Diurnal and seasonal changes in bovine respiratory functions in a subtropical climate

Abstract

The respiratory responses of four Israeli-Holstein heifers sheltered from direct solar radiation were measured every 4 hr during 4 consecutive days in early spring and again in late summer. Two of the animals were fed according to Fredricksen's standard, and two ad libitum. Average atmospheric conditions were 18.4°C and 65° R.H.

In spring and 27.5° and 75° in summer.

In spring no marked diurnal variation was evident in either ventilation rate or respiration rate, whereas in summer a well-defined cycle was evident. Tidal volume showed a distinct cycle in spring with the lower volumes occurring in the day-time, while in summer the variation was almost nil and absolute levels were lower. In both seasons the animals fed ad libitum had lower tidal volumes and a more restricted cycle.

In both seasons, ventilation rates and respiration rates were not correlated with diurnal variation in climatic parameters. Ventilation rates increased by only 4% from spring to summer in both feeding groups, while respiration rate was doubled. The response of respiration rate to temperature was of the order of Q₁₀ = 2, for changes from season to season only.

In both seasons respiratory vaporization followed very similar and pronounced diurnal cycles, the maximum value being attained at 0830 hours. Higher rates were found in both seasons for the group fed ad libitum, owing mainly to consistently higher vaporization rates in the afternoon and evening hours. The diurnal cycle in vaporization was due mainly to changes in moisture content of the expired air.

The partial effect of respiration rate on the moisture content of expired air and on moisture exhaled per minute was highly significant, while that of ventilation rate was not significant. The increase in absolute atmospheric humidity from spring to summer was associated with an increase in absolute humidity in the exhaled air, but this did not prevent a 12.6% reduction in respiratory vaporization.