Particle size in compound feeds and digestive tract activity in the rabbit.

Dietary particle size plays a specific role in the rabbit's digestive tract activity In the colon, small particles (less than 0.1 mm) tend to return to the caecum during hard faeces elaboration, and in the same time, large particles (more than 0.3 mm) are preferentially included in hard faeces. A new measurement method of the size of particles included in pelleted feeds, made the demonstration of the grinding activity of the pelleting machines possible. As a consequence, knowledge of particle size of a meal before pelleting is of scarce interest. In addition, the physiological effects of particle size are observed mainly in the hind part of the digestive tract (caecum, colon), i.e., after digestion of a variable proportion of the particles observable in the pellets before ingestion. Fortunately, most of the published experiments related to particle size effects, were performed after various grinding intensities of the same diet. A fine grinding modifies the propulsive activities in the digestive tract and specially increases the digesta retention time in the caecum. A very fine grinding (holes of 0.25 mm) may increase organic matter digestibility by 7 points. But with the different grinding facilities generally available in compound feeds factories (holes varying from 2 to 10 mm), effects of grinding intensity on the diet's digestibility are very small or absent. A fine grinding may reduce feed intake without modification of weight gain, but it also increases the risk of fatal diarrhoea in fattening rabbits. From the various studies on the effects of dietary particle size distribution, one may conclude that physiological effects on digestive tract activities are easy to demonstrate. But from a practical point of view, only scarce effects on feed utilisation by the rabbit may be expected of particle size control of the diet.