Gut parasites
Gut parasites often reduce food intake by sheep and this is discussed in Chapter 17.
Tannins can offer some protection from gut parasites, as animals feeding on plants with tannins show lower nematode burdens, lower faecal egg counts and higher body weight gains than those eating similar plants without tannins.
Animals can learn to eat foods high in tannins and nutrients in order to overcome internal parasites (Hutchings et al., 2003). Gut immune function is also increased by the increase in protein passing through the rumen undegraded – caused by tannins in the diet.
Anti-emetic drugs
Daily treatment with the anti-emetic metoclopramide increased intake of endophyte1-infected, but not uninfected, tall fescue by sheep, suggesting that intake of the infected material is limited by feelings of nausea (Aldrich et al., 1993). In addition, anti-emetic drugs attenuated the aversion to several cereals induced by LiCl in sheep (Provenzaet al., 1994a), which does induce nausea in many species. Also, sheep receiving anti-emetic drugs ate more grain than those not receiving them; it thus appears that intake of cereal-based foods is limited by mild aversion to abdominal discomfort induced by overeating.
Lameness in chickens
Lameness is a common problem in intensively reared poultry, and administration of the analgesic carprofen allows lame birds to negotiate obstacles more quickly than untreated lame birds, although not as fast as normal birds, evidence that lameness causes pain. In order to discover whether broiler chickens could self- medicate with carprofen, lame and sound broilers were trained to discriminate between different-coloured feeds, one of which contained carprofen (Danburyet al., 2000). Lame birds selected significantly more drugged food than sound birds and, as the severity of the lameness increased, lame birds consumed a significantly higher proportion of the drugged feed. Clearly, they were selecting a dose of the analgesic that relieved the pain or discomfort of lameness.
grains (Henuk and Dingle, 2002). Standard feeding equipment can be used, as birds can select within a trough as long as there are clear visual differences between the foods. Such a system is also widely used in large-scale commercial production, both for laying hens and growing broilers, as detailed below.
Broiler chickens
In a comparison of feeding a complete diet with the choice of whole or ground maize or sorghum to male broilers from 21–56 days of age, no differences in body weight, food consumed, food efficiency, carcass ash, dressing percentage or mortality were found (Olver and Jonker, 1997). The main benefit of choice feeding was said to be the financial savings of feeding whole grains as the main energy source.
Birds trained with alternate days of access to whole wheat and a standard starter food for 4 days, when given the choice between the two, ate a significantly smaller proportion of wheat than birds that had been given a choice throughout (Covasa and Forbes, 1996). It is likely that offering whole wheat alone for 24 h at a time rendered this food aversive, due to its low content of protein and other essential nutrients. In a subsequent experiment, the period of exposure to whole wheat alone was reduced to 2 or 6 h and was followed by significantly higher whole wheat intake than for birds fed to choice throughout. It was concluded that a training period is not necessary as long as birds have experience of whole wheat from an early age.
In a classic experiment, Gous and Swatson (2000) offered broilers choices of two or three foods containing different protein sources: fishmeal, sunflower oilcake meal and soyabean oilcake meal. Their growth and food intake were monitored from 7–21 days of age and compared with those of birds fed 13 different mixtures of the three foods. The design is shown in Fig. 8.1a, with 13 combinations of foods 1, 2 and 3 represented. Fig. 8.1b shows the results in terms of growth rates, and it can be seen that birds chose mixtures that gave them growth very close to optimal; they did not choose mixtures that gave optimal FCE, however, although supplementing the soyabean food with DL-methionine in a second experiment did allow the choice-fed birds to optimize FCE. As a result of this ‘severe test’, it is possible to state unequivocally that broilers choose to maximize weight gain and FCE when given the opportunity to do so.
WHOLE-GRAIN FEEDING Wheat and barley are attractive ingredients for broiler foods in view of their low price but, because of their low protein content and imbalanced amino acid composition, they must be supplemented, and this necessitates grinding and mixing with supplements before pelleting into a complete manufactured food. Whole grains of cereal can, however, be viewed as low-protein foods that can be offered in free choice with high-protein concentrate pellets, but the extra expense of providing two feeding systems has prevented its large-scale adoption. There is, however, no need to offer the two foods separately, for as long as they are mixed in roughly the right proportions they are visually different and can be separately prehended by the birds. The concentrate is made by removing the cereal from a conventional formulation
with added premix and calcium (note that it is not necessary to formulate a special high-protein food to use as a balancer for wheat. Standard commercial foods are designed to provide sufficient protein for 95% of the birds, which means that the great majority are being over-provided).
