- Response to selection in a group-breeding scheme for Merino Sheep
|Last update: March 30, 2012 01:57:05 PM|
Response to selection in a group-breeding scheme for Merino Sheep
G.J. Erasmus and C. V. Pettit, S.A. Fleece Testing Centre,
Grootfontein. P B X529. Middelburg Cape, 5900
THE "Lowestoffe Sheep Improvement Group" was formed in 1971. Together with "The Downs" group, it was the first group-breeding scheme initiated in South Africa. Both operate with Merino sheep and are situated in the Cathcart district. The "Lowestoffe" group consists of a nucleus of approximately 750 ewes in a total ewe population of approximately 12 000. After visual culling, selection is carried out on measured body mass and clean fleece mass with a restriction on mean fibre diameter. Half of the annual ewe replacement requirements of the nucleus are derived from group members while the other half are selected from the ewe progeny born in the nucleus, affording a culling percentage of approximately 60% in the latter case.
As in any commercial enterprise, it is difficult to measure actual genetic progress, owing to the lack of a genetic stable control. However, the average phenotypic values of the four traits with their fitted trend lines plotted from 1974 (when numbers started to stabilize), shown in FIG. 1, do suggest that a fair amount of genetic change has taken place in the case of the "Lowestoffe" group. A similar analysis of data from "The Downs" group has not yet been made.
From FIG. 1 it is clear that both traits under positive direct selection, viz. body mass and clean fleece mass, showed a marked response. This could, to a large degree, be attributed to environmental change, but, if an improvement in environment had been solely responsible, an increase in mean fibre diameter, one of the components of clean fleece mass, would be expected. A marked increase (in the absence of direct selection) was obtained in staple length, which is also a component of clean fleece mass as is body mass which also increased, but mean fibre diameter showed no increase. It is, therefore, fair to suggest that selection has been effective in increasing fleece and body mass without an increase in mean fibre diameter or, conversely, if the increase was due to environmental improvement, the relative contribution of mean fibre diameter to fleece mass has been reduced.
Irrespective of how these results are interpreted, the economic significance of the genetic change should be viewed against the background of the growing importance of mean fibre diameter in determining the price paid for wool (Erasmus, 1981).
ERASMUS, G.J., 1981. Die teeltverbetering van Merinoskape- op pad na 2000. Karoo Agric. 2, (2).
Karoo Agric 2 (3), 17