4 - Digestive system, Digestion and Feeding behavior

The stomach is an elongated sac with a mucous lining. The oesophagus is connected to the stomach by the cardia. The "blind" part of the stomach is the fundus (fundus in Latin) and the opposite region is the pyloric antrum (antrum pyloricum), which ends in the pylorus. It is endowed with a powerful sphincter that regulates the transfer of food towards the small intestine.

The wall mainly secretes hydrochloric acid, pepsin and minerals (Ca, K, Mg and Na). The pH is always very acid but varies noticeably during the day, especially in the fundic region (figure 12) The highest pH (the least acid) is observed when there are caecotrophs (pH 3.5) whilst the most common pH is between 1.5 and 2.0. The secretion of pepsin and electrolyte varies in the same way and depends, in particular, on the ingestion rate of caecotrophs (Beauville and Raynaud, 1964; Salse et al., 1982). The stomach content of a 9-week-old rabbit is 90-120 g of fresh matter, depending on the time. The dry matter content varies between 16 and 21 % (Gidenne and Lebas, 1984).

The stomach wall of a very young kit (1 week old) secretes pepsin, the optimum pH àf which is between 1.8 and 2.4, as well as another peptidase, the rennin or chymosin (optimum pH is between 3.4 and 3.8). From 21 days on, the optimum pepsin pH drops to 1.2-1.8 whilst rennin is not found in a 45-day-old or 60-day-old rabbit (Angelo and Srivastava, 1979).

During lactation, endopeptidase is responsible for milk coagulation in the stomach because break of the chain of kappa-casein Pepsin secretion is important only from the age about of 30 days. In contrast, the gastric lipase secreted by a small part of the stomach wall around the cardia reaches a maximum when the kit is 30 days old. It then decreases rapidly between the ages of 30 and 60 days and until 180 days. It cannot be observed in adults. (Bernadac et al., 1991).

Between 3 and 5 weeks of age the anterior segments of the digestive tract (stomach and small intestine) become less important, whilst the posterior ones increase significantly (figure 14). From the age of 7 weeks, the organs in the digestive tract develop less rapidly in relation to the live weight of the rabbit, in particular the colon which is always delayed by two weeks. In adult does the importance of the digestive tract decreases at the end of gestation but rapidly increases during lactation until it reaches a relative size similar to that of a 5-7-week-old rabbit, (figure 14), an evolution which demonstrates its plasticity.

Taking into account the evolution of the digestive mass and the relative proportionality existing between the weight of the organ and that of its contents, the slaughter yield systematically improves with age, at least for kits older than 6 weeks (figures 15 and 16). The weight of the raw skin, which is another important element of carcass yield, only increases slightly from 14.5% up to 15.1% of the live weight between 50 and 150 days, whereas the digestive mass decreases in the sames interval of age from 23 % of the live weight at 50 days, to less than 15 % at 150 days (Combes et al., 2000). The evolution of slaughter yield may be a little bit different from one rabbit line to the other ((Figure 16)

If a rabbit has no access to drink water at all and there is only dry feed available (less than 14 % water), dry feed intake will stop completely within 24 hours. Under these conditions of water lack and depending on the environmental temperature (temperature and hygrometry), an adult rabbit can survive 4 to 8 days without its vital functions undergoing any irreversible alterations. However its weight can decrease by 20 to 30 % in less than a week. If only clean water is freely available a rabbit will survive 3 to 4 weeks without solid feed. As for what is considered as "normal", water ingestion increases 4 to 6 fold in a few days. Sodium chloride distributed in the water (0.45 %) reduces this increase in consumption; but potassium chloride does not (sodium is lost through the urine, not potassium). Rabbits are therefore very resistant to fasting and relatively resistant to lack of water but it should be pointed out that any limitation to the amount of water in relation to the rabbit's needs, means a proportional drop in the dry material ingested, and consequently, alterations in productivity.

In some cases, decreased water ingestion occurs because the animals cannot reach the drinking troughs. Suckling kits sometimes have to climb onto the mother to do this because they are so high up (over 25 cm between the floor of the cage and the waterer). As long as they ingest milk (70 to 75 % water in the milk) and the mother is tolerant, the kits will have no problems with the drinkers. But the situation can become dramatic after weaning, (no more mother to climb on) especially in the case of early weaning (before the end of the 4th week). The risk is further magnified for small breeds.

Figure 25: The dinker must be accessible for the young rabbits: 25 cm maximum from the floor of the cage and close to a wall

Table 12. Consumption of rabbits raised in groups of 4 or 30, from 29 to 71 days of age,  at the same density of 15.6 rabbits / m².
Source: Maertens and Van Herck (2000).

Nevertheless, rearing growing rabbits in very large collective cages (30 subjects in 1.9 m²) authorize more movement for the animals which reduce their daily ingestion compared to that of rabbits reared at a rate of 4 per cage at the same animal density (125 vs 135 g / day). Unlike the effect of the number in the same cage (table 11), the reduction in consumption with large numbers of rabbits in larges cagess at fixed density (15.6 rabbits / m²) is evident even from the start of fattening (table 12).

Finally, the number of places in front of the feeder (1 to 6 places) for a group of 10 rabbits fed ad libitum, does not have a measurable influence on the level of the pellets consumption. It is the same for rabbits rationed not too drastically (at least 70% of consumption at will).