Although barley is generally cheaper than wheat, the high content of
-glucans in barley, which slows the rate of food passage through the digestive tract, means that replacement of wheat with barley in a broiler food reduces intake. Treatment of ground barley with a -glucanase enzyme increases the proportion selected by choice-fed broilers, presumably by overcoming the aversive properties of untreated barley.
Performance of broiler chicks given a choice of concentrate pellets and sorghum grains is about equal to that of pellet-fed controls, but the choice-fed birds ate less protein and converted protein more efficiently. Broilers given a choice of a cereal and a high-protein concentrate performed just as well as controls, whether sorghum, wheat or both were the cereals and, when both wheat and sorghum were on offer, they ate more sorghum than wheat (Cumming, 1983). It is appropriate to provide a small proportion of whole grains from an early age, to give the birds familiarity with their appearance and, hopefully, with their metabolic effects.
Many broiler producers in Northern Europe are now including whole grain in their foods. Adequate mixing takes place during the normal handling of the food through the augers, bins and feeders. Successive batches of food are made with increasing proportions of whole wheat to give a steady increase in the proportion of whole grain, calculated to meet the requirements of the average bird. Results in terms of growth rate and carcass quality are reported to be at least as good as with the commercial grower food fed on its own and, in Scandinavia and other European countries, it is now common practice to feed
(a) soybean (b) soybean
sunflower
fishmeal fishmeal sunflower
Fig. 8.1. (a) Thirteen combinations of three components making up the optimal design for a 3-component mixture; (b) Contours in a 3-component mixture representing different
combinations of soybean oilcake meal, fishmeal and sunflower oilcake meal that result in similar daily weight gains (maximum growth rate (X) and combinations chosen when broilers offered the various 2-way (thick bars) and 3-way (
*
) choices); the contours are at 1 g/day intervals (from Gous and Swatson, 2000).a starter food containing 240 g protein/kg throughout and to add increasing amounts of wheat, up to 40%.
While there are scientific grounds for controlling the proportion of whole grain more accurately to match the birds’ potential and actual growth, and these are utilized in a comprehensive, computer-monitored and -controlled complete housing and feeding system (the Flockman system, Filmer, 1991), the expense of installing such a system seems unjustified at the moment in view of the good results being achieved with ad hoc methods.
Laying hens and pullets
The large-scale application of choice feeding of caged hens was studied by Tauson and Elwinger (1986) using two narrow, flat-chain feeders, one distributing a mash concentrate, the other whole grain. In a semi-choice treatment the mash was given as a layer on top of the grain, which was provided ad libitum. Two experiments with over 5000 birds showed greater egg size with choice and semi- choice feeding than with conventionally fed controls, with no difference in the number of eggs laid. Egg shell quality and cracks tended to be worse in the choice treatments, however. Profit margin was higher over the two production cycles for both choice-fed groups than control, and these authors concluded that choice feeding from flat feeders is feasible but that further studies would be necessary before similar systems were used in practice.
R.B. Cumming (personal communication) has been a great proponent of choice feeding in practice, and emphasized the importance of exposing pullets during rearing to all the grains that they may be offered later in life. He also observed that water consumption has been found to be lower in choice-fed layers, and the droppings have tended to be drier and to cone more readily under the cages. This may be due to the better gizzard development in birds fed some whole grains, and requires further investigation.
Pullets of a layer strain were given a choice of pelleted protein concentrate (as pellet or mash), whole yellow maize and limestone powder from 7–16 weeks of age and compared with birds fed a complete pullet grower diet in mash form (Olver and Malan, 2000). Choice-fed birds were significantly heavier at first egg than those fed the control diet, even though they ate less food (66.2 versus 68.2 g/day). During the laying period (16–80 weeks), choice-fed hens laid eggs that were significantly heavier, had thicker shells and darker yolks, with better food conversion ratios than those fed the control diet. It was concluded that choice feeding was beneficial for laying hens, and it was suggested that for optimal results the birds should have experience of choice feeding well before the onset of laying.
