The purpose of this thesis was to investigate, in a series of four experiments, the factors that are important for the control and prediction of the voluntary food intake of growing pigs fed poor quality foods.
The aim of Experiments 1 and 2 was to provide a severe test of two current conceptual frameworks available for the understanding and prediction of food intake. Framework 1 assumes that intake will be that which allows genetic performance to be achieved. If this is not achieved then it is assumed to be because there is a constraint to food intake (e.g. the bulk content of the food) operating. Framework 2 sees intake as being a consequence of a process of optimisation, such that biological efficiency (rate of net energy ingested per litre of oxygen consumed) is maximised. The experiments tested areas where the frameworks differed in their predictions of intake on poor quality foods. Experiment 1 tested the effect of ambient temperature on the intake of foods of differing bulk content, Experiment 2 tested the effect of a period of reduced growth on the subsequent intake of foods of differing bulk content. The results of both experiments were in closer agreement with the predictions of Framework 1. It was concluded that the results provided no evidence that the idea that intake on poor quality foods in young growing pigs is constrained by gut capacity should be rejected.
Experiment 3 investigated the effect of giving foods of differing bulk content on the short term feeding behaviour (STFB) of growing pigs. It was expected that differences in bulk content between the foods would result in different levels of intake that would be reflected as differences in STFB. Three hypotheses were developed based on ideas about the way in which a physical constraint to intake might operate. HI; there would be less diurnal variation in feeding on high bulk foods that limit intake. H2; feeding patterns on bulky foods would be less flexible than those on a control food when feeding time is limited by reducing time of access to the feeder. H3; when the composition of the food offered is changed intake and hence STFB will be rapidly altered to suit the new food. The results of the experiment were in agreement with H2 and H3. Pigs fed higher bulk foods did not maintain intake or performance when time of access to the feeder was reduced, this was mainly due to the absence of any adaptive change in STFB. In agreement with H3, when the composition of the food offered was changed there was a rapid change in STFB so that it became appropriate to the new food. These results are in agreement with the idea that intake on bulky foods is physically constrained. Contrary to expectation, a higher bulk content had no significant effect on the diurnal intake pattern of pigs given 24 hours access to food, the reason for this remains unclear.
The final experiment investigated the effect of degree of maturity on the capacity to consume food bulk. This was done by feeding a constraining food (a food high in bulk that was intended to limit the growth of the pigs to which it was fed) at one of three different degrees of maturity (12, 36 or 108kg). The aim of this final experiment was to determine how capacity for bulk (bulk units/ kg day) changed with increasing maturity and to determine if the hypothesis that the capacity to consume food bulk is proportional to liveweight can be extended for use in pigs greater than 45kg. The idea that constrained food intake is proportional to liveweight did not hold for pigs that were greater than 40kg, as the scaled capacity for bulk -was not constant but decreased with an increase in liveweight beyond 40 kg. The relationship between liveweight (W, kg) and the absolute capacity for bulk (Cap, kg water holding capacity/day) was accurately described by the quadratic function Cap = (0.230.W) - (0.000476.W2).
A change in food type to one with a higher bulk content may have important consequences for both intake and performance. Initially intake and performance will be reduced. Over time, as the animal becomes adapted to the new food, intake will increase until an equilibrium intake appropriate to that food is reached. Current models that predict intake over time assume that the animal is always fully adapted to the food that it is on. A model was developed with the aim of predicting intake and performance during the adaptation period to a new food. The model is based upon the assumption that that an animal has a desired rate of intake (DFI) which is determined by the requirements for protein and lipid deposition and maintenance. DFI may not be met if a bulk constraint to intake exists. Where a bulk constraint operates intake is calculated as the constrained intake (CFI) which is a function of the animals capacity to consume food bulk and the bulk content of the food. The time taken to adapt to a new food is assumed to depend on the difference in bulk content between the first and second food. The initial capacity to consume food bulk is related to the bulk content of the current food. Thereafter, capacity will increase over time to a maximum. The rate at which capacity is increased is assumed to be the same for all pigs regardless of the type of food being fed, and the increase in capacity over time is assumed to be linear. The magnitude and direction of response of the model is in close agreement with relevant observed data, the underlying theoretical assumptions of the model are concluded to be reasonable.
Taken together the results of the work completed in this thesis show that the idea that food intake on high bulk foods is constrained by the capacity of the gastrointestinal tract in relation to the bulk content of the food can account for observed food intakes on high bulk foods. Physical constraints to food intake are thus concluded to be important for the control and prediction of food intake on poor quality (high bulk) foods. Food intake prediction models that are based on this assumption will therefore have a good theoretical basis and should provide an accurate description of change in intake over time.