All these results correspond to the average calculated over the entire growing period after weaning. If, on the other hand, the evolution of the effect of the restriction of the access time on the actual consumption is of interest , it was seen that there is an adaptation over time. For example, after 8 weeks of rationning the rabbits arrive, even with only 8 hours of acces, to consume as much as those fed ad libitum. This adaptation over time has been confirmed by more recent works. For example, Foubert et al. (2007) have shown that with an access time limited to 8 hours per day, the effective restriction is 64% then 73% and finally only 83% for each of the 3 consecutive weeks following a weaning at 32 days.

The Hungarian team from Kaposvar also measured the number of meals made by the rabbits according to the number of hours of access to the feeder each day. It appears that, if they have at least 9 hours to consume their solid feed, rabbits systematically realize 30 to 35 meals per day. If they have all the day to eat, they eat at an average rate of 1.3 meals per hour (figure 28). But if the access time is restricted, this number of meals can increase to almost 4 meals per hour, during "working" hours of course. On the other hand, the duration of each meal is not significantly affected by the duration of access to the feeder over 24 hours: 3.2 minutes on average, nad the interval between meals is reduced. In total over the day, 12-week-old rabbits fed ad libitum spend about 1 hour 45 minutes eating, while if they have only 9 or 10 hours to consume their solid feed, they spend only 1 hour 20 minutes approximately.

The wild rabbit can feed on a very wide range of plants. However, it clearly prefers graminaceous (Fescue, Brachypodium, or Digitaria for example) and consumes few dicotyledons if other plants are available. Among the dicotyledons it prefers certain leguminosae (Fabaceae, Mimosaceae, Papillonaceae, ...) and asteracea (formerly Compositae). But it should be noted that if rabbit have the choice, the consumption of carrots (Daucus carotta) is very low. Contrary to popular belief, carrots (roots or leaves) are not among the plants sought after by rabbits.

The proportion of dicotyledons and even mosses can increase during some seasons when plant availability is low. In winter and early spring, grazing by rabbits of young cereals grown can completely compromise a harvest, especially in the 30 to 100 m area from the burrow. When rabbits can choose between winter cereals grown with or without mineral fertilization (phosphorus and / or nitrogen) they clearly prefer cereals without artificial fertilization.

In a situation of choice, the rabbit can be very selective in its feeding behavior. Like many herbivores, it will prefer to ingest the leaves rather than the stem of a plant, and generally young plants or "green and tender" parts rather than dry ones. For example, he will choose a part of the plant with a high nitrogen concentration. Likewise, in a trial conducted in Ireland, rabbits grazed more intensively a particular variety of spring barley compared to 4 other winter barley varieties. This is possibly related to the composition of the plant. However, the differences in the sugar content of the 5 varieties did not fully explain this varietal choice.

At the end of winter, the rabbit has a strong appetite for the buds and young stems of some woody plants. The browsing of very young trees or their shoots can thus completely compromise the regeneration of a forest, or more specifically the regeneration of certain shrubs such as juniper or the Scotch broom (Cytisus scoparius) as is observed in France in Sologne. In winter the rabbit likes to eat the bark of some cultivated trees (not just the young stems), especially apple trees and also peach or cherry trees. The bark of apricot, pear and plum trees is generally less attacked.

In the forest, rabbits clearly prefer deciduous trees, but can also attack the bark of conifers (mainly firs and some types of pines). When the trees are very young, rabbits prefer to eat the apical or lateral shoots of fir trees, rather than those of oaks.

The basic reasons for the rabbit's feed choices remain unclear, even if they are constant. It can only be said that this behavior is under hypothalamic regulation, since lesions of the hypothalamus clearly modify the rabbit's feed choices.

Many experiments have been undertaken particularly in Australia and New Zealand to study the behavior of wild rabbits, with a view to develop baits (the end goal being the extermination of imported wild rabbits in the 19th century). It was observed a lot of variation depending on the type of bait, but also depending on the season. For example, pellets made from milling by-products (middlings + bran) are well consumed throughout the year. In contrast, the acceptability of carrots or oats changes seasonally. The addition of salt (1% or 5% NaCl) or alfalfa meal (15%) in the bait pellets made with wheat by-products significantly reduces their consumption.

If a rabbit can choose between a feed with or without an appetite stimulator, it usually chooses the feed with the "apetizer". But, if each of these 2 feeds is offered alone, there is no difference in ingestion, or in growth. The same phenomenon has also been shown with the addition of a repellent in the feed such as formalin.

On the other hand, the rabbit seems to appreciate a certain degree of bitterness in its diet. When rabbits are presented with feeds containing dehydrated alfalfa with variable levels of saponin, and therefore variable bitterness, their choice is fixed on those feeds which have a relatively high degree of bitterness (up to 3 mg/ g of saponin in the diet - figure 34). All these feeds are neglected for example by rats or pigs. On the other hand, if only feeds with a variable rate of saponin (1.8 to 6.4 mg / g of feed) are proposed, feed intake and growth are independent of the saponin concentration.

When the diet contains a toxin, such as aflatoxin, the rabbit totally refuses to consume it, or he ingests it in very small quantities. When an energetic feed (low in fiber) is distributed with a free choice with a fibrous feed, the rabbit generally prefers the first. It is probably the consequence of a specific search for energy sources (rare in nature), which is the regulatory system dominating the ingestion of the rabbit. On the other hand, for the rabbit, this can lead to an increase in the frequency of digestive disorders (lack of fibre and consequently slow digestive transit) and therefore to enhanced health risk, particularly in growing young.