In summary, under commercial conditions laying hens usually perform well although efficiency is sometimes reduced. There is uncertainty about the optimum methods of training and feeding.
The potential advantages of whole-grain feeding
If animals do make nutritionally wise choices between foods, the following advantages may be seen:
1. There is a saving by not having to grind and pellet as much food and the fact that grain stores better whole, especially in hot, humid areas. Trained birds modify the ratio of intakes of the two foods within a few hours of a change in the energy content of the grain or of the protein content of the concentrate.
Under heat stress, they decrease only their grain intake, but not that of concentrate. In the cool of the night they once again eat more grain, and eggs are generally a bit heavier than for birds given a single food, especially in hot weather. Choice diets are cheaper, but broilers take a bit longer to reach a given weight as they lose a day or two while they learn. They may be less susceptible to coccidiosis and usually more efficient financially.
2. Expensive determination and laborious calculations of nutrient requirements for use in food formulations could be avoided by offering a choice of two or more foods and allowing animals to choose a combination that will reflect their needs.
3. Separate-sex feeding will be unnecessary: within a mixed-sex, choice-feeding flock the males and the females will be able to select different diets that reflect the different requirements of the sexes. Differences between individual birds and different strains with different growth potential will be compensated for.
4. Food changes will not be needed: two foods, offered as a choice, could be used throughout the growing and finishing period. In addition, nutrient under- supply (with consequent loss in output) or nutrient oversupply (with no resulting benefit but increased cost and waste) will be avoided since the diet selected by each individual will precisely meet its requirements. Changes in environmental temperature will be accommodated by the birds without the need for reformulation of the foods.
5. Excretion of nitrogenous and other waste will be reduced as individual animals select diets to meet their nutrient requirements. There is potential for a significant reduction in the pollutants generated by intensive animal units.
The potential disadvantages of whole-grain feeding
1. It is an additional task for managers to check that both cereal grains and concentrate are available to all birds to ensure an adequate mixture is provided.
2. The proportion of whole grains eaten tends to increase with the amount of grains provided – in some cases birds seriously overeat whole grain, especially if it is very freely available. There is an advantage in providing the ratio of grain:concentrate that is optimal for the birds in question, i.e. increased management input.
3. Risk of poorer performance: in a comprehensive experiment in which broiler chicks were offered either a single balanced food or a choice of two foods that differed in protein and energy, growth rate and efficiency were reduced by choice feeding, especially in the faster-growing strain (Siegel et al., 1997).
Choice-fed birds selected a diet that was lower in protein:energy than optimal, had heavier abdominal fat pads and lighter breasts relative to body weight than those fed a single diet. In another case, although total food intake to 6 weeks of age was not significantly affected by the feeding treatments, broilers on choice
feeding selected diets with a lower percentage of CP and had lower live body weights at 6 weeks than those fed the complete diet (Yoet al., 1998).
Gizzard development and coccidiosis
Grit is not usually offered in intensive poultry production and foods are low in fibre, as a result of which the gizzard is small and the proventriculus may be dilated. This results in food passing very quickly through the stomachs and arriving in the duodenum in a particulate form in which penetration of digestive juices might be slow. This might be conducive to coccidiosis, and Cumming (1987) observed that a higher-fibre food (62.5 g/kg) given to groups of cockerels from 1-day-old to 4 weeks reduced the incidence of coccidiosis compared with a low- fibre food (28.5 g/kg). All birds were dosed with coccidiosis oocysts, and 7/50 died in the low-fibre group, 3 in the high-fibre and none in a third, choice-fed group.
The oocyst outputs were 293, 96 and 6, respectively, at 6 days post-infection. It is also advantageous for gizzard development and coccidiosis prevention to offer limestone grit to ensure large particles that stay in the gizzard, rather than as powder incorporated in complete food. Gizzards were significantly heavier in choice-fed birds compared with those given a single complete food.
