GENETIC PARAMETER ESTIMATES FOR FITNESS TRAITS IN SOUTH-AFRICAN MERINO SHEEP

The main objectives of this study were to estimate heritabilities and the range of sire breeding values for reproductive traits and survival rate in two Merino hocks using a threshold model. A GFCAT set of prog rammes was used to analyse reproductive data collected on the Grootfon tein Merino stud (from 1968 to 1996) and the Carnarvon Merino flock (f rom 1964 to 1983). Fitness traits analysed were fertility (whether a e we lambed or not; 0 or 1), fecundity (number of lambs born to a ewe th at lambed; 1, 2 or 3), litter size at birth (number of lambs born per ewe mated; 0, 1, 2 or 3), litter size at weaning (number of lambs wean ed per ewe mated; 0, 1, 2 or 3) and survival rate (whether a lamb born alive, was dead or alive at weaning; 0 or 1). The estimated heritabil ities on the underlying scale for the respective traits were 0.072, 0. 173, 0.131, 0.092 and 0.000 for the Grootfontein Merino stud and 0.203 , 0.311, 0.201, 0.183 and 0.000 for the Carnarvon Merino flock. The he ritability estimates and the range in sire breeding values indicate th at it would be possible to improve reproduction, but not survival rate , genetically through selection. (C) 1998 Elsevier Science B.V.