The UK pig industry has shown an ability to respond quickly to changing market requirements and improving efficiency of pig meat production. Rapid genetic improvement in breeding stock, particularly in carcass quality and growth traits, together with the adoption of new production strategies, have been the main factors. These changes have had major implications for the way pigs are fed at all stages of the production cycles. Market forces have resulted in a pig population with reduced fat reserves and low appetite. Food intake is a limiting factor at several stages of the production cycle including post weaning, in the case of early-weaned piglets, and the management of young lactating sows. Knowledge of the daily nutrient requirements of the individual animal far exceeds the ability to ensure that those requirements are satisfied by feeding systems in commercial use. Manipulation of food intake to achieve optimum profit is the weakest link in the UK producers' knowledge.

The central nervous system is the integrator of most of the actions of the animal and as such plays a vital rôle in the control of voluntary food intake. Much of the work to understand how intake is controlled has been carried out with rats but that which has been done with pigs is included. The first experiments used electrolytic lesions in the designation of the ‘hunger centre’ and the ‘satiety centre’. Recent work has identified the paraventricular nucleus as a sensing site for experimental manipulations. Chemical stimulation of the brain has also been carried out to try to gain understanding of the rôle of neurotransmitters. Noradrenaline (NA) stimulates intake when given into many sites. Serotonin (5-HT) inhibits intake and has been claimed to play a rôle in the selection of macronutrients but 5-HT must now be interpreted in the light of the existence of several different subtypes of 5-HT receptors. Dopamine appears to moderate the hedonic response of eating. Numerous peptides are active in the brain where their rôle as neuromodulators may be quite different from their function in the periphery and at least three types of opioid receptors are implicated with kappa antagonists producing the most potent facilitatory effects. Neuropeptide Y and peptide YY produce massive orexigenic effects which readily overcome peripheral satiety factors. The brain cannot control intake in isolation. It receives inputs in the blood stream, such as glucose, as well as via the nervous system, both from the special senses and from visceral organs such as stomach, intestines and liver. Taste and olfaction are important in diet selection and a specific appetite for protein has been demonstrated in the pig.

The short-term regulation of intake is thought to be achieved at least in part by signals from the gastrointestinal tract. Infusions of glucose, protein hydrolysate or emulsified fat into the stomach, and of glucose or protein hydrolysate into the duodenum, jejunum or ileum caused pigs to reduce their intake to compensate approximately for the energy infused, probably through the regulation of gastric emptying. Protein or protein hydrolysate infusions also caused a long-term inhibition of intake for 30 h by up to three times the energy infused. During feeding gastric emptying occurred at a constant rate of calories, the rate increasing linearly with body weight; duodenal infusion of glucose slowed gastric emptying to compensate for the infused energy and satiety occurred at a reduced food intake but at the same stomach volume. Duodenal infusion of emulsified fat caused a greater than caloric inhibition of intake, which was blocked by the cholecystokinin (CCK) antagonist L-364,718 and appeared to be mediated through a mono-glyceride-induced CCK release. Emulsified fat infused into the duodenum slowed gastric emptying by more than the energy infused and the gastric volume at satiety was decreased suggesting that the effects of fats on intake are possibly independent of gastric emptying. Since L-364,718 did not increase intake on a normal diet (186 g protein, 20 g fat per kg), CCK may only contribute to satiety in pigs eating high-fat diets.

The rôle of voluntary food intake, measured as daily food consumption on ad libitum feeding from 30 to 90 kg live weight, in future selection strategy is reviewed. Intake shows a heritability of 0.3, and genetic correlations of 0.6 with growth rate and –0.4 with leanness. Low genetic correlations between test station and commercial farm performance are reported for growth rate (0.27) and backfat (0.41) which arise either from genotype × feeding level interactions, or from individual feeding at stations. Selection for rate of lean growth appears to lead to a primary increase in rate of protein deposition, whereas selection for efficiency of lean growth appears to lead to a reduction in rate of fat deposition via a decline in intake. Continued reduction in intake may limit further improvement in lean growth rate and sow productivity. As optimum fatness is approached, the selection emphasis is expected to swing towards rate of lean growth to reduce total food used for maintenance. To determine the optimum selection regime, a knowledge is required of the genetic relationship between intake and lean growth rate. Meanwhile, the optimum selection regime may involve ad libitum group feeding with electronic recording of individual food intake. In the long term, exogenous or endogenous growth promoters could remove the need for selection against backfat, and necessitate a radical genetic increase in intake.

