Angus cattle at high elevation: Comparison of models to estimate breeding values of yearling pulmonary arterial pressure

Abstract Text: As an indicator of pulmonary hypertension, pulmonary arterial pressure (PAP) is widely used in the selection of cattle to reduce the incidence of high altitude disease (HAD). In initial analyses of yearling PAP data, a violation of normal distribution of residuals (i.e. skewed right tail) was observed. To remedy, alterative expressions of yearling PAP were investigated in this study with a goal of determining the effect of alternative expressions on genetic evaluation outcomes. Yearling PAP records (42.46 ± 0.58mmHg) were collected from 5,296 Angus cattle from 280 sires from Colorado State University Beef Improvement Center (2,150m elevation). The alternative phenotypes included power-transformed (PT) PAP records, an ordinal three-category phenotype (CAT3), an ordinal two-category phenotype (CAT2) and the non-transformed PAP observations (RAW). The CAT3 observations were defined as low risk (PAP < 41mmHg), moderate risk (41mmHg ≤ PAP ≤ 49mmHg) and high risk (PAP > 49mmHg) for HAD. The CAT2 observations were constructed by combining low and moderate risk categories of CAT3. Univariate linear and threshold animal models were applied in analyses of RAW and PT; CAT3 and CAT2, respectively. The fixed effects for PAP phenotypes included sex, age of dam, date and age (covariate) of PAP measurement. All fixed effects were significantly (P < 0.05) associated with each PAP phenotype. The estimated heritabilities were 0.24, 0.24, 0.26, and 0.31 for RAW, PT, CAT3 and CAT2, respectively. Sire EBV accuracies from univariate models of RAW, PT, CAT3 and CAT2 ranged from 0.03 to 0.67, 0.03 to 0.68, 0.01 to 0.65 and 0.01 to 0.58 with means of 0.31, 0.31, 0.28 and 0.21, respectively (pooled sd = 0.13). The Rank correlations between EBV from RAW and PT, CAT3 or CAT2 were 0.92, 0.84 and 0.77, respectively. The lowest Rank correlation (0.69) was identified between PT and CAT2, while Rank correlation of 0.91 was obtained between PT and CAT3. All phenotypes resulted in decreasing genetic trends. Results suggested similar heritability, accuracy and rank of animals based on EBV from RAW and PT, yet losses in EBV accuracy and some re-ranking of sires was observed in ordinal categorical phenotypes compared to continuous PAP scores. In conclusion, violation of normality had limited influences on the genetic evaluation of yearling PAP measurements. Ordinal categorical phenotypes can be alternative dependent variables in studying susceptibility of HAD, however, they would cause some re-ranking of sires relative to non-transformed PAP scores.  

Keywords: Cattle, Pulmonary Arterial Pressure, Genetic Evaluation