The features of a food material that are sensed before it is swallowed, of which an animal is consciously aware, are often collectively called the ‘palatability’ of that food. Palatability is often confused with acceptability or preference, but it cannot be considered to be simply a quality of the food because it depends so heavily on the experience and metabolic status of the animals in question. A good definition is: ‘a response measure which is based on the outcome of the central nervous system’s integration of taste and internal-state signals combined with cues arising from previous associations’ (Grill and Berridge, 1985).
There is no single way to measure palatability, but several methods have been used: (i) intake tests, which often give very different results depending on whether the two foods are offered simultaneously or sequentially; (ii) brief exposure tests with minimal ingestion; (iii) initial rate of eating on first exposure (lick analysis); (iv) oesophagostomy or stomach fistula tests; and (v) taste reactivity observations.
It might be thought that a palatable food will be eaten in greater quantities than an unpalatable food. When a flavour is added to the single food on offer, there is sometimes a short-term increase or decrease in intake but rarely a prolonged change. It is not realistic to expect to be able to stimulate voluntary food intake in the long term simply by changing the flavour of the food.
For example, the addition of Bitrex (the most bitter substance according to the human palate) to the food being offered to growing pigs immediately caused the animals to stop eating (Blair and Fitzsimons, 1970). However, after a few hours they became hungry, sampled the food again but finding it still very ‘unpalatable’ avoided it for a while longer; eventually their hunger got the better of them and, as there had been no adverse consequences of nibbling it earlier, they increased their intake, returning to normal within 3 days of the start of addition of Bitrex.
Thus, a completely ‘unpalatable’ food had become ‘palatable’ within a few days. Although not tried in the reported experiment we can be confident that,
were the same animals to be confronted with Bitrex-adulterated food at some later date, they would recognize it as safe and eat it readily.
Greenhalgh and Reid (1967) developed an ingenious method of separating how much of the difference between intakes of different diets is due to post-ingestional differences rather than to differences in palatability. Two foods of very different sensory and chemical properties were offered to sheep without involving any differences in the composition of the digesta. This was done by matching the weight of food eaten voluntarily with an equal weight given through the ruminal fistula. When straw was given both by mouth and by fistula, the total intake was 13.8 g/kg0.73/day and the digestibility of organic matter (OMD) was 0.41; when dried grass was given by both routes, intake was 59.4 g/kg0.73and OMD was 0.74. When equal amounts of the two foods were given, one by mouth and the other by fistula, the digestibility was intermediate (0.55–0.57) but the voluntary intakes were very different: 23.5 when straw was given by mouth and 48.8 g/kg0.73 when dried grass was given by mouth. This was said to demonstrate a powerful influence of palatability on voluntary intake.
However, given the discussion above, it is quite possible that these sheep were eating according to learned rather than to innate preferences, as they had experienced eating straw before the experiment started. As the experiment progressed they ate less straw – they learned that it provided less nutrients than the dried grass once they had had a chance to experience the effects of eating both.
An example of the lack of correlation between intake when a choice of foods is on offer and when they are given singly is the work in which pregnant ewes were offered one of three silages or else free choice of the three silages (Forbes et al., 1967). Silage B was of poor quality but was eaten in quantities only slightly less than silages A or C by ewes which had access to only one food. However, the group that had access to all three ate equal amounts of silages A and C but very little of silage B.
Gherardi et al. (1991), studying the short- and long-term responses in intake of sheep to additions of chemicals thought to influence palatability, concluded that palatability effects are not important in determining the level at which a single forage is eaten, but can have marked effects on the relative intakes when two forages are on offer. Addition of butyrate plus monosodium glutamate increased preference for a forage by sheep, while magnesium oxide had the opposite effect.
Adding a pleasant flavour can increase the preference for a particular food, and there is sometimes a short-term increase in intake or rate of eating.
Unusually, a prolonged 10% increase in silage intake was obtained with the addition of Simax 100 (a mixture of several flavours with a predominantly orange taste) (Weller and Phipps, 1989) but, more commonly, animals revert to their former level of intake once they learn that there has been no change in the nutritive value of the food.
Sudden changes in the formulation of concentrate feeds, even though the nutrient composition is unchanged, make cows hesitant in eating the new food and suspicion falls on the manufacturer. If the same flavour is included in all
batches of food then changes in the underlying formulation go unnoticed, rate of eating is not affected and the farm staff are kept happy. For example, when rapeseed meal was included in a dairy supplement there was a reluctance to eat on the first few occasions, but this was largely prevented by inclusion of a constant flavour (Fredericket al., 1988).
Drinking water can be used to carry nutrients to cows; ammonium acetate would provide both non-protein nitrogen and an energy source, but it is not readily accepted by some cows. Jackson et al. (1968) found that addition of molasses, sodium cyclamate or ethyl acetate caused cows to drink more whereas saccharin, vanilla or aniseed gave little or no improvement.
Thus, in summary, the extent to which an animal finds a particular food
‘palatable’ depends on innate factors (e.g. bitter/sweet) and on the previous history, if any, of interactions between the animal and that food, or foods with similar sensory properties.
