2. Nutrition and feeding

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Anatomy and physiology
Feeding behaviour
Nutritional needs
Types of feed

Anatomy and physiology

In an adult (4-4.5 kg) or semi-adult (2.5-3 kg) rabbit the total length of the alimentary canal is 4.5 to 5 m. After a short oesophagus there is a simple stomach which stores about 60-80 g of a rather pasty mixture of feedstuffs.

The adjoining small intestine is about 3 m long and nearly I cm in diameter. The contents are liquid, especially in the upper part. Normally there are small tracts, about 12 cm long, which are empty. The small intestine ends at the base of the caecum. This second storage area is about 40-45 cm long with an average diameter of 3-4 cm. It contains 100-120 g of a uniform pasty mix with a dry matter content of about 20 percent.

Very near the end of the small intestine, at the entrance to the caecum, begins the exit to the colon. The caecum thus appears to be a blind pouch branching off from the small intestine-colon axis (Figure 2). Physiological studies show that this blind pouch-reservoir forms part of the digestive tract: the contents circulate from the base to the tip passing through the centre of the caecum, then return towards the base, along the wall. The caecum is followed by a 1.5 m colon: this is creased and dented for about 50 cm (proximal colon) and smooth in the terminal section (distal colon).

The alimentary canal, which develops rapidly in the young rabbit, is nearly full size in an animal of 2.5 kg, when it has reached only 60-70 percent of adult weight.

Two major glands secrete into the small intestine: the liver and the pancreas. Bile from the liver contains bile salts and many organic substances but no enzymes. Bile aids digestion catalytically. The reverse is true of pancreatic juice which contains a sizable quantity of digestive enzymes allowing the breakdown of proteins (trypsin, chymotrypsin), starch (amylase) and fats (lipase).

DIGESTIVE -TRACT AND CAECOTROPHY

Feed eaten by the rabbit quickly reaches the stomach. There it finds an acid environment. It remains in the stomach for a few hours (3-6), undergoing little chemical change. The contents of the stomach are gradually "injected" into the small intestine in short bursts, by strong stomach contractions. As the contents enter the small intestine they are diluted by the flow of bile, the first intestinal secretions and finally the pancreatic juice.

After enzymatic action from these last two secretions the elements that can easily be broken down are freed and pass through the intestinal wall to be carried by the blood to the cells. The particles that are not broken down after a total stay of about 11/2 hours in the small intestine enter the caecum. There they have to stay for a certain time, from 2 to12 hours, while they are attacked by bacterial enzymes. Elements which can be broken down by this new attack (mainly volatile fatty acids) are freed and in turn pass through the wall of the digestive tract and into the bloodstream.

FIGURE 2.-The digestive system of the rabbit

The content of the caecum is then evacuated into the colon. Approximately half consists of both large and small food particles not already broken down, while the other half consists of bacteria that have developed in the caecum, fed on matter from the small intestine.

So far, the functioning of the rabbit's digestive tract is virtually the same as that of other monogastric animals. Its uniqueness lies in the dual function of the proximal colon. If the caecum content enters the colon in the early part of the morning it undergoes few biochemical changes. The colon wall secretes a mucus which gradually envelops the pellets formed by the wall contractions. These pellets gather in elongated clusters and are called soft or night pellets (more scientifically, caecotrophes). If the caecal content enters the colon at another time of day the reaction of the proximal colon is entirely different.

Successive waves of contractions in alternating directions begin to act; the first to evacuate the content normally and the second to push it back into the caecum. Under the varying pressure and rhythm of these contractions the content is squeezed like a sponge. Most of the liquid part, containing soluble products and small particles of less than 0.1 mm, is forced back into the caecum. The solid part, containing mainly large particles over 0.3 mm long, forms hard pellets which are then expelled. In fact, due to this dual action, the colon produces two types of excrement: hard and soft. Table 15 shows the chemical composition of these pellets.

The hard pellets are expelled, but the soft pellets are recovered by the rabbit directly upon being expelled from the anus. To do this the rabbit twists itself round, sucks in the soft faeces as they emerge from the anus, then swallows without chewing them. The rabbit can retrieve the soft pellets easily, even from a mesh floor. By the end of the morning there are large numbers of these pellets inside the stomach, where they may comprise three quarters of the total content.

From then on the soft pellets follow the same digestive process as normal feed. Considering the fact that some parts of the intake may be recycled once, twice and even three or four times, and depending on the type of feed, the rabbit's digestive process lasts from 18 to 30 hours in all, averaging 20 hours.

