Genetic analyses of lamb survival in Rambouillet and Finnsheep flocks by linear and threshold models

Data on lamb survival from birth to weaning of Rambouillet and Finnsheep we re analysed with sire-maternal grandsire linear (LM) and threshold (TM) mod els. Models for the Rambouillet included the effects of year, type of birth , age of dam and sex as fixed effects and sire, maternal grandsire and resi dual as random effects. For the Finnsheep, fixed effects were year-age of d am combination, type of birth-rearing and sex, and random effects were sire , maternal grandsire and residual. Restricted maximum likelihood (REML) and marginal maximum likelihood (MML) estimates of variance and covariance com ponents were obtained under LA I and TM, respectively. The performance of L M and Till was assessed in terms of goodness of fit and predictive ability. Within the Rambouillet breed, heritabilities of additive direct (0.06), ad ditive maternal (0.04) and total effects (0.13) obtained with Till were 2, 1.3 and 1.9 times greater than those obtained with LM, respectively. For th e Finnsheep data, estimated heritabilities of direct (0.17), maternal (0.26 ) and total (0.34) effects using TM were 1.9, 1.4 and I ti times greater th an the estimates using LM, respectively Estimated genetic correlation betwe en direct and maternal effects was 0.14 for the Finnsheep with both the LM and TM. Higher genetic correlations between direct and maternal effects (0. 44 and 0.62 with LM and TM, respectively) were obtained for the Rambouillet data. Goodness of fit and predictive ability of the models used with the F innsheep data were better than for the models used with the Rambouillet dat a, but within breed, no sizable or significant differences were detected be tween LM and Till. Results indicate that maternal effects were important in lamb survival to weaning; especially in the more prolific Finnsheep breed.