Survival analysis from birth to slaughter of Ripollesa lambs under semi-intensive management

The survivability from birth to slaughter of 1,487 Ripollesa lambs with a preslaughter overall mortality of 9.6% was studied under the proportional hazards framework, assuming a Weibull distribution for the baseline hazards function. A sire frailty model was fitted, with the common environment received by the lamb as an additional random source of variation. Common environment was considered time-dependent and was characterized by the dam and the contemporary lamb group during the preweaning and fattening periods, respectively. Only 3 fixed effects were statistically significant: the linear and quadratic effects of birth weight (P [less than or equal to] 0.001), the relative position of the delivery within the lambing season (P [less than or equal to] 0.001), and the presence of stillbirths or mummified fetuses within the litter (P [less than or equal to] 0.05). Birth type and parity of the ewe were significant only when birth weight was removed from the model (P [less than or equal to] 0.001 and P [less than or equal to] 0.05, respectively). Nevertheless, the model including birth weight became preferable according to the Akaike's information criterion. Survivability dramatically decreased with extreme birth weights, although it reached a survival probability greater than 93.5% within the 3.3 to 5.4 kg range, indicating an optimum birth weight range of Ripollesa lambs for survival purposes. The hazard ratio (HR) increased for births occurring within the last third of the lambing period (HR = 1.70; P [less than or equal to] 0.05), as well as for primiparous ewes that lambed in December and January (HR = 5.36; P [less than or equal to] 0.001). Survival probability decreased for lambs born from litters with 1 or more stillbirths or mummified fetuses (HR = 1.61; P [less than or equal to] 0.05). The variance component estimated for sire variance (0.07) was clearly lower than that of the common environment (1.87), with a heritability estimate of 0.027. Key words: lamb survival, proportional hazard, Ripollesa breed, survival analysis