The relation of fasting and resting metabolic rates to heat tolerance in the domestic fowl

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Abstract
1. Resting metabolic rate (RMR) and fasting metabolic rate (FMR) were periodically determined, using an open air circuit master system, from January to September in 10 White Leghorn, 10 Plymouth Rock and 20 New Hampshire × White Leghorn mature laying birds fed ad libitum. 2. The relationship of metabolic rate (l.O2/hr.) to body weight (kg.) in the pooled data followed the 0.795 W0.61 equation for the fasting level, and 0.916 W0.59 for the resting level. The expression of metabolic rates as litres oxygen per kg. body weight can lead to serious errors if birds of differing body weights are compared. 3. Heat increments of feeding (estimated by RMR‐FMR) gradually increased with rising environmental temperature, with only slight initial change in the FMR, followed by marked reductions in both the FMR and the RMR at the higher temperatures. It is suggested that a decrease in the FMR may allow for a reduction in the RMR in the hotter season, without this necessarily being linked with a depression in the heat increment of feeding and productivity. 4. In the lower temperature range the three breeds had similar metabolic rates. At the highest temperatures the White Leghorns and New Hampshire x White Leghorn had lower FMR than the Plymouth Rocks, thereby maintaining similar heat increments, but at lower RMR. It is suggested that this may be the basis for their better heat tolerance.