Half the soft pellets consist of imperfectly broken-down food residues and what is left of the gastric secretions, and half of bacteria. The latter contain an appreciable amount of high-value proteins and watersoluble vitamins. The practice of caecotrophy therefore has a certain nutritional value.

The composition of the soft pellets and the quantity expelled daily are relatively independent of the type of feed ingested, since the bacteria remain constant. In particular, the amount of dry matter recycled daily through caecotrophy is independent of the fibre content of the feed (Table 16). The higher the crude content of the feed and/or the coarser the particles, the sooner it passes through the digestive tract.

TABLE 15.-COMPOSITION OF HARD AND SOFT FAECES. AVERAGE AND RANGE FOR 10 DIFFERENT FEEDS:

SOURCE Proto, 1980.

¹Balanced concentrate feeds, green and dry forages.

TABLE 16.-INTAKE AND EXCRETION OF DRY MATTER BY GROWING RABBITS EATING ISONITROGENOUS FEEDS COINTAINING 2 LEVELS OF STRAW IN PLACE OF MAIZE STARCH

Source: Dehalle. 1979
¹Average ± 1standard deviation from the mean.

On the other hand, this particular function requires roughage. If the feed contains few large particles and/or it is highly digestible, most of the caecal content is pushed back to the caecum and loses elements which nourish the ' normal" bacteria living in the caecum. This would appear to increase the risk of undesirable bacteria developing in this impoverished environment, some of which might be harmful.

It is thus advisable to include a minimum of roughage in the feed, enabling the rabbit's digestive process to be completed fairly rapidly. In theory, roughage is provided by the crude fibre content of the feed, as this is normally rather hard to digest. However, certain fibre sources (beetroot pulp, fruit pulp in general) are highly digestible (digestibility of crude fibre varies from 60 to 80 percent). For this reason recommendations are now made on quantities of indigestible crude fibre to be fed.

Table 17 gives the chemical composition of various raw materials which can be fed to rabbits.

Caecotrophy regulation depends on the integrity of the digestive flora, and is governed by intake rate. Experiments have shown that caecotrophy starts 8- 12 hours after the feeding of rationed animals, or after the intake peak of animals fed ad lib. In the latter case the intake rate and hence the function of Caecotrophy are governed by the light regime to which the animals are subjected.

Caecotrophy also depends on internal regulatory processes as yet not understood. In particular, the removal of the suprarenal glands halts caecotrophy. Cortisone injections of animals without suprarenal glands causes the resumption of normal behaviour. The digestive process of the rabbit appears to be highly dependent on adrenalin secretions. Hypersecretion associated with stress slows down digestive activity, and entails a high risk of digestive ailments.

Caecotrophy first starts to function in young rabbits (domesticated or wild) at the age of about 3 weeks, when they start eating solid feed in addition to mother's milk.

Feeding behaviour

Feeding behaviour studies have basically involved rabbits receiving balanced concentrates or fed ad /lb. on dry feed (cereals, straw, dry forage).

The feeding pattern of newborn rabbits is imposed by the dam. A doe in fact feeds her young only once every 24 hours (though some does will nurse their young twice). Suckling lasts only 2-3 minutes. If there is not enough milk the young try to feed every time the doe enters the nest box, but she will hold back her milk.

From the third week of life the young rabbits begin to move about, taking a few grams of mother's milk and a little drinking water if available. In a few days the intake of solid feed and water will exceed the milk intake. During this period the changes in feeding behaviour are remarkable: the young rabbit goes from a single milk feed a day to a large number of alternating solid and liquid feeds distributed irregularly throughout the day.

TABLE 17.-CHEMICAL COMPOSITION ON DIFFERENT RAW MATERIAL SUITABLE FOR FEEDING RABBITS

Table 18 gives an example of changing feeding behaviour in New Zealand White rabbits, aged from 6 to 18 weeks.

The number of solid meals, stable up to 12 weeks, tends to decrease slightly thereafter. The total time spent on feeding every 24 hours exceeds 3 hours at age 6 weeks. It then drops off rapidly, to less than 2 hours. At any age, feed containing over 70 percent water, such as green forage, will provide rabbits with ample water at temperatures under 20°C.

The consumption of solid and liquid intake fluctuates over a 24 hour period, as shown in Figure 3. Much more is consumed in the dark than in the light of both liquid and solid feed.

Intake in experimental hutches is very high just before the lights are switched off. As the rabbit grows older the nocturnal nature of its feeding habits becomes more pronounced. The number of feeds during light periods drops and the morning "feeding rest" tends to lengthen. The feeding habits of wild rabbits are even more nocturnal than those of domesticated rabbits.

