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

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/delta T, kJ/h per m2 per degrees delta T, where H is total heat production, m2 is the surface area calculated as 0.097 W kg0.633 (Brody, 1945) and degrees delta T is the difference between rectal temperature, taken as 39 degrees, 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/delta T obtained during feeding periods were, on average, 4.5 kJ/h per m2 per degrees delta T higher than the mean values. 4. The maximum value for Tcl during the immediate post-weaning period was 25.9 degrees. The mean Tcl at 17, 24 and 31 d were respectively 21.7, 18.4 and 18.6 degrees for pigs fed almost to appetite.