There are various ways of monitoring food intake, but some of the indirect ones have a large amount of variability associated with them. Automation allows the collection of large amounts of data without the presence of a human observer to disturb the animals. It is possible to monitor the feeding behaviour and food intake of individual animals kept in a group, which allows monitoring under realistic farm conditions.
This has paved the way for the use of food intake and efficiency of utilization in selecting breeding stock. It has also allowed better statistical
Feeding Behaviour 39
Fig. 2.12. Time spent feeding (blank) and performing stereotypies (filled) by lactating cows fed on a total mixed ration. Period 1, weeks 3–14 of lactation; period 2, weeks 17–26. AL, ad libitum throughout; AL-R, ad libitum in period 1, restricted in period 2; R, restricted throughout (from Redbo et al., 1996).
methods of defining inter-meal intervals to be developed. Feeding is characterized by relatively stable food intakes over periods of several days, but also by widely different meal patterns between individuals on the same food and within individuals with different foods.
The feeding behaviour of an animal in a barren experimental environment might be quite different from that of the same animal in more natural surroundings, and it has been suggested that the use of operant systems that make the animal work to obtain food encourage the animal to adopt a more natural feeding pattern. Unnatural feeding-related behaviour is seen in animals that are frustrated, either by lack of food or by being kept in a restricting environment.
There are strong circadian rhythms of feeding, but these are not inflexible:
for example, chicks will eat during the dark if that is the only time food is available, even though they normally eat nothing at night. Short periods without food can be compensated for when food becomes available, but longer periods of fasting result in a significant reduction in daily intake.
3 Feedbacks from the Gastrointestinal Tract
In searching for hunger/satiety signals we are looking for changes in the body that go in one direction during a meal, may continue in that direction for some time after the meal, but eventually return to the pre-meal level. There are numerous changes that fit these criteria, including physical and chemical factors in the gastrointestinal tract, and hormones and metabolites in the bloodstream. These changes, and the routes by which information concerning them is carried to the brain, are referred to as negative feedback pathways. It is unlikely that, under normal circumstances, only one factor is involved in the termination of feeding, or satiety. Rather, satiety is proposed to occur when the combined strength of signals from gastrointestinal and liver receptors reaches a threshold (MTD hypothesis, see Chapter 10).
Even this is a simplification, however, as it does not allow for long-term balancing of food intake with requirements; in order to achieve this match, feeding must terminate long before the animal knows how much of the various nutrients have been absorbed. In order to provide ways of predicting nutrient availability, learned associations between the organoleptic properties of a food and its eventual nutritive value are developed (see Chapter 6). However, the considerable variation in meal size and number within a day, both between and within animals, throws great doubt on the importance of short-term negative feedback signals as the only determinant of the amount of food eaten over periods of 1 day or more. This has already been alluded to in Chapter 2, and we will return to this theme in later chapters. This chapter deals with the gastrointestinal tract, while metabolites and hormones are covered in Chapter 4.
There is particular emphasis in this chapter on ruminant animals because of the great economic importance of their having evolved a large and complex set of stomachs in which digesta are stored for many hours for microbial fermentation (Church, 1988). This long period of storage makes the physical capacity of the stomachs a potential limiting factor to intake and gives considerable importance to the rates of digestion, breakdown and onward passage of particles of food. In
© J.M. Forbes 2007.Voluntary Food Intake and Diet Selection 41
in Farm Animals 2nd Edition (J.M. Forbes)
addition, there are several unusual products of digestion, particularly the volatile fatty acids (VFAs), which the host animal must metabolize and which are therefore potentially important controllers of voluntary intake.
Altering the digestibility and rate of passage of a forage food causes parallel changes in intake (Van Soest, 1994). For example, supplementation of low- protein forages with urea increases the rates of digestion and passage and allows a greater intake. Grinding a forage food also increases its rate of flow out of the rumen and allows increased voluntary intake. Although the digestibility of the food is reduced by grinding, since the food is in the rumen for a shorter time, the total weight of nutrients absorbed daily is increased. Such treatment of poor forages, accompanied by pelleting, is sometimes practised commercially.
Relationships between the digestible energy concentration of food and level of intake by ruminants and simple-stomached animals are discussed in Chapter 11.