range of 130–160 g CP/kg, i.e. within the optimum range. When the low- protein food contained 172 g CP/kg they avoided it almost completely, apparently to avoid a toxic excess of nitrogen in the rumen. The motivation for selection of an adequate protein concentration is strong, as sheep are willing to make at least 30 responses in an operant-conditioning situation to obtain a food reinforcement in order to obtain a ‘balanced’ diet (Houet al., 1991a).
An attempt to resolve the question as to whether ruminants select diets to optimize rumen degradable protein (RDP) intake was made by James et al.
(2001), who fed sheep on basal foods formulated and demonstrated to be deficient, adequate or excessive in RDP, and then gave choices between their basal food and the same food with urea (which provides RDP) added. In every case the animals ate more of the urea-supplemented food, even though in the cases of the RDP-sufficient and -excessive foods this provided them with a great excess of RDP. Clearly, these sheep were not managing to control their intake of RDP in order to prevent an excess.
The need for sensory differentiation
Clearly, animals must be able to differentiate between two or more foods if they are to select proportions in order to make up a balanced diet. If the nutrient in question is required only in trace amounts, and especially if it is colourless, it is necessary to give a cue by means, for example, of artificial flavouring and/or colouring. This is not usually a problem with foods differing markedly in protein content, but is surely necessary when a single amino acid is deficient in one food and in excess in the other.
Given that there are these sensory differences, the animal might have an innate preference for one food, e.g. because of its sweet taste. However, nutritional value is not always closely correlated with sweetness, so that the associations the animal learns, between sensory properties and nutritive value, are much more valuable to it than innate preferences. Thus, a bitter flavour imparted to food will cause initial rejection but, if the nutritional value remains balanced, then normal intakes are resumed within a few days. A good example of this is the lack of effect of inclusion of Bitrex, the most bitter substance known to man, on the long-term intake of food by pigs (see Chapter 6). Thus, there is no need to be concerned about the exact nature of the flavours used to differentiate between two foods, as animals soon learn to associate them with the nutrient yields of the foods in which the flavours are incorporated and to eat for nutrients rather than just for taste.
A clear example of the need for sensory differentiation and learning is provided by work with young broiler chicks (Kutlu and Forbes, 1993c), in which mild heat stress increases the requirements for ascorbic acid (vitamin C), offered a choice between a normal food with no supplementary ascorbic acid and an exactly similar food containing 200 mg/kg of protected ascorbic acid.
When neither food was coloured, the birds did not differentiate between them and ate at random. When each food was given a different colour (red or green), however, and the birds were trained to associate each colour with the nutritive value of the food by giving them separately in half-day periods for 6 days, they subsequently selected significantly more unsupplemented food when in the thermoneutral environment and significantly more supplemented food when in the hot environment. A sudden change in environmental temperature was followed by a gradual change in the proportions of the two foods eaten as the birds learned that the red (or green) food was no longer the one that made them feel metabolically most comfortable.
In summary, farm animals can select a diet appropriate to their metabolic needs as long as the foods offered are clearly differentiated by flavour or colour and the animals have the opportunity to learn the nutritional difference between the foods.
Training and experience
In order for an appetite for a specific nutrient to develop, it is necessary for animals to learn to associate the sensory properties of each food with its
content of the nutrient in question. In many cases they will learn about two foods if they are introduced simultaneously, but they may learn more quickly if each food is given in turn for a few days. It is evident that some form of
‘education’ about the foods on offer prior to a choice may aid animals in their later discrimination between foods, but in many cases no advantage of such training has been noted. In group situations, there may be opportunities for animals to learn about food in other ways, and ‘training’ periods may be less important than for individual animals. A training period may be a hindrance to choice feeding being accepted commercially, due to the high levels of management involved.
Poultry
While large farm animals have been given alternating days on two foods to be offered later as a choice, half-day alternating periods have been used for broiler chickens, on the basis that smaller animals, with a higher rate of turnover of nutrients, need shorter periods of exposure to learn the characteristics of different foods. It takes hens only a few hours to recognize a change in protein content of the food.
