An investigation into beef calf mortality on five high-altitude ranches that selected sires with low pulmonary arterial pressures for over 20 years

Abstract

Producer reports from ranches over 2,438 meters in southwest Colorado suggest that the mortality of preweaned beef calves may be substantially higher than the national average despite the selection of low pulmonary pressure herd sires for over 20 years. Diagnostic investigations of this death loss problem have been limited due to the extensive mountainous terrain over which these calves are grazed with their dams. The objective of the current study was to determine the causes of calf mortality on 5 high-altitude ranches in Colorado that have been selectively breeding sires with low pulmonary pressure (<45 mmHg) for over 20 years. Calves were followed from branding (6 weeks of age) in the spring to weaning in the fall (7 months of age). Clinical signs were recorded, and blood samples were taken from sick calves. Postmortem examinations were performed, and select tissue samples were submitted for aerobic culture and/or histopathology. On the principal study ranch, 9.6% (59/612) of the calves that were branded in the spring either died or were presumed dead by weaning in the fall. In total, 28 necropsies were performed: 14 calves (50%) had lesions consistent with pulmonary hypertension and right-sided heart failure, and 14 calves (50%) died from bronchopneumonia. Remodeling of the pulmonary arterial system, indicative of pulmonary hypertension, was evident in the former and to varying degrees in the latter. There is a need to better characterize the additional risk factors that complicate pulmonary arterial pressure testing of herd sires as a strategy to control pulmonary hypertension.

Angus calves born and raised at high altitude adapt to hypobaric hypoxia by increasing alveolar ventilation rate but not hematocrit1

“…Hematocrit, conversely, did not show a dynamic relationship with altitudes ≥ 1,470 m. Despite the increase in ventilation rate with altitude, calves became increasingly hypoxemic. These physiological responses, or lack thereof, may influence the susceptibility of suckling calves at high altitudes to diseases such as "summer pneumonia" and cor pulmonale (Neary et al, 2013b).…”

Pulmonary arterial pressures, arterial blood-gas tensions, and serum biochemistry of beef calves born and raised at high altitude

“…High calcium and phosphorous concentrations may predispose calves to develop soft-tissue mineralization under certain pathologic states, such as pulmonary hypertension. 5,32 Alkaline blood conditions, tissue injury, and hypoxia are all predisposing factors for mineralization of tissues. 33…”

“…4 An investigation of beef calf mortality on high-altitude ranches revealed a high risk of calf mortality due to respiratory disease and/or pulmonary hypertension. 5 In order to meet oxygen requirements in a high-altitude environment, the stress on a growing calf's cardiopulmonary system may be considerable. Despite the health and economic livelihood implications, there has been little attention paid to the effect of high altitude on bovine calf physiology.…”

Hypoxic Pulmonary Hypertension

“…Neonatal calves develop supra-systemic PH that can lead to right heart failure. They also develop excessive proliferation of the vasa vasorum, the systemic blood vessels providing circulation to the adventitial (Stenmark Malherbe et al 2012;Neary et al 2013). It is possible that these animals represent a natural model of "outof-proportion" PH which may be associated with an as yet undefined "second hit.…”