Impaired alveolar-arterial oxygen transfer is associated with reduced milk yield in primiparous post-partum dairy heifers at moderate altitude.

Abstract

Domestic cattle have limited cardiopulmonary reserve for their body size and oxygen requirements. Therefore, it is plausible that impaired alveolar-arterial gas exchange may be detrimental to energetically expensive traits such as milk production which, like all aerobic processes, requires oxygen. The degree of alveolar-arterial oxygen transfer impairment can be determined by estimating the alveolar-arterial oxygen (A-a O2) pressure gradient from arterial blood-gas tensions. The degree of oxygen transfer impairment is proportional to the A-a O2 pressure gradient: the higher the A-a O2 pressure gradient the less oxygen is transferred to the blood for a given ventilation rate. In this study two cohorts of Holstein-Friesian heifers were followed on one northern Colorado dairy farm. Arterial blood-gas analyses were performed up to 9d post-calving. Heifers were grouped into quartiles based on A-a O2 pressure gradient so that relative comparisons could be made. Heifers in the lowest (Q1) and highest (Q4) quartile had the least and greatest impairment of alveolar-arterial oxygen transfer, respectively. We hypothesised that milk yield over 60d would be greatest for heifers in Q1 and would decrease with quartile increments. Hyperventilation, as indicated by hypocapnia, was notable. Despite hypoxia, haematocrit was low. Alveolar-arterial O2 pressure gradient was associated with milk production (P=0·03) when controlling for cohort, treatment for disease and calving difficulty score. Heifers in Q1 produced 1992 kg (95% CI=1858, 2127kg) of milk when controlling for all other variables. Relative to heifers in Q1, heifers in Q2, Q3 and Q4 produced 130kg (95% CI=313, -52kg; P=0·45), 285kg (95% CI=474, 96kg; P=0·004) and 169kg (95% CI=395, -57kg; P=0.14) less milk, respectively. In conclusion, efficacy of alveolar-arterial oxygen transfer was associated with milk yield in dairy heifers on one farm at moderate altitude.