Training birds by accustoming them to whole grains at an early age appears to confer benefits at the later stages of growth, in terms of ability to select foods to meet nutrient requirements. Broilers trained from 10–21 days after hatching by giving whole sorghum and protein pellets showed no difference in weight gain compared with complete-fed or untrained choice-fed birds, but the trained birds were significantly more efficient because the inexperienced choice-fed birds ate much greater excess of the protein concentrate in the first 10 days of the experimental period than those with previous experience (Covasa and Forbes, 1993).
No effect was found on subsequent selection for whole wheat of time of access to wheat nor deprivation during the rearing phase, but those birds given wheat alone for part of each day during rearing ate significantly more wheat during the growing phase (Covasa and Forbes, 1995a). They had heavier proventriculus and gizzard than those given mixed wheat and starter crumbs during the rearing phase, but growth and abdominal fat pad weight were unaffected by treatment. Thus, it may be beneficial for subsequent diet selection to remove the standard food while offering whole grains to young chicks during the training period.
Pigs
Kyriazakis et al. (1990) trained growing pigs to recognize the difference between HP and LP by offering them as single foods on alternate days for 1 week before offering both in a choice-feeding situation but they did not compare their performance with untrained animals, so whether it is helpful or necessary is not known. Newly weaned pigs do not seem to benefit, in terms of subsequent ability to select between HP and LP, from a period of training such as that advocated by Kyriazakis et al.(Dalby et al., 1995). Whether they were
given alternate daily exposure to the foods for 6 days, given one for 3 days and the other for 3 days or given free choice for 6 days, there was no difference in food intake or growth rate over the next 21 days of free-choice feeding, compared with controls fed a single, adequate food throughout, but all choice- fed pigs ate significantly less protein than controls.
Comparisons have been made between selection for protein by trained and untrained growing pigs (Morganet al., 2003). Groups of four pigs were formed containing either an individual trained to select between two foods or an untrained control animal, and all were offered LP and HP foods. Over a 2-week period there was no effect of the trained pig on the growth rate. However, for the first few hours, the groups with the trained pig were more consistent in selecting a balanced diet close to that of the trained pig, while the untrained groups showed much more variation initially, but stabilized within 3 days. Thus, although this form of training does influence piglets, the untrained piglets learn to achieve a balanced diet so quickly that there is no effect on long-term performance.
Cattle
In a study of training in dairy cows, they were offered two mixtures of grass silage and concentrates, one with a high concentration of protein, the other low in protein (Tolkamp and Kyriazakis, 1997). Some cows were offered both at the start of the experiment, while others had only one food for the first 3 days followed by just the other for the next 3 days. Yet other animals had two such periods of alternate access before being offered both as free choice. After the first week, untrained cows selected 0.66 HP, significantly different from random.
From the third week onwards, those given one or two periods of 3-days access to each food alone established similar diet choice: 0.70 HP. It was concluded that, under the circumstances tested, training was not required for cows to distinguish between two mixed foods with different calculated MP/ME ratios and to select proportions significantly different from random.
Identifying ‘nutrient requirements’ of animals (the need for adequate control) For the proper design and interpretation of diet selection experiments, we first have to be clear about what food resource(s) it is we are studying. Then we need to formulate foods that provide a greater and a lesser ratio of resource:energy than the animal requires. This entails knowing what the animal’s ‘requirement’ is for the resource in question and for energy, which typically entails the feeding of a range of single foods with different ratios of resource:energy (made as mixtures of the two foods to be given as a choice), building up a dose–response curve that allows us to define the optimum.
In many other diet selection studies, the optimum resource:energy ratio has been taken from the literature, which may be satisfactory in the case of intensively managed animals in which genotype, diet and environment are well characterized. It can, however, be easy to make unwarranted assumptions. It is generally considered, for example, that chicks do not require supplementary
ascorbic acid in their diet, yet they choose a significant proportion of a supplemented food (see Chapter 8), which might be taken as evidence for unwise selection had it not been demonstrated that, under the conditions of the experiments in question, dietary ascorbic acid was beneficial to growth and food intake, especially at high environmental temperatures.
