Age trends in economically important traits of Merino ewes subjected to 10 years of divergent selection for multiple rearing abi

S.W.P. Cloete, A.R. Gilmour, J.J. Olivier and J.B. van Wyk
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Data were recorded for annual reproduction, wool traits and pre-joining live weight of between 992 (for staple strength) to approximately 1200 production records of Merino ewes over the period from 1997 to 2002. The resource population has been divergently selected from the same base since 1986, either for (H line) or against (L line) maternal multiple rearing ability. When expressed relative to mean L line performance, the advantage in reproduction for the H line amounted to 34% for number of lambs born per ewe, 49% for number of lambs weaned per ewe and 56% for weight of lamb weaned per ewe. Overall trends in reproduction with an increase in age from 2 - 7+ years were consistent with corresponding trends in the literature. The shape of these curves did not differ between selection lines, i.e. no significant interaction between selection line and ewe age was found. Maiden H line ewes were heavier than L line contemporaries. Ewe joining weight increased with age in both lines. Line differences were reduced to a tendency in 3-year old ewes, and no subsequent line differences were found. The random non-linear component of the spline for ewe age also interacted with selection line for clean fleece weight. Conclusive advantages in favour of L line ewes were obtained at 3 - 6 years of age. In maiden ewes the line difference was restricted to a tendency in favour of the L line while no line difference occurred in 7-year old ewes. Ewes in the L line outperformed H line contemporaries for staple strength at four years of age, but no line difference was found at other ages. Coefficient of variation of fibre diameter and mean fibre diameter did not differ between lines. Coefficient of variation of fibre diameter generally declined curvi-linearly with an increase in ewe age, while fibre diameter showed a near linear increase. Estimates of h² for reproduction traits were 0.09±0.05 for number of lambs born per ewe, 0.05±0.05 for number of lambs weaned per ewe, and 0.05±0.05 for weight of lamb weaned per ewe. Corresponding h² estimates for wool traits were 0.34±0.08 for clean fleece weight, 0.05±0.05 for staple strength, 0.74±0.02 for coefficient of variation and 0.76±0.02 for fibre diameter. A h² estimate of 0.37±0.09 was obtained for ewe live weight at joining. Ewe permanent environment (c² ewe) for the reproduction traits were 0.15±0.05 for number of lambs born per ewe, 0.19±0.05 for number of lambs weaned per ewe and 0.19±0.05 for weight of lamb weaned per ewe. The corresponding c² ewe estimate for ewe joining weight was 0.40±0.09, while those for annual wool production traits were 0.31±0.08 for clean fleece weight and 0.12±0.05 for staple strength. Coefficient of variation and mean fibre diameter were not influenced by the ewe permanent environment. Line differences in age trends for wool traits and ewe joining weight were attributed to the drainage by reproduction on the reserves of H line ewes.