Genetic and phenotypic parameters in New Zealand Romney sheep

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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 from a wide range of industry sources and selected at random from within the flock. Traits analyzed were growth from weaning (3 mo.) to 16 mo. of age, fleece wt of lambs (4 mo.) and of hoggets (13 mo.) and number of hogget estrus occurrences. The principal environmental sources of variation for live wt and fleece wt were age (birth date), birth-rearing rank and age of dam; the effects of these diminished as animals grew older. The effects of the sex management group increased with age. Date of birth significantly influenced number of hogget estruses. Interactions among environmental factors were of little importance. For genetic analyses records were 1st adjusted for environmental effects. Heritability is discussed in relation to the expected genetic progress from direct and indirect selection and 2 stage selection. To improve weaning wt more progress per generation can be expected by selecting on yearling wt than weaning wt itself. Two stage selection (preliminary selection at weaning and final selection at the yearling stage) is expected to lead to the same genetic progress as selection on yearling wt alone and may be more acceptable in practice.