The influence of diurnal variation in animal activity and digestion on animal heat production

Dimensioning of air conditioning plants for animal houses is traditionally based on steady-state conditions with constant animal heat and moisture production over a 24-hour cycle, despite the fact that the heat and moisture production varies diurnally. The heat production is normally highest in daytime and lowest during the night and therefore, there will be a discrepancy between the actual and the calculated indoor temperature and humidity, when using steady state models to control ventilation rate.  This article deals with the two main factors influencing on the diurnal heat production, namely the animal activity and the digestion of food. In experiments carried out over three years in respiration chambers at Research Centre Foulum, Denmark, the diurnal influence of feeding and other behavioural activities have been analyzed for weaned pigs, growing pigs, dry sows and broilers. These experiments show that most of the diurnal variations in animal heat production can be explained by animal activity (R2 between 0.58 and 0.89), for examined species but heat production due to digestion processes does also contribute. Because of coincidence between high animal activity and the feeding schedules, the animal heat production due to digestion is confounded with that caused by physical activity. By applying a model that takes into account both animal activity and the time after feeding for pigs fed twice a day, the correlation is improved (R2 between 0.80 and 0.90). This model shows for growing pigs at 35, 65 and 90 kg, fed twice daily, that animal activity and digestion each can explain half of the animal heat production above basal heat production. For dry sows with low feed intake the impact of digestion on the heat production is negligible compared to the impact due to activity of the animals. The investigations also show that even under experimental conditions in climatic chambers, there is a typical diurnal variation in animal heat production. Expressed by a sinusoidal function, the diurnal variation in heat production (amplitude), measured in climatic chambers was in the range from 9 to 21%, which is lower than that found in other investigations for conventional animal houses with normal group size and human activities in the animal house during the daytime. The analysis shows that the measured animal heat production in the laboratory investigations was in fairly good agreement with the calculated total heat production according to CIGR Equations, with deviations from -6.3 to 6.8 % for fattening pigs and dry sows, but greater deviations were measured for piglets fed by an artificial sow and for broilers