GENETIC AND NON-GENETIC ASPECTS OF EARLY GROWTH TRAITS IN NEW ZEALAND ROMNEY SHEEP

ABSTRACT
The aim of the present study was to analyze the effect of genetic and non-genetic factors on the early growth traits of New Zealand Romney sheep. Early growth data of 1629 lambs (863 males and 766 females), progenies of 20 rams and 905 ewes, were used to test the factors affecting birth weight (BW), adjusted weight for 90 days (W90) and average daily gain from birth to weaning (ADG) using the general linear model of SAS software (SAS, 2000). The fixed effects were the year of birth, the parity of ewe, the gender of lamb and the type of birth. Bi-interactions between the fixed effects were included in the statistical model. The co-variance components, direct (h2a) and maternal (h2m) heritabilities, and genetic (rG) and phenotypic (rP) correlations among the studied traits were estimated using MTDFREML (Boldman et al., 1995).
Year of birth had no effect on all the studied traits. The parity of ewe proved significant effect (P ˂ 0.01) on both W90 and ADG. The gender of lamb and type of birth contributed significantly (p ˂ 0.001) to the variation of the studied traits. The parity of ewe interacted with gender of lamb to significantly (P ˂ 0.01) affect ADG and also interacted with type of birth to significantly affect BW (P ˂ 0.001), W90 (P ˂ 0.001) and ADG (P ˂ 0.05). Significant effects were observed on BW (P ˂ 0.05), W90 (P ˂ 0.001) and ADG (P ˂ 0.01) due to the interaction between gender of lamb and type of birth.
The estimates of h2a for BW, WW and ADG were 0.15 ± 0.02, 0.08 ± 0.03 and 0.16 ± 0.05, respectively; however, the corresponding estimates of h2m for these traits were 0.30 ± 0.02, 0.20 ± 0.02 and 0.16 ± 0.05, respectively. The additive genetic variance (σ2a) for BW, WW and ADG were 0.106, 1.419 and 0.0004, respectively, and the corresponding maternal genetic variance (σ2m) for these traits were 0.213, 6.535 and 0.0004, respectively. The estimates of environmental variance (σ2p) were 0.357, 11.289 and 0.0012 for BW, WW and ADG, respectively, which contributed to the higher proportion of total phenotypic variance for all these traits.According to these findings, it is important to consider the role of various environmental factors in New Zealand Romney sheep production, since they affected the early growth performance of lambs. The low to moderate estimates of h2a for early growth traits indicate to the possibility of improving them using traditional or genomic selection. Furthermore, selection for any of the studied traits would be expected to have a large effect on the other growth traits as a result of their moderately positive genetic correlations.