Selection for production and reproduction in South African Merino sheep is
always combined with selection based on visual appraisal and will, in all p
robability, remain so for many years to come. Heritabilities for acceptabil
ity were estimated using a threshold model to analyse data from two parent
Merino studs. Effects included in the analysis were sex, birth status, age
of dam and year of birth. An acceptable animal was defined as one that was
present in the stud at 18 months of age, implying that such an animal would
either be retained or sold for breeding purposes. This provided one thresh
old and two classes (acceptable or not acceptable). Records from the two st
uds used comprised 3011 progeny of 31 sires and 6495 progeny of 70 sires. E
rror variance was taken as unity and heritability estimated as h(2) = 4 sig
ma (2)(s)/1+ sigma (2)(s) where sigma (2)(s) is the sire variance. Heritabi
lity estimates were 0.210 and 0.448. The results suggest that selection imp
rovement for acceptability is possible, and that breeding values for this t
rait should be supplied for AI sires.