Conclusions
Animals are highly motivated to eat and drink, and this motivation is increased as metabolic demands increase. All the senses are used in the identification of food and the absence of one sense does not, therefore, result in a loss of selective or ingestive ability. However, while mammals rely primarily on taste to identify foods, birds use vision, and quickly learn to associate the sensory properties of a food with the metabolic consequences of eating that food.
Thus, the term ‘palatability’ is difficult to define as it depends not only on the taste and appearance of the food, but also on the nutrient requirements and history of the animal. For example, a food that is initially preferred becomes aversive when its ingestion is coupled with injections of a toxic substance. Such learned aversions and preferences are long-lived and are likely to influence animals’ behaviour for many years.
In addition to innate preferences/aversions and those learned by the animal from its own experience, it can also learn from conspecifics without ever having itself ingested the food in question. Thus, a mother can pass on to her offspring some information about food before the young have themselves started to eat solid food.
As will be made clear in Chapter 13, these learned associations form the basis of the ability of animals to make appropriate selections when a choice of foods is offered, and may also play an important part in determining how much of a single food an animals eats, i.e. its voluntary food intake (see Chapter 10).
7 Diet Selection: Principles
In the previous chapter we have seen the overwhelming evidence that animals can learn to associate the taste, smell or colour of a food with the feelings they experience after they have eaten that food. It was hinted that this is a powerful ability, which allows animals to select from a range of foods to best meet their nutrient requirements, and it will be proposed later that learning is also of importance in determining how much they eat if only one food is available.
This chapter reviews the evidence for diet selection, defines the prerequisites for animals to demonstrate its occurrence and discusses the possible mechanisms whereby animals can make nutritionally wise food choices. The following chapter deals with effects of differences between animals, between foods and in different environments on food choice and shows how diet selection might be exploited in farming practice, while Chapter 13 deals with appetites for specific nutrients.
In modern farming systems, animals are typically presented with a single food. This is not a situation in which most species of birds and mammals have evolved and must be considered unnatural. The ancestors of our farm animals had the opportunity to select from a range of available foods and obviously were able to select a mixture that allowed them to grow and reproduce. It is possible that by eating at random from a variety of foods they obtained sufficient nutrients to survive, but this would not be enough in some situations, e.g. when toxic food sources make up a significant part of what is available. It should be recognized that some species of animal have evolved to eat only a single food, i.e. they have an innate inflexible diet selection. However, many species eat a variety of foods and they need to learn the properties of these foods.
Because animals do not always select from between two or more foods on offer to produce what the observer considers to be an optimum mixture, there has been considerable scepticism as to whether the ability to select is widespread, either across a wide range of species or between food and physiological situations. Sih and Christensen (2001) have collated information
144 © J.M. Forbes 2007.Voluntary Food Intake and Diet Selection
in Farm Animals 2nd Edition (J.M. Forbes)
on what they call optimal diet theory (ODT) from across the animal kingdom and concluded that:
Contrary to our predictions, forager types (invertebrate versus vertebrate ectotherms versus vertebrate endotherms) did not differ significantly in their fit to ODT. Apparently, even organisms with relatively low energy demands or with simple information gathering and processing abilities (e.g. insects, snails or even protozoa) are capable of adaptive foraging.
It is justifiable, therefore, to start from the assumption that farm animals have the characteristics necessary to enable them to select wisely between different foods. Sometimes it is the observer’s prejudices that hinder the correct interpretation of observations made in experiments or in the wild (Forbes and Kyriazakis, 1995).
There are currently new opportunities in diet selection, both from scientific and commercial points of view. The realization that it is necessary for animals to be able to differentiate between foods with different nutrient compositions by colour, taste and/or position, and that they need to be able to learn to associate the sensory properties of foods with the metabolic consequences of eating them, has made it possible to envisage a learned appetite for each of the essential nutrients. Thus, there is now more certainty that if animals can be taught an appetite then this can be used in a choice-feeding situation to improve the balance between their nutrient requirements and their nutrient intake.
Most of the raw materials used in animal foods have concentrations of available energy within a fairly narrow range, 9–13 MJ/kg DM, whereas the content of nutrients such as protein, minerals and vitamins varies over a much wider range. An animal can control its energy intake by varying the amount of DM it consumes but can then only control its intake of nutrients independently of energy if it has access to two or more foods that differ in the content of the nutrient in question. Because of its quantitative importance, high cost and the polluting nature of its excretory products, protein has been widely studied in terms of diet selection and is used as evidence for diet selection in this chapter.
An appetite for protein seems to be very primitive in evolutionary terms, as trained locusts eat more of a high-protein food after preloading of carbohydrate, and vice versa.
Some species of animal rely on a narrow range of foods for their sustenance, and the recognition of these food sources is genetically predetermined. Other species will sample a wide range of potential foods and must learn by experience those that are palatable and nutritious. Whereas deficiency in an animal of a nutrient for which there is a specific appetite will induce increased preference for a food that contains that nutrient, the reverse is not necessarily the case: that is, intake of a modestly greater amount of that nutrient than is necessary is not detrimental and may not induce a reduction in selection for a food containing it in high concentrations.
Taste and smell are the most important senses in feeding situations; visual and auditory cues are not normally part of the food experience and so are not conditioned as well in mammals, while the reverse is true for birds (see Chapter 6).