Last update: August 18, 2011 08:20:05 AM E-mail Print




S.W.P. Cloete, G.J. Delport*, G.J. Erasmus**, J.J. Olivier+,

H.J. Heydenrych++ & Elizabeth du Toit+++

Elsenburg Agricultural Centre, Private Bag, Elsenburg 7607


A random control population is generally maintained to evaluate genetic change in selection experiments. The assessment of realized genetic change in selection lines is thus sensitive to drift in such a control flock. Data regarding 18 months clean fleece mass (CFM) and fibre diameter (FD) of progeny from the Merino flock selected for increased CFM (Selection Flock; 3070 individuals) and the Control Flock (2529 individuals) at Tygerhoek were analysed to investigate genetic change over the interval 1969 - 1988. For this purpose an animal model was used to obtain predicted breeding values (PBV's) for all individuals by mixed model methods. Average PBV’s of Selection Flock progeny within birth years were taken as a measure of genetic change, rendering trends in this flock independent from genetic drift in the Control Flock. Due to slight genetic drift in CFM in the Control Flock, the genetic trend derived from PBV’s for Selection Flock individuals alone (1,09 % p.a.) differed (P ≤ 0,01) from that obtained by deviating phenotypic means of the Selection Flock from the Control Flock (0,93% p.a.). With regard to FD there was also evidence of genetic drift in the Control Flock. This led to a marked underestimation (P ≤ 0,01) of genetic change derived from phenotypic deviations from the Control Flock in the Selection Flock (0,03 % p.a.), compared to the trend based on average PBV’s of the Selection Flock only (0,14 % p.a.). While the underestimation of genetic change in the Selection Flock may not be serious in the case bf CFM, it is of more importance to note that the trend in FD would have remained undetected if mixed model methods did not become available.


*S.A. Fleece Testing Centre, Grootfontein Agricultural College, Middelburg 5900.

**Department of Animal Science, University of the O.F.S., Bloemfontein 9300.

+Karoo Region, Grootfontein Agricultural College, Middelburg 5900.

++Department of Animal Science, University of Stellenbosch. 7600.

+++Tygerhoek Experimental Farm, P.O. Box 25, Riviersonderend 7250.



Proceedings 29th SASAP congress