Submitted to: Journal of Animal Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/23/2001
Publication Date: 9/2/2001
Citation: Southey, B.R., Rodriguez-Zas, S.L., Leymaster, K.A. 2001. Survival analysis of lamb mortality in a terminal sire composite population. Journal of Animal Science. 79:2298-2306. Interpretive Summary: Lamb survival is an important factor that influences profitability of sheep production. Seedstock producers could select to improve survival genetically, but this approach generally is not practiced due to the low heritability of about 10% typically reported for lamb survival. Lambs are routinely classified as either dead or alive at some phase of production, such as weaning. This classification method contributes to low estimates of heritability because information about age at death is ignored for lambs that do not survive to weaning. For example, a lamb that dies at 2 days of age is treated the same as a lamb that dies just prior to weaning. An alternative approach to the classification method is called survival analysis. Survival analysis accounts for the more continuous nature of mortality by considering age at death. Data from a terminal sire composite population were analyzed using both methods. Heritability estimates of lamb survival from survival analysis were about twice as great as estimate using the classification method. Survival analysis is a more effective method to increase lamb survival, leading to improved productivity, profitability, and welfare.
Technical Abstract: Mortality records of 8,642 lambs from a composite population at the U.S. Meat Animal Research Center were studied using survival and logistic analyses. The traditional logistic approach considers whether or not a lamb survived until a particular time point, thus disregarding information on the actual age at death. Survival analysis considers the precise age at death while accounting for possible record censoring. Lamb mortality was studied across five periods: birth to weaning, birth to 120 d of age, birth to 365 d of age, weaning to 365 d of age, and 120 to 365 d of age. Explanatory variables included in the models were sex, type of birth, age of dam, and type of upbringing (under nursery conditions or not). The survival analysis was implemented using Weibull and Cox proportional hazards models with sire as random effect. The logistic approach included sire, animal, and maternal models. Lambs that were culled were treated as censored in the survival analyses and assumed alive in the logistic analyses. Similar estimates of explanatory variables were obtained from survival and logistic analyses, but survival analyses had lower standard errors than logistic analyses. Heritability estimates were generally consistent across all periods ranging from 0.15 to 0.21 in the Weibull model, 0.12 to 0.20 in the Cox model, 0.08 to 0.11 in the logistic sire model, 0.04 to 0.05 in the logistic animal model, and 0.03 to 0.07 in the maternal effects logistic model. Maternal effects were important in the early stages of lamb life but maternal heritability was less than 0.07 in all stages with a negative correlation (-0.86 to -0.61) between direct and maternal effects. Estimates of additive genetic variance indicate that use of survival analysis in breeding schemes could allow for effective