In well-controlled experiments using Eimeria acervulina, it has not been possible to confirm the reduction in oocyst output and improvement of bird performance obtained by Cumming, however, even though gizzard weight was significantly increased by the use of whole wheat, rather than ground, in the diet of broiler chickens (Banfield and Forbes, 2001). It had been proposed that the beneficial effects of whole cereals on coccidial infection were due to the increase in the viscosity of gut contents compared with ground cereals.
However, when carboxymethyl cellulose was added to the diet to increase greatly the viscosity of gut contents, there was no benefit in terms of reduced Eimeria oocyst output, with the additional disadvantage that food conversion efficiency was reduced (Banfieldet al., 2002).
When grit is available to broilers offered whole wheat and a high-protein food, they select a higher proportion of whole wheat as presumably they are better able to grind it in the gizzard in which the grit is stored for this purpose;
when access to grit is denied there is a reduction in the intake of the whole grain. When the balancer food for broilers was given in mash form the birds selected more wheat when ground than whole but, if the balancer was pelleted, they selected more wheat when whole. Presumably physical form influences gizzard development and pelleted food improves its ability to grind food and allows birds to make better use of whole wheat. Thus, they select more whole wheat when offered concentrate in the pelleted form rather than mash.
Sequential feeding
One way to cope with the logistical difficulties of providing two foods simultaneously under commercial conditions would be to feed the two alternately. The length of time each food is offered will affect the outcome, as
there is limited storage for many nutrients in the animal’s body. If one food is eaten to satiety and another, contrasting, food is then offered, animals usually eat again: this is known as sensory-specific satiety. On the face of it, therefore, offering two or more foods in sequence might be expected to increase total intake but this is not usually the case.
Offering HP and LP to broilers for alternate half-days gave food intakes and growth very similar to birds fed the same two free-choice foods (Forbes and Shariatmadari, 1996), and it appears that alternate feeding would be an effective way of ‘choice feeding’. Birds given HP and LP on alternate days had somewhat lower total intakes and significantly less fat in the carcass than those offered free choice. When whole wheat and balancer are given to broilers during alternate 8-h periods, there is good selection: the higher the protein content of the balancer, the greater the proportion of wheat is eaten.
Gous and DuPreez (1975) gave layer cockerels two foods that were individually poorly balanced but complementary in their amino acid com- position, in alternating periods of 6 or 12 h. There were no significant differences in food intake or weight gain, either between the two alternating treatments or compared with controls fed the two foods mixed together. Thus, the growing bird appears to have the ability to compensate for short periods on amino acid-imbalanced foods.
Chicks offered a choice between high- and low-protein foods during the 12 h of daylight but a food with an adequate protein content at night adopted a nocturnal feeding pattern, but did not completely eliminate daytime meals.
Thus, although normally a diurnal feeder, if necessary the chick will eat at night in order to get a single, complete food. This suggests they prefer to eat a single, balanced food rather than making up their own balanced diet. Chicks exposed to cold conditions at night ate more and selected a higher energy:protein ratio than controls kept continuously in the warm, and chicks kept in permanently cool conditions ate more food, with a higher energy:protein ratio, than those put in the warm at night (Hayne et al., 1986). The chick has the ability, therefore, to compensate for changed nutrient requirements at one time of day by altering its diet selection at other times.
In a comprehensive series of experiments, Rose et al. (1995) compared alternating periods of access to whole wheat and a balancer food with free access to both or a single, complete food. The proportion of the diet taken as whole wheat was low with choice- and 4-h sequential-feeding, but higher with sequential periods of 8, 12 or 24 h (see Fig. 8.2). Weight gains were similar for the complete, choice, 4-h and 8-h treatments, but lower for the 12-h and lower still for the 24-h sequential treatments.
When the foods were given in alternating 4-h periods, it can be assumed that the chicks learned that they did not have to wait very long before the preferred balanced food became available again, whereas with the 12-h and 24-h alternating periods they became hungry and this encouraged them to eat more of the whole wheat. When the 8-h sequential treatment was compared with a single complete food, both in small group cages and in larger groups in floor pens, the sequential treatment gave growth rates that were 7% and 5%
lower, respectively. However, because of the reduced need for food processing