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Abstract
1. The improvement in heat tolerance acquired during the acclimatisation of laying hens to intermittent heat stress was lost or considerably reduced when energy intake increased following the addition of maize oil to the diet.
2. The oxygen consumption (VO2) of acclimatised hens increased after the addition of maize oil to the diet.
3. Reducing the environmental temperature (Ta) from 30 ° to 10 °C prevented the development of acclimisation if the heat stress commenced soon after the change of Ta; if the heat stress commenced 5 weeks after the change of Ta acclimatisation took place normally.
4. Reducing the Ta from 30 ° to 10 °C resulted in an increased VO2 and food intake.
5. The administration of triiodothyronine increased VO2 and reduced the heat tolerance of acclimatised hens.
6. It is concluded that heat tolerance, despite being reinforced by regular exposure to heat stress, remains susceptible to changes in energy metabolism brought about by dietary or environmental means.