Sheep
It might be expected in ruminant animals that products of digestion in the rumen would affect diet selection. Infusion of sodium acetate or sodium chloride intra- ruminally in sheep gave large reductions in the intake of concentrates but had no effect on hay intake (Engku Azahan and Forbes, 1992). The effect was almost as great for chloride as it was for acetate, so the major reason for the decreased intake was probably osmotic (see Chapter 3).
Poultry
As animals grow, the increased cost of maintenance and the increase in rate of fat deposition mean that the optimal ratio of protein:energy in the diet declines.
It is to be expected, therefore, that broilers offered a choice of foods higher and lower in protein:energy yield would progressively eat a greater proportion of the food lower in protein; this has been observed to occur with several different combinations of protein content in the two foods (Shariatmadari and Forbes, 1993). Males grow more quickly than females and it is not surprising, therefore, that they choose a high proportion of high-protein food when given a choice with a low-protein food (see Fig. 7.11).
Correlations between selection for protein and rates of deposition of protein and fat in the body might be the cause rather than the result of different growth potentials at different ages or in the two genders. In attempts to resolve cause and effect, corticosterone has been used to reduce the ratio of muscle:adipose tissue being deposited in broiler chickens. In a short-term experiment in which 2-week-old broilers were offered a choice between whole wheat and a balancer food, corticosterone injection at 4 mg/day for 5 days induced a significant preference for the whole wheat and thereby a reduced protein:energy ratio in the diet, as predicted (Covasa and Forbes, 1995b).
In a more comprehensive experiment, male broiler chickens were given free choice of three foods in each of which protein, lipid or carbohydrate was isocalorically substituted for one other macronutrient (Malheiros et al., 2003).
0.4 0.5 0.6 0.7 0.8 0.9
0 1 2 3 4 5
Week of treatment
HP as proportion of total intake
Fig. 7.11. The effect of corticosterone on the proportional intake of HP food in male (circles) and female (squares) broiler chickens. Closed symbols, control; open symbols,
corticosterone (from Sahin, 1998).
Treatment with corticosterone (45 mg/kg of food) increased liver and fat pad weight and tended to reduce the dietary content of protein and increased that of fat; there was no effect on daily food intake.
Not all results are as predicted, however. Male and female broilers were offered high- (HP, 285 g CP/kg) and low-protein (LP, 70 g CP/kg) foods and given daily intramuscular injections of 4 mg/kg of corticosterone and had, as a result, a decrease in protein accretion and an increase in total body fat content (Sahin and Forbes, 1997b) (Fig. 7.11). Corticosterone significantly increasedthe intake of HP as a proportion of total food intake from the third week of treatment in males (from 195 to 245 g/kg) and from the second week in females (from 183 to 203 g/kg). Rather than reducing the protein:energy ratio of the diet selected, corticosterone treatment increased the preference for the HP food. This is likely to be due to the increased protein turnover and/or to increased protein degradation to supply the carbon skeleton used for fat synthesis.
Similar increases in the ratio of protein:energy intake were found with inclusion of corticosterone in the diet of broilers (Sahin and Forbes, 1997a) and laying hens, in which it decreased the egg production (Sahin and Forbes, 1999). This might be due to the fact that corticosterone changes the oestradiol profile in plasma, resulting in the change in the route of lipid transport from the yolk to the abdominal fat pad, and then builds the new body profile with an increase in fat anabolism. Therefore, the demand for protein is increased rather than reduced because of the wasteful usage of dietary protein.
Pigs
As with broiler chickens, growing pigs show a decrease in the protein:energy ratio of the diet they choose from high- and low-protein foods (Kyriazakiset al., 1990).
Stimulation of the rate of protein deposition – and depression of fat deposition – in choice-fed growing pigs by daily injections of 4 mg STH significantly depressed food intake. This decrease was accounted for entirely by a reduction in the amount of the low-protein food (120 g CP/kg) selected (the other food contained 240 g CP/kg), so that energy intake was reduced but total protein intake was unaffected (Roberts and Azain, 1997). When the treatment was stopped intake increased, so that within 3 days it had reached pre- treatment levels, but this was by increased consumption of both high- and low- protein foods rather than specifically by normalization of intake of the low- protein food.
Sheep
It has been observed that growing lambs offered high- and low-protein foods chose a diet well matched to their requirements for growth and, even when one of the foods required up to 30 responses to obtain a reinforcement, the lambs responded accordingly to maintain this balanced diet (Houet al., 1991b).
Diet Selection: Principles 169
Pregnant ewes select a significantly greater proportion of a food of high CP concentration than non-pregnant ewes, reflecting their enhanced demand for protein (Cooper et al., 1994). Thus, diet selection is driven not only by the composition of the foods on offer but also by the requirements of the animal, which change in a systematic manner with growth and reproductive cycles.
Conclusions
The theory that animals can select, from a choice of foods, a diet that meets their requirements is difficult to prove, but it is clear that there are many situations in which they show considerable ‘nutritional wisdom’. The most widely studied situation is one in which animals are offered two foods, with higher and lower protein contents than required in relation to the energy concentration of the diet. Manipulating the protein content of one or both foods is accompanied by diet selection to maintain a fairly constant protein intake.
A training period, in which the foods are offered alternately, can be useful in ensuring that animals unambiguously associate each food with its nutritional value. Individuals can also learn about the differences between foods indirectly, from conspecifics, particularly the mother.
It is far from clear what internal mechanisms are involved in controlling diet selection; very few studies have been made in this area with farm animals.
What is clear is that the principles established in this chapter for protein apply also to many other components of the diet, as will be demonstrated in Chapters 8 and 13.
8 Diet Selection: Practicalities
In Chapter 7 we presented evidence for the ability of animals to select between foods in order to obtain a diet balanced for energy and protein; this is based on learned associations between internal and external stimuli, covered in Chapter 6.
We have also discussed the prerequisites for successful studies of diet selection and effects of changes in physiological state, especially growth. In this chapter we bring the subject closer to application by covering those aspects of diet selection that can be utilized in practice and, again, selection for protein is the main focus of research because of the economic and biological importance of protein intake, in relation to energy intake. Examples of appetites for nutrients, including protein, are given in Chapter 13.
The controversial subject of self-medication is discussed; this is of great current interest with the banning of the routine use of antibiotics and other drugs in livestock production and the consequent search for natural remedies for the diseases associated with intensive agriculture. The potential for using applied selection methodology to the commercial situation is also considered.
It has been demonstrated in Chapter 7 that, where two foods differ in the concentration of a nutrient such that one contains more and the other less than optimal, in relation to energy requirements, then animals eat the two in such a ratio that the intake of the nutrient in question is optimized. The higher the demand for the nutrient, the greater should be the proportion of the food high in that nutrient. Note that the animal can cope with mildly imbalanced diets but at a metabolic cost; where possible, it should avoid this cost by eating that amount and mixture of foods that minimizes the cost (discomfort, Chapter 10) of metabolism.
There is a large literature on selection by herbivores at pasture, not surprising given the agricultural and ecological significance of herbivores, especially ruminants. Not only do most grazing animals have to make choices between different species of plant, they also can choose which part of the plant to eat – this ability varies depending on the width of the incisor arcade – and whether to
© J.M. Forbes 2007.Voluntary Food Intake and Diet Selection 171 in Farm Animals 2nd Edition (J.M. Forbes)
select against old herbage. As this book does not cover herbage intake, this important subject is omitted, but see the proceedings of relevant conferences (Hodgson and Illius, 1998; Lemaire et al., 2000) and examples of experimental (Parsonset al., 1994) and modelling (Thornleyet al., 1994) reports.