|Martinez, Gonzalo - UNIV. OF NEBR.-LINCOLN|
|Kachman, Steven - UNIV. OF NEBR.-LINCOLN|
|Van Vleck, Lloyd|
Submitted to: Applied Statistics In Agriculture Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: July 1, 2003
Publication Date: October 1, 2003
Citation: Martinez, G., Kachman, S.D., Van Vleck, L.D. 2003. Nested threshold sire models for estimating genetic parameters for stayability in beef cows [proceedings]. Applied Statistics In Agriculture Conference. p. 248-257. Interpretive Summary: The longevity of a beef cow is a complex trait that reflects performance over her total herd life and is determined principally by her fertility, maternal ability, and health. Longevity is a trait that affects overall profitability. One problem with working with longevity is that longevity can be measured only after cows have been culled or have died (actual length of life). An alternative way to measure of herd life is by use of stayability traits. Stayability is defined as the probability of surviving to a specific age, given the opportunity to reach that age. Cows are evaluated in opportunity groups where the score of an animal with the opportunity to survive to given age is recorded as one (1) if she survives and zero (0) if she fails to survive to the given age. Analyses assuming a threshold model for discrete traits such as stayability and fertility in beef cattle have not been common. Previous studies have generally used threshold sire models for single trait analyses. Knowledge of genetic relationships among stayability traits, however, is important for designing economically optimum recording and selection programs. This study estimated heritability and genetic correlations between stayability traits measured at an early age and stayability traits measured at older ages with a two-trait threshold model. Observations were from 1,868 Hereford cows. This study showed that two-trait threshold models can be used to estimate heritability and genetic correlations between stayability measured at early and at late ages. Selection for stayability would be possible but would be expected to be slow due to low estimates of heritability on the observed (binomial) scale. Estimates of genetic correlations indicate that selection for stayability to an early age would have limited impact on improvement for stayability to later ages. More research is needed in this area especially with other definitions of stayability such as stayability to calving and stayability to weaning.
Technical Abstract: Stayability is the ability of a beef cow to remain in production to a specified age. In this study, the interest was in determining the genetic relationship between stayability to an early age with the stayability to a later age. A nested threshold sire model for stayability was used to estimate the genetic correlations between stayability to two different ages. Genetic correlations were estimated among all pairs of six different stayability traits using records from 1,868 Hereford cows. The model included period (trait) and year of birth as fixed factors and sire as a random factor. The numerator relationship matrix accounted for all known relationships among sires. Penalized quasi-likelihood estimates were obtained using a probit link function. Estimates of heritability on the original scale were small and ranged from 0.09 to 0.17. Estimates of genetic correlations were low to moderate and variable in sign. Results indicate that selection for stayability to an early age would have a limited impact on stayability to later ages.