Just because two foods offered in a choice situation intentionally differ in their content of a nutrient, e.g. protein, according to some calculation or limited analysis, does not restrict animals to selecting only on the basis of that nutrient.
This is especially true for protein, where it is notoriously difficult to formulate foods with different protein contents but the same ratio of amino acids. Pigs given choices between pairs of foods containing 220/180, 220/140, 220/100 or 180/100 g CP/kg grew well and efficiently, but it was found that the isoleucine content of the chosen diet was almost exactly what would be predicted for optimal growth, suggesting that the pigs might have been selecting in order to obtain the correct amount of this most limiting amino acid rather than CP as intended (Bradford and Gous, 1991). Proper interpretation was not possible, therefore – a situation existing with most protein selection experiments.
Even when diets selected by choice-fed animals are compared with
‘control’ diets, care must be taken not to draw unwarranted conclusions. For example, broiler chicks given free access to foods containing 456 and 86 g protein/kg selected proportions that gave the same growth as controls (Kaufmanet al., 1978). As they grew, the proportion of HP taken in the diet fell to give a decline in the protein content of the selected diet from 250 to 140 g/kg. The authors pointed out that this was a lower protein content than the single food given to controls, implying that the birds had not selected appropriately but, as the control food contained an excessive amount of protein (265 g/kg), this was not surprising.
Previous nutritional history
Animals rendered deficient in a nutrient will, when given access to a choice of foods one of which provides an excess, eat more of that food than do non- deprived control animals. For example, pigs made fat through feeding a low- protein food subsequently selected a much higher protein content when choice-fed (233 g CP/kg) than those previously given a high-protein food (175 g CP/kg); this diet selection enabled them to deposit fat at a slower rate and protein at a faster rate than the latter animals and return their body composition to a ‘desired’ one (Kyriazakis and Emmans, 1991). It must also be borne in mind that gender influences growth potential and that males chose a diet higher in protein than did females (228 versus 181 g CP/kg).
As the nutrient requirements of animals vary with stage of growth, gender, breed and nutritional history, so diet selection should not be expected to be static or uniform across animals. Great care is needed, therefore, in deciding which is more ‘correct’, the choices made by selecting animals or the expectations of the human observer, when interpreting the results of diet selection studies. The animal’s ‘target’ might be different from that assumed by
us on their behalf; for example, choice-fed animals are often somewhat fatter than those given a single, commercially optimal food, the implication being that we often ‘force’ animals to be a little leaner than they ‘want’ to be.
Concentrations of the resource being studied
It is short-sighted to provide two foods with high and low contents of the nutrient in question such that the optimum diet consists of half of one and half of the other, because equal intakes of the two foods can be interpreted as random eating or directed choice where the optimal is 50:50 (see Lawson et al., 2000). Better to design foods for which an optimum mixture is well away from 50:50, as in the study of Tolkamp and Kyriazakis (1997), so that a statistical comparison of the observed proportion of one food can be tested against the nul hypothesis of 0.5.
For animals to make nutritionally meaningful choices, all foods on offer should be nutritionally imbalanced, otherwise there is no benefit to be had in choosing. Many experiments have offered a choice between two foods, neither of which is demonstrably imbalanced. For example, several experiments on amino acid appetite in growing pigs have shown that the pigs could grow perfectly well on either given alone so that, when given a choice between the two, individual animals adopted different choices and overall no significant selection was apparent.
There are instances where animals have been given a choice between two foods in both of which the content of the nutrient under study is below their requirements, either due to miscalculation of their requirements or intentionally.
The expectation is that the animal will consume the less limiting food (e.g. the food with slightly lower protein over a food very low in protein) and avoid the more limiting one. With laying hens (Holcombe et al., 1976), broiler chickens (Shariatmadari and Forbes, 1993), growing pigs (Kyriazakis et al., 1990) and sheep (Kyriazakis and Oldham, 1993), animals consumed appreciable amounts of the more limiting food, i.e. more than just occasional sampling. This is puzzling and suggests that they are confused, because neither food provides sufficient protein. Choosing between two limiting foods may be more difficult than choosing between two which can, together, provide a balanced mixture.
