raised, particularly regarding certain aspects of skeletal and muscular development, and these are discussed in light of the present findings. Sexual differences were clearly apparent, both in absolute and relative terms. The males grew faster than the females and were later maturing, as indicated particularly by delayed fat deposition. With respect to carcass proportions, masculinity was expressed in superior development of the neck and thorax arising from all constituent tissues. By contrast, the females were better developed in the hind quarter, which gave them an advantage in muscle weight distribution over the normal range iti slaughter age.

While genotype differences in live weight gain and feed conversion efficiency were inconclusive, significant effects were demonstrated on the relative development of the different body parts and tissues at any given age or weight. In Iceland, the selection for a short cannon bone and compact body form had grossly reduced skeletal weight, increased muscular thickness, the muscle: bone ratio and the proportion of back muscles, at the cost of neck and thorax muscles, and brought forward fat deposition, resulting in earlier attainment of marketable condition. The Edinburgh selection was less effective in altering carcass shape or composition, due to the break-down of intraskeletal proportions, the greatest selection effects being exerted on the lower limb bones and gradually diminishing in approaching the body trunk. The practical implications of these findings are discussed, and it is concluded, that breeding can effectively alter the animal's constitution, both as regards the tissue composition and the area to area distribution of the carcass.