During the 1st week of life the energy contained in the body of the average piglet increases by a factor between four and five. To achieve this increase the piglet must ingest metabolizable energy (ME) at a rate which is about four times its maintenance requirement. Well over half the ME supplied in the milk of the sow is in the form of emulsified fat. Whilst the piglet is being suckled, its intake is controlled by litter size, availability of teats and productiveness of the mammary gland. At 3 weeks of age the normal piglet requires about 7.8 MJ digestible energy to sustain a live growth rate of 280 g/day. At this stage the growth comprises about 40 g protein and about 68 g fat. This means that the required intake for a newly weaned piglet on a typical starter diet should be about 475 g/day. In practice such intakes are rarely achieved for several days, therefore weaning causes severe disruption of intake and of the growth curve and there are other factors involved including social disturbance and stress. Further problems are caused by the provision of diets which are unsuited to the physiology of the piglet's immature digestive tract. The sudden loss of the IgA component and indeed other protective factors contained in the dam's milk can be extremely serious, and the reduction in food intake is effectively a defence strategy by the piglet to cope with its new circumstances and try to maintain physiological homoeostasis. In some circumstances the use of antibacterial agents, organic acids or probiotics may be beneficial, but the approach is less reliable than careful formulation of the diet to avoid provocative ingredients such as soya-bean and rapeseed meal.

The amount of food eaten by the pig is a balance between the needs of the animal and the ability of the food to meet those demands. Major factors influencing the needs of the animal are live weight, genotype and sex. Equations have been established to predict the intake of pigs at different live weights but variation exists amongst these equations which reflect a change to genotypes with smaller appetites. Rapid growth may be limited by the appetite of the pig and differences in intake between the sexes are notable, the biggest being castrated males compared with boars and gilts. Intakes of boars and gilts can be similar but often gilts eat slightly more. Dietary energy has a marked effect, with the pig attempting to adjust its daily energy intake by eating less of high-energy diets. The extent to which it can exert this physiological control is limited when a stage is reached where it is unable to compensate by eating more of a low-quality diet because of physical capacity. The influence of protein on intake has received much less attention but there are clear indications that pigs eat less of diets which are very high or very low in crude protein. Intake is also influenced by protein quality and there is evidence from work at the University of Nottingham that quite small variations in lysine level relative to other essential amino acids can markedly affect intake.

The lactating sow requires 5 to 7 kg food daily if it is to maintain body weight and condition in lactation but even on ad libitum intake sows fail to achieve the required food consumption and so lose both weight and tissue. The problem is most acute in primiparous sows and low lactation food intake is recognized as a major factor in post-weaning anoestrus and impaired reproductive performance. Studies of sows from the Moorepark herd are reported. Older animals, those with high suckling load and thin animals all tend to eat more with maximum daily intake occurring at 4 to 5 weeks of lactation. Although intake reduces at high environmental temperatures, this has been alleviated by wet feeding, offering a high energy-density diet or a high-protein diet. Other factors observed to influence voluntary food intake are breed, lighting pattern, dietary calcium and phosphorus levels.

In pigs, variation in appetite under ad libitum feeding conditions is often associated with variation in carcass composition and meat quality. Breeds, strains and pigs types possessing high appetites are frequently fatter and have better meat quality (e.g. lower PSE incidence and more juicy meat) than those possessing low appetites, although for different physiological reasons. These conclusions are based on published reports of performance, carcass and meat characteristics in the following pig types: carriers of the halothane gene, Pietrain and Duroc breeds, animals selected for low backfat thickness and entire males. Selection schemes which reduce carcass fatness yet increase appetite are preferable to those which depress appetite so far as meat quality is concerned.

The thermal components of the environment, through their effects on the animals' heat exchange, influence growth, metabolism and voluntary food intake. The environmental components considered include air temperature, either constant or fluctuating, air movement, relative humidity, group size, stocking density and atmospheric concentrations of various gases and dusts. For the growing pig, it has been calculated that each 1°C change in temperature is associated with a 0.65 MJ metabolizable energy (ME) per day change in energy intake; for the other environmental components the variation was between 0.36 and 0.65 MJ ME per day. From the relation between environmental temperatures, food intake and growth, it was shown that for young pigs the optimum temperature range was between 20 and 25°C; for older animals it was between 10 and 20°C. Knowledge of the extent to which food intake changes with the environmental circumstances, both cold and hot, allows feeding and managemental strategies to be developed to ensure optimum growth and efficiency of food utilization.