Using Random Regression Models to Genetically Evaluate Functional Longevity Traits in North American Angus Cattle

Abstract

This study aimed to propose novel longevity indicators by comparing genetic parameters for traditional (TL; i.e., the cow's lifespan after the first calving) and functional (FL; i.e., how long the cow stayed in the herd while also calving; assuming no missing (FLa) or missing (FLb) records for unknown calving) longevity, considering different culling reasons (natural death, structural problems, disease, fertility, performance, and miscellaneous). Longevity definitions were evaluated from 2 to 15 years of age, using single- and multiple-trait Bayesian random regression models (RRM). The RRM fitting heterogenous residual variance and fourth order Legendre polynomials were considered as the optimal models for the majority of longevity indicators. The average heritability estimates over ages for FLb (from 0.08 to 0.25) were always higher than those for FLa (from 0.07 to 0.19), and higher or equal to the ones estimated for TL (from 0.07 to 0.23), considering the different culling reasons. The average genetic correlations estimated between ages were low to moderate (~0.40), for all longevity definitions and culling reasons. However, removing the extreme ages (i.e., 2 and >12 years) increased the average correlation between ages (from ~0.40 to >0.70). The genetic correlations estimated between culling reasons were low (0.12 and 0.20 on average, considering all ages and ages between 3 and 12 years old, respectively), indicating that longevity based on different culling reasons should be considered as different traits in the genetic evaluations. Higher average genetic correlations (estimated from 3 to 12 years old) were observed between TL and FLb (0.73) in comparison to TL and FLa (0.64), or FLa and FLb (0.65). Consequently, a higher average proportion of commonly-selected sires, for the top 1% sires, was also observed between TL and FLb (91.74%), compared to TL and FLa (59.68%), or FLa and FLb (61.01%). Higher prediction accuracies for the expected daughter performances (calculated based on the pedigree information) were obtained for FLb in comparison to TL and FLa. Our findings indicate that FLb is preferred for the genetic evaluation of longevity. In addition, it is recommended including multiple longevity traits based on different groups of culling reasons in a selection sub-index, as they are genetically-different traits. Genetic selection based on breeding values at the age of four years is expected to result in greater selection responses for increased longevity in North American Angus cattle.

Keywords: beef cattle; lifespan; longitudinal trait; productive life; stayability; survival.

Figure 1

Number of cows by culling age (cumulative; (a)) and by number of calvings (b). Age at culling equal to 15 included cows culled from 15 to 20 years-old.

Figure 2

Residual variance estimated across different ages (from 2 to 15 years), considering all groups of culling reason (i.e., natural death, structural problems, disease, fertility, performance, miscellaneous, and all culling reasons together) and longevity definitions. The longevity definitions are: (a) traditional longevity, (b) functional longevity assuming 0 after the cow was culled or if the cow did not record a calf at the specified age, and (c) functional longevity assuming 0 only after the cow was culled, and missing records when no information of calving was found at the specific age.

Figure 3

Heritabilities estimated over the different ages, for all groups of culling reason and longevity definitions, using heterogeneous residual variance. The groups of culling reasons are: natural death (), structural problems (), disease (), fertility (), performance (), miscellaneous (), and all reasons together (). The longevity definitions are: (a) traditional longevity, (b) functional longevity assuming 0 after the cow was culled or if the cow did not record a calf at the specific age, and (c) functional longevity assuming 0 only after the cow was culled, and missing records when no information of calving was found for the specific age.

Figure 4

Genetic correlations estimated across ages, for all groups of known culling reason and longevity definitions. The groups of culling reason are: natural death (a–c); structural problems (d–f); disease (g–i); fertility (j–l); and performance (m–o). The longevity definitions are: traditional longevity (TL); functional longevity assuming 0 after the cow was culled or if the cow did not record a calf at the specified age (FL_a); and functional longevity assuming 0 only after the cow was culled, and missing records when no information of calving was found at the specific age (FL_b), respectively.