Genetic and phenotypic parameters in New Zealand Romney sheep

Abstract

Genetic and phenotypic parameters were estimated from 4 annual lamb drops in an experimental flock of 1050 Romney ewes. Progeny were sired by 133 different rams drawn initially from a wide range of industry sources and subsequently selected at random from within the flock. Traits analysed were: growth from weaning (3 months) to 16 months of age, fleece weights of lambs (4 months) and of hoggets (13 months), and number of hogget oestrus occurrences. Relevant records were available on all live progeny as well as on parents born in the flock in the first 2 years. Within years, the principal environmental sources of variation for live weights and fleece weights were age (birth date), birth—rearing rank, and age of dam, all of whose effects diminished as animals grew older; and sex-management group, whose effects increased markedly with age. Of these factors, only date of birth significantly influenced number of hogget oestruses. Interactions among environmental factors were of little importance. For genetic analyses, records were first adjusted for environmental effects. Heritabilities of live weights increased up to about 10 months of age (0.34) and thereafter declined. Likewise, they were about twice as high for hogget as for lamb fleece weights.The estimates were comparable with previous published New Zealand findings except for higher values for hogget fleece weight (0.29) and lower values for weaning weight (0.08), hogget live weight (0.27), and live-weight gain (range 0–0.19). The number of hogget oestruses showed a heritability of 0.31. A striking and consistent finding was the higher heritability estimates for female than male progeny. For July (10 month) live weight the values were 0.46 and 0.21 respectively; corresponding estimates for hogget fleece weight were 0.41 and 0.27. Higher environmental and lower genetic variances among males were implicated. Heritabilities calculated from offspring–sire regression agreed with those from paternal half-sib analyses, whereas offspring—dam regression estimates were substantially higher for most live weights but similar for hogget fleece weight. Both phenotypic and genetic correlations among live weights and number of hogget oestruses were lower than in a previous New Zealand study. Phenotypic and genetic (paternal half-sib) correlations between fleece weight and post-weaning live weights were in the range 0.38–0.52, but genetic correlations estimated from offspring—parent relationships were much lower. Lamb and hogget fleece weights showed a strong genetic correlation (0.80). The results are discussed in relation to the expected genetic progress from direct and indirect selection and two-stage selection. To improve weaning weight, more progress per generation can be expected by selecting on yearling weight than weaning weight itself. Two-stage selection, with preliminary selection at weaning and final selection at the yearling stage, is expected to lead to the same genetic progress as selection on yearling weight alone and may be more acceptable in practice.