Behaviour and Welfare THE IMPROVEMENT OF LAMB SURVIVAL OF MERINO SHEEP AS A CORRELATED RESPONSE TO DIRECT SELECTION FOR REARING ABILITY

Data were available for peri-natal lamb survival (PNS), lamb survival from 4 days to weaning (S4DTW), birth weight (BW) and birth coat score (BCS) in South African Merino sheep that were selected divergently from the same base population for their ability to rear multiples. These data spanned the period 1986-2008, and were subjected to genetic analyses involving two three-trait linear-threshold analyses, entailing PNS and S4DTW analysed along with the other traits. The lamb survival trait considered was treated as a categorical trait in both instances. The line selected in the upward direction was denoted the high line (H line) and the line selected downward as the low line (L line). Heritability was estimated at 0.15 for PNS, 0.20 for S4DTW, 0.16 for BW and 0.60 for BCS. Corresponding estimates for maternal heritability amounted to 0.16, 0.10, 0.25 and 0.06 and to 0.11, 0.05, 0.08 and 0.03 for the dam permanent environment. Expressed relative to the overall mean, direct and maternal breeding values for PNS improved at respectively 0.61% p.a. and 0.46% p.a. in the H line. Corresponding genetic trends in the L line amounted to respectively 0.26% p.a. and -0.96% p.a. Direct and maternal genetic trends for S4DTW amounted to respectively 0.70% p.a. and 0.16% p.a. in the H line and to respectively -0.26% p.a. and -0.26% p.a. in the L line. It was concluded that genetic change in lamb survival would accrue when selection is based on a trait like maternal multiple rearing ability.