Evaluation of moderate to high elevation effects on pulmonary arterial pressure measures in Angus cattle1

Altitude-induced pulmonary hypertension is a disease once thought to only occur at extremely high elevations (>1,600 m), but recently, it has been observed at moderate elevations of 1,200 to 1,600 m. Pulmonary arterial pressure (PAP) has been used as an indicator of tolerance to high altitude in mountainous beef production systems for over 30 yr. The trait is typically measured on yearling bulls and heifers with values ≤ 41 mmHg being favorable. These observations were historically only considered valid when they were recorded at elevations ≥ 1,600 m; however, if observations from lower (i.e., moderate) elevations were reliable indicators, a greater number of cattle records could be used in genetic improvement programs for high-altitude beef systems. The objectives of this study were to evaluate the relationship between PAP and elevation, as well as to determine whether PAP measures obtained at moderate elevations (ME) less than 1,600 m have a genetic relationship with PAP observations obtained at high elevations (HE) 1,600 m or greater. Elevation and PAP data from purebred Angus cattle (n = 14,665) from 349 contemporary groups were used in the analyses. Elevation and PAP averaged 1,887 ± 1.8 m and 43.0 ± 0.1 mm Hg, respectively. A univariate model containing the effects of sex, age, elevation category (HE vs. ME), elevation (continuous), and elevation category by elevation interaction along with a random direct genetic effect was utilized to determine the relationship between PAP and elevation. In this model, all main effects were found to be significant contributors of variation in PAP (P < 0.001). The interaction between elevation category and elevation was not a significant contributor to variability of PAP (P > 0.05). A bivariate animal model was then used to evaluate the relationship between PAP observations obtained between HE and ME groups. Heritability estimates for these 2 groups were 0.34 ± 0.03 and 0.29 ± 0.09, respectively, and their genetic correlation was 0.83 ± 0.15. Even though this is a strong genetic relationship, results of this study support the hypothesis that PAP observations collected at HE and ME are not perfectly, genetically related. Results suggest that PAP measures collected from 1,219 to 1,600 m may be useful as a correlated trait in a multitrait genetic evaluation to produce EPD useful for selection of animals with reduced susceptibility to pulmonary hypertension.