Environmental factors
Farm animals are easy to train in operant-conditioning situations in which they have to ‘work’ for a food by pressing a coloured panel. If the animal has to work harder to obtain one food than the other, then it is likely to bias selection in favour of the food that is obtained with less effort.
Because two foods offered in choice are usually given in different troughs, sometimes one trough will be in a more favoured position than the other. We have observed such an effect due to the positioning of a heater for newly weaned piglets, in which food in the trough closer to the source of heat was
eaten in greater quantities than food further away, to the extent of biasing diet selection against the choice of an optimum diet.
Another constraint on completely free choice between foods would be if one food were to be more difficult to obtain than the other. Ginane and Petit (2005) offered heifers a leafy (L) and a coarse (C) hay, together with a physical and a temporal accessibility constraint given singly or combined. The physical constraint involved covering the trough containing L with a steel grid of 4 cm mesh size, and the temporal constraint limited the daily access time to both hays to 4 h. The physical constraint made the heifers decrease their choice (proportion of feeding time or intake) for L regardless of access time, whereas the temporal constraint had no significant effect on choice (see Fig. 7.8). The presence of mesh over food L caused a reduction in the proportion of L eaten and increased the rate at which L was eaten. Reduction in time of access from 24 to 4 h reduced total food intake while increasing the rate of eating. Even when they were not constrained, the heifers continued to eat some C, thereby showing preference for a mixed diet.
0 2 4 6 8 10
open 24 mesh 24 open 4 mesh 4 (a)
0 100 200 300 400 500
open 24 mesh 24 open 4 mesh 4 (b)
Feeding time (min/day)DM intake (kg/day)
Fig. 7.8. (a) Daily dry matter (DM) intake and (b) feeding time on leafy (open bars) and coarse (solid bars) hays when offered in a choice situation, according to physical (open or 4 cm mesh) and temporal constraints (24 or 4 h of access daily) (from Ginane and Petit, 2005).
We learn from this example that there are complex interactions between rate of eating and food choice that could make it difficult to interpret the results unless we were aware of the effects of the constraints.
Social interactions
When animals are in a group in which a social hierarchy has developed, they often have to compete for access to food(s). An animal low in the pecking order might not be able to gain equal access to both foods, while a dominant animal might feel obliged to maintain its position by eating more of the ‘better’ food than is optimal for its nutritional status. This situation could be accentuated when the trough space is not sufficient for all animals in the group to eat simultaneously, so that free and continuous access to both foods cannot be achieved. Such bias can be overcome by keeping animals in individual pens or cages. However, it has been shown in some situations that animals learn about foods more quickly when they are in groups than in individual cages (see Chapter 6).
We know that the behaviour of animals in groups can differ markedly from that of individuals, and there is strong evidence of nutritional wisdom being passed between individuals (e.g. mother to offspring, older to younger animals, see Chapter 6). A serious study should be made of the effect of single-penning and the social environment on diet selection, and particularly on the variation in selection between individuals. If the potential to use diet selection methodology to study the optimum nutrition of farm animals is to be realized, then practical problems such as optimum group size and the need for a training period need to be resolved.
Poultry
Most chicks in a choice situation begin to eat from both foods on offer within a fairly short time, but there is often a minority that is slow, and there may be a few individuals who fail to select close to an appropriate diet. However, animals living together in a group tend to copy from each other and there is usually a leader that guides the others to the desired food. Individually caged broilers that did not voluntarily consume any wheat, when given a choice with a standard grower food, immediately started to eat significant amounts when put in pairs, irrespective of whether the partner was formerly a wheat-eater or not.
Within 5 days of being given a choice between a calcium-deficient food and calcite, broilers consumed enough calcium when kept in groups (Joshua and Mueller, 1979). However, individual caging inhibited this ability even when there was visual contact between birds, but birds caged individually after learning to eat calcium in a group took an adequate amount of calcium.
Pigs
One of the ‘rules’ for successful choice feeding is that animals have access to both choices ad libitum. Clearly, this will not always occur when animals are