Tests of two theories of food intake using growing pigs - 2. The effect ofa period of reduced growth rate on the subsequent intake of foods of differing bulk content

The effect of a period of feeding on a high bulk food, upon the subsequent intake of foods of differing bulk content, was investigated in two experime nts of the same design. The intention was to provide a severe test of the t wo current conceptual frameworks available for the prediction and understan ding of food intake. In each experiment 40 male Manor Meishan pigs were ran domly allocated to one of four treatment groups at weaning. Each experiment was split into two periods, P1 (12 to 18 kg) and P2 (18 to 32 kg). The tre atments, all with ad libitum feeding, were: a control food (C) given throug hout (treatment CC); a medium bulk food (M) given throughout (treatment MM) ; a high bulk food (H) given in P1 and then C in P2 (treatment HC); H given in P1 and M in P2 (treatment HM). C was based on micronized wheat with 13. 4 MJ digestible energy and 243 g crude protein per kg fresh food. In experi ment 1 M contained 350 g/kg and H 560 g/kg of unmolassed sugar-beet pulp an d in experiment 2 M contained 500 g/kg and H 700 g/kg of unmolassed sugar-b eet pulp. Framework 1 predicted that food intake on the medium bulk food (M ) would not be increased, whereas framework 2 predicted that intake on M wo uld be increased after a period of feeding on H, compared with when M was o ffered continuously. In P1, both food intake (P < 0.01) and growth (P < 0.001) were severely lim ited on H compared with C. In experiment 1 growth was limited on M compared with C during the first 7 days of P1 (P < 0.01) only. In experiment 2 inta ke (P < 0.001) and growth (P < 0.001) on M were limited throughout P1, comp ared with C but not thereafter. Therefore, in neither experiment did M caus e a lower growth rate than C from 18 to 32 kg. In experiment 1 there was fu ll adaptation to M after about 10 days from 12 kg. In experiment 2 adaptati on was complete by the end of the first 7 days from 18 kg. In P2, food intake (P < 0.001) and live-weight gain (P < 0.05 and P < 0.001 in experiments 1 and 2, respectively) were increased on HC compared with C C. By the last 7 days of P2 intake was still higher (P < 0.01) but growth r ate was no longer different to CC. Intake and gain were increased in P2 on HM compared with MM but, in general, these differences were small and not s ignificant. In the first 7 days of P2, in experiment 1 pigs on HM had highe r intakes (P < 0.001) and gains (P < 0.05) than those on MM, but in experim ent 2 only intake was higher (P < 0.01) with no difference in gain. By the last 7 days of P2 there was no difference in either intake or gain between these two groups in either experiment. Pigs on HC increased intake by more than those on HM. There was, therefore, a significant interaction for food intake (P < 0.05, in experiment 1 and P < 0.001, in experiment 2) between p rior and present food. The unexpected failure of either M food to limit growth throughout the expe rimental period meant that the results of these experiments could not be us ed as a strong test to reject either one of the frameworks. However, the ab ility of the pigs to compensate on M was less than that on C. The data prov ide some evidence that under conditions of compensation foods such as M may be limiting. This is in closer agreement with the framework that predicted that consumption of a limiting food will not increase after a period of fe eding on a high bulk food (framework 1).