Genetic and environmental parameters for early growth and yearling traits of the Elsenburg South African Mutton Merino resource flock

The Elsenburg South African Mutton Merino (SAMM) resource flock has been maintained at the Elsenburg Research Farm in the Western Cape, South Africa since 1955. Early growth and lamb survival data (record numbers from 9512 for weaning weight or WW to 13101 for lamb survival LS) over the 66-year period from 1955 to 2020 and yearling weight and wool data (record numbers from 1351 the coefficient of variation of fibre diameter or CVFD to 3505 for clean yield or CY) over a 38-year period from 1983 to 2020 were used to estimate genetic and environmental parameters for the flock. The ASREML package was used to estimate fixed effects and genetic parameters. Birth year, sex, birth type and age of dam significantly (P < 0.001) influenced the weight traits and LS. The interaction between birth year and sex was evident in most early growth traits. Direct single-trait heritability estimates amounted to 0.11 +/- 0.02 for birth weight (BW), 0.04 +/- 0.01 for WW, 0.03 +/- 0.01 for LS, 0.28 +/- 0.04 for yearling weight (YW), 0.62 +/- 0.04 for CY, 0.36 +/- 0.04 for clean fleece weight (CFW), 0.33 +/- 0.05 for staple length (SL), 0.56 +/- 0.06 for CVFD, and 0.65 +/- 0.04 for fibre diameter (FD). Maternal heritability estimates were 0.17 +/- 0.02 for BW, and 0.06 +/- 0.01 for WW. Maternal permanent environmental effects amounted to 0.08 +/- 0.02 for BW, 0.04 +/- 0.01 for WW, 0.04 +/- 0.01, 0.07 +/- 0.02 for YW and 0.05 +/- 0.02 for CFW. Litter effects were 0.22 +/- 0.01 for BW, 0.18 +/- 0.02 for WW and 0.14 +/- 0.01 for LS. Multi-trait heritability estimates compared well with the single-trait estimates. Genetic correlations for the weight traits in the three-trait analyses ranged from -0.23 +/- 0.12 for BW and YW to 0.81 +/- 0.09 for WW and YW. Genetic, environmental, and phenotypic correlations amongst yearling weight and wool traits in the six-trait analyses were low to moderate and mostly within the ranges from previous literature for wool breeds. It should be feasible to select for a balanced breeding objective based on economic considerations in the flock.