et al. (1990) included choices between two foods with different protein concentrations. When one food contained more, and the other less, than the likely optimum protein level, the pigs selected a mixture which gave almost identical overall protein contents of about 205 g CP/kg (see Fig. 7.4). When both foods had less protein than required (125 and 171 g CP/kg, respectively), more was eaten of the one with the higher protein level; it is, perhaps, surprising that they did not eat entirely or almost entirely from the higher- protein food. When both foods provided too much protein, intake was almost exclusively from the one with the lower protein content, suggesting a strong drive to avoid an excessive protein intake. As shown in Fig. 7.2 (Dalbyet al., 1995), there is variation in the selection pattern between a low-protein food (L) and one high in protein (H) by different animals.
In order that animals can make sensible choices between foods they must be able to differentiate between them before they eat them, and the foods used here were clearly different in appearance, smell and taste because of the different ingredients used to achieve the different protein contents. These pigs had the opportunity to balance both energy intake (as a result of their total intake) and protein intake (as a result of the choice they made between HP and LP), whereas pigs given a single food could only meet their energy and protein requirements exactly if the food was perfectly balanced. Even if the diet allowed them to meet their requirements for both energy and protein at one point in time, the same food would no longer be well balanced some time later when the growing animal’s protein requirements, relative to energy requirements, had fallen.
0 200 400 600 800 1000
65:115 65:225 115:225 225:280 280:320
CP contents of LP and HP (g/kg)
112 217 195 227 282
Proportion chosen (g/kg)
CP content of chosen diet (g/kg)
Fig. 7.3. Proportions of HP (solid bars) and LP (open bars) chosen by broiler chickens offered pairs of foods varying in protein content. The CP contents of the two foods on offer are shown at the bottom of each column and the resulting CP content of the diet chosen at the top (from Forbes and Shariatmadari, 1996).
Thus, an animal offered a single food has to compromise for much of its life by either: (i) controlling its energy intake and suffering the consequences of eating too much or too little protein; (ii) controlling its protein intake and suffering the consequences of eating too much or too little energy; or (iii) compromising between the two. We will consider in Chapter 10 which of these is the most likely.
Cattle
Experiments with several different pairs of foods given to different individuals, as described elsewhere in this chapter for chickens, pigs and sheep, have not been performed, but a number of demonstrations of the ability of dairy cows to select for CP have been provided by Tolkamp and colleagues, and are detailed in Chapter 13. These have offered two foods of mixed silage and concentrates, the composition of the latter used to manipulate protein content.
A different approach to the study of cows’ ability to choose appropriately for protein was adopted by Lawson et al. (2000), mindful of the difficulties in practice of providing two mixed foods for large numbers of cows. They offered 24 cows free access to grass silage and, for 3 weeks, a choice between differently flavoured concentrates with 90 or 39 g digestible undegradable protein (DUP)/kg DM, up to a maximum of 5.4 kg DM/day, via computer-controlled out-of-parlour feeders such as could feasibly be provided on a commercial dairy farm. The amounts of the high-protein food eaten as a proportion of total concentrate intake were 0.47, 0.45 and 0.50 for the 3 consecutive weeks, with a much higher standard deviation in the first week (0.372) than in the second (0.265) or third week (0.252).
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0 200 400 600 800 1000 1200
125:171 125:217 125:266 171:217 171:266 217:266 CP contents of LP and HP (g/kg)
CP content of chosen diet (g/kg)
160 208 204 202 205 218
Proportions chosen (g/kg)
Fig. 7.4. Proportions of HP (solid bars) and LP (open bars) chosen by growing pigs offered pairs of foods varying in protein content. The CP contents of the two foods on offer are shown at the bottom of each column and the resulting CP content of the diet chosen at the top (from Kyriazakiset al., 1990).
This greater initial variation in selection is shown clearly in Fig. 7.5, which includes the diet selection pathways for eight of the 24 animals. Two cows ate almost entirely HP; another cow ate almost only LP for the first 6 days; the remaining five ate closer to equal amounts of HP and LP. In all animals but one, however, the selection paths eventually became approximately horizontal, confirming that approximately equal amounts of LP and HP were being eaten once the animals had become accustomed to the choice-feeding situation (and had learned to associate the sensory properties of each food with the metabolic consequences of eating it). The fact that the preference ratio was not significantly different from 0.5 could be due to an indifference on the cows’
part as to which concentrate they ate (no selection) or because a roughly equal mixture of the two provided an optimal diet.
Based on previous research on factors affecting food choice in both ruminants and non-ruminants, it would be expected that individuals with high protein requirements would select a higher proportion of HP than animals with low protein requirements. The major need for protein in lactating cows is to support milk protein output, so choice between concentrates – expressed as the proportion of HP in the total intake of concentrates – was regressed against milk protein output immediately before the period of choice feeding began.
It can been seen from Fig. 7.6 that there was a positive relationship (P<
0.05); the cow that ate only HP was the one with by far the highest yield of milk protein. Because the regression was carried out with milk protein output measured before the choice-feeding period, when cows were given a single
Fig. 7.5. Diet selection pathways for eight lactating cows with access to high (HP) and low (LP) protein concentrates for 21–29 days in mid-lactation. See text for interpretation (from Lawsonet al., 2000).
balanced concentrate food, this relationship can be interpreted as causal, i.e.
the higher pre-existing milk protein output driving the selection of a higher proportion of HP concentrate.
Silage DM intake was significantly higher when, in other periods of the experiment, LP concentrate was offered alone (14.7 kg/day) than it was when HP was given (14.1 kg/day) or when there was a choice (14.2 kg/day). Milk yield was significantly reduced when the concentrate was LP and body condition score significantly increased compared with the HP or choice treatments. This suggests that cows given LP concentrates were trying to increase their protein intake by eating more silage, but that the increase was insufficient to provide the protein intake needed to maintain milk protein output and provided more energy, which was utilized for body fat deposition.
Sheep
Growing lambs have been observed to switch from little discrimination among foods of different digestible energy concentrations, when first introduced to them, to a preference for high-energy foods as they learned the consequences of eating each one (Glimp, 1971). More recently, it has been shown that sheep select proportions of low- and high-protein foods to give a protein intake matched to their presumed requirements for growth (Kyriazakis and Oldham, 1993).
Figure 7.7 shows the results of an experiment in which individually penned growing lambs were offered a high-protein food containing 235 g CP/kg and a lower-protein food containing either 78, 109, 141 or 172 g CP/kg. In the cases of the first three treatments, the lambs chose a mixture providing CP in the
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0 0.2 0.4 0.6 0.8 1.0
0.4 0.6 0.8 1.0 1.2 1.4
Milk protein output (kg/day) Proportion of HP in total concentrate intake
Fig. 7.6. Relationship between milk protein output and the proportion of a high-protein concentrate selected when given in choice with a low-protein concentrate (from Lawson et al., 2000).
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.