Circadian rhythm in heat production of limit-fed growing pigs of several breeds kept at and below thermal neutrality.

Circadian rhythm in total and activity-free heat production (H and Hacf, respectively) was studied in Norwegian Landrace (N), Finnish Landrace (F), Dutch Landrace (D), and Great Yorkshire (Y) barrows. Animals, weighing 26 kg at the start of the study, were kept in groups for 18 2-d periods in climate respiration chambers at environmental temperatures (Tenv) between 11 and 26 degrees C. Measurements of H and physical activity of the pigs were done in 12-min intervals for 36 h per breed per Tenv. Feeding level of animals in a group was 93 g.kg-.75 x d-1 (2.5 times maintenance) and based on mean BW. Circadian rhythm in H and Hacf with data excluding the feeding periods was assessed using a sine wave with a 24-h periodicity per breed and per Tenv. With respect to H, the intercepts and amplitudes for the three Landrace breeds were similar, but the Y pigs had a lower intercept and a lower amplitude. The intercepts increased with decreasing Tenv. In N pigs, but not in F, D, or Y pigs, the amplitude increased with decreasing Tenv. With respect to Hacf, all breeds had similar intercepts and amplitudes. The Hacf varied less within a day than did H. In all breeds, the intercepts were increased with decreasing Tenv. The amplitudes were not affected by Tenv.