Studies on diurnal variations of heat production and the effective lower critical temperature of early-weaned pigs under commercial conditions of feeding and management

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Abstract
1. The heat production of groups of pigs, weaned at 10 d of age, was determined in an open-circuit respiration chamber at various ages between 10 and 33 d at temperatures above and below the lower critical temperature (Tcl).2. The heat production was lowest on the second or third day post weaning when pigs were given feed increasing by 25 g/pig per d from day 2. There was a marked diurnal pattern in heat production, the lowest values being recorded between 24.00 and 08.00 h.3. The mean thermal conductance (H/ΔT, kJ/h per m2 per °ΔT, where His total heat production, m2 is the surface area calculated as 0.097 W kg0.633 (Brody, 1945) and °ΔTis the difference between rectal temperature, taken as 39°, and air temperature) below Tcl was calculated as 18.0, 16.9, 18.5 and 21.2 respectively at 10, 17, 24 and 31 d of age. Maximum values of H/ΔT obtained during feeding periods were. on average, 4.5 kJ/h per me per °ΔT higher than the mean values.4. The maximum value for Tcl during the immediate post-weaning period was 25.9°. The mean Tcl at 17, 24 and 31 d were respectively 21.7, 18.4 and 18.6° for pigs fed almost to appetite.