TABLE 18.-CHANGING FEED HABITS OF 9 NEW ZEALAND WHITE MALE RABBITS AGED FROM 6 TO 18 WEEKS.

Source:Prud'hon, 1975

The intake of feed and water depends on the kind of feed, and also on the type of rabbit and its age and stage of production. Taking as a reference animal an adult fed ad lib. ( 140-150 g of dry matter per day, for example, for a 4 kg New Zealand White): at 4 weeks a young rabbit eats a quarter of the amount an adult eats but its liveweight is only 14 percent of the adult's. At 8 weeks the relative proportions are 62 and 42 percent; at 16 weeks they are 100 to 110 and 87 percent.

A doe's ad lib. feeding during the reproduction cycle varies greatly (see Figure 4). The intake during the final days of pregnancy drops off markedly. Some does refuse solid food just before kindling. Water intake, however, never stops completely. After kindling, the feed intake increases very rapidly and can exceed 100 g dry matter/kg liveweight a day. Water intake is also high at that time: from 200 to 250 g a day per kg of liveweight. When a doe is both pregnant and lactating, she will eat the same amount as a doe that is lactating only, not more.

FIGURE 3.Hourly distribution of daily intake of water and balanced pelleted feed of a 12-week-old rabbit over a period of 24-hours

FIGURE 4.-Changing intake of balanced concentrate feed (89 percent DM) in a doe during gestation and lactation (Lebas, 1975)

FEEDING AND ENVIRONMENT

The rabbit's energy expenditure depends on ambient temperature. Feed intake to cope with energy needs is therefore linked to temperature.

Laboratory tests on growing rabbits showed that at temperatures between 5°C and 30°C intake of pelleted feed dropped from 180 to 120 g a day and water intake rose from 330 to 390 g (Table 19).

TABLE 19.-CHANGING FEED AND WATER INTAKES OF GROWING RABBITS IN CHANGING TEMPERATURES

SOURCE: Eberhan, 1980.
¹Balanced pelleted feed containing 20 percent crude protein and 11 percent crude fibre, rich in protein and energy.

A closer analysis of feeding behaviour shows that as temperature rises the number of solid meals eaten in 24 hours drops. From 37 solid feeds at 10°C the number drops to only 27 at 30°C (young New Zealand White rabbits). The amount eaten at each meal drops with high temperatures (5.7 g from 10°C to 20°C down to 4.4 g at 30°C) but the water intake goes up, from 11.4 to 16.2 g between 10°C and 30°C.

If drinking water is not provided and the only feed available is dry with a moisture content of less than 14 percent, dry matter intake drops to nil within 24 hours. With no water at all, and depending on temperature and humidity, an adult rabbit can survive from 4 to 8 days without any irreversible damage, though its weight may drop 20-30 percent in less than a week.

Rabbits with access to drinking water but no solid feed can survive for 34 weeks. Within a few days they will drink 4-6 times as much water as normal. Sodium chloride in the water (0.45 percent) reduces this high intake, but potassium chloride has no effect (sodium loss through urination). The rabbit is therefore very resistant to hunger and relatively resistant to thirst; but any reduction in the water supply, in terms of water requirements, causes a proportional reduction in dry matter intake, with a consequent drop in performance.

FEEDING PREFERENCES

Given a choice of several feeds rabbits are often unpredictable. When dehydrated alfalfa and dry grain maize are offered the ratio chosen is 65 percent alfalfa to 35 percent maize. With alfalfa and oats the ratio is 60 to 40. But if the maize grains are rather moist, say with a 14-15 percent moisture content which could cause storage problems, the proportion of maize rises to 45-50 percent. When rabbits are offered rations containing dehydrated alfalfa with a variable saponin content, which gives the feeds varying degrees of bitterness, they choose the relatively bitter feeds. Such feeds are ignored by rats and pigs, as shown by Cheeke's tests in the United States.

Feeding rabbits forage plus supplementary concentrate feed raises problems when the forage is not very palatable. The experimental findings given in Table 20 demonstrate that in ad lib. feeding of both high-bulk and high-energy pelleted feeds rabbits are unable to adjust intake for maximum growth. A breeder faced by such a situation should limit the daily dose of concentrate feed or, generally speaking, the proportion of the more palatable feed. The problem sometimes arises with certain low-value green forages.

Growing rabbits receiving a pelleted feed lacking in sulfur amino acids or Iysine, and with access to pure water as well as these missing amino acids in solution, choose the amino acid solution over pure water. Thus they grow as well as control rabbits receiving balanced feed.

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