Quantitative review of the effects of environmental temperature on food intake, egg output and energy balance in laying pullets

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Abstract
1. Data from 30 published experiments have been analysed to examine the relationships between environmental temperature and the long‐term, adapted responses of laying pullets, measured as meta‐bolisable energy intake, egg output and body weight change. Heat production was also estimated indirectly from the other three variables. 2. The majority of experiments employed White Leghorns, but there were 8 large trials in which brown crossbred pullets had been compared directly with White Leghorns. These trials were used to estimate differences in energy intake and heat output between brown and white birds. 3. A total of over 26 000 birds was involved in the analysis. Individual trials varied in scale from 9 birds to 2 280 birds per treatment and in duration from 8 to 61 weeks. The constant temperatures investigated ranged from 10 °C to 34 °C, but there were no data for brown pullets beyond 30 °C. 4. The relationship between temperature and metabolisable energy intake is curvilinear, with food intake declining more steeply as ambient temperature approaches body temperature. Adapted heat production per bird is also a curvilinear function of temperature, tending towards a value of zero when extrapolated to the point at which ambient temperature equals normal body temperature. However no satisfactory data are available for fully adapted responses in the range 34 to 42 °C because egg production declines continuously when these temperatures are maintained for long periods. 5. When energy intake and heat output are expressed as functions of metabolic body size (kg °75) they can be represented as linear functions of temperature within the range 15 to 30 °C, but the slope must change outside this range. 6. It is calculated that the energy available for production is at a maximum at 23 °C for brown birds and at 24 °C for White Leghorns. Gross energetic efficiency is at a maximum at 30 °C, but egg output is reduced at this temperature. The optimum operating temperature for laying houses will depend upon the local cost of modifying ambient temperature and on the cost of supplying diets of appropriate protein content.