Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/30/2003
Publication Date: 6/20/2003
Citation: Southry, B.R., Rodriguez-Zas, S.L., Leymaster, K.A. 2003. Discrete time survival analysis of lamb mortality in a terminal sire composite population. Journal of Animal Science. 81:1399-1405. Interpretive Summary: Lamb survival is an important factor that influences profitability of sheep production. Genetic improvement of lamb survival requires accurate and precise estimates of the genetic influence on survival. The traditional method to estimate heritability results in values of 0.1 or less, giving little optimism for genetic improvement of survival. An alternative approach to estimate heritability was compared to the traditional method. Estimates of heritability from the alternative approach were about twice as large as estimates based on the traditional model. The alternative approach may be a useful method to improve lamb survival.
Technical Abstract: Mortality records during the first year of life of 8,642 lambs from a composite population at the U.S. Meat Animal Research Center were studied using discrete survival analyses. Lamb mortality was studied across periods from birth to weaning, birth to 365 d of age and weaning to 365 d of age. Animal-time datasets were created for each period using different interval durations. Each dataset was analyzed using logistic and complementary log-log sire, animal, and maternal effects models. Explanatory variables included in the models were interval duration, sex, type of birth, contemporary group, age of dam, and type of upbringing (nursery or not). Similar estimates of explanatory variables were obtained within the same period regardless of model and interval duration. Heritability estimates from the complementary log-log models were greater than the comparable logistic models due to the difference in variance of these functions. Heritability estimates from the complementary log-log sire model ranged from 0.13 to 0.21 for all periods. These estimates were greater than the complementary log-log animal model estimates that ranged from 0.04 to 0.12. Maternal effects were important early in life with maternal heritability slightly greater than direct additive heritability and a negative correlation (-0.72 to -0.65) between direct additive and maternal effects. The similarity of the results to other survival analysis methods demonstrates that the discrete methodology is a viable alternative to estimate variance components in livestock survival data.