Phenotypes

Data for this study consisted of records from 1,365 first parity cows from a Composite Gene Combination breed (CGC; 50% Red Angus, 25% Charolais, 25% Tarentaise; Newman et al. 1993a, 1993b) born between 2002 and 2011 at USDA-ARS, Fort Keogh Livestock and Range Research Laboratory, Miles City, MT. The pedigree file consisted of 5,374 animals including 128 sires and 1,723 dams. Fertility phenotypes consisted of age of puberty (AOP), age of first calving (AOC), and heifer pregnancy status (HPS). AOP was predicted as described by Roberts et al. (2009) and is subject to be left and right censored (animal reached puberty before testing date or animal did not reach puberty at testing date). Rate of censoring increases with the increase in the age at first evaluation. In our study, age at first evaluation for puberty ranged between 301 and 354 d. Right censoring will not be an issue in the multivariate analyses as nonobserved phenotypes will be imputed based on the observed other correlated traits. However, left censoring could be more problematic as it is hard to account for through the statistical model because it is seldom the case that the exact AOP is known. The first 2 traits (AOP and AOC) were treated as continuous responses, and HPS was treated as binary. Missing AOP records are largely due to right censoring. In fact, left and right censoring for AOP were 10% and 34%, respectively. A summary description of the data is presented in Table 1. Further information concerning management and production records on the population are given in Roberts et al. (2016)

Summary of descriptive statistics for age at first calving (AOC), age of puberty (AOP), and pregnancy status (HPS) of CGC¹ heifers