I. Germ-free (GF) and conventional (CV) rabbits wearing collars to prevent coprophagy were fed an autoclaved diet with added cellulose. Their faecal excretion was analysed to determine nutrient digestibility.
2.Clearly distinguishable hard faeces were excreted by the GF rabbit only if the diet contained at least 15 yo cellulose. Unlike CV rabbits, the GF rabbits did not consume their soft faeces even when permitted to do so. Soft faeces made up a larger proportion of the total output of GF than of CV rabbits. Food intake and total dry-matter excretion per kg body-weight were similar in both groups.
3.Although digestibility of dry matter was similar in the two groups, in the GF rabbits there was a higher digestibility of crude fat and true protein and a lower digestibility of crude fibre and nitrogen-free extract. GF rabbits excreted a higher percentage of ingested calcium and phosphorus in the urine than did CV rabbits.4. The results suggest that intestinal microbes, even without the enhancing effect of coprophagy, aid in the digestion of carbohydrate by rabbits. The greater faecal excretion of crude fat and true protein by CV rabbits could result from poorer digestion and absorption, but could also represent nutrients synthesized by microbes from simpler materials. The reingestion of faecal crude fat and true protein might therefore improve the quality of the total nutrient intake. The results suggest ways of assuring an adequate dietary intake by GF rabbits in the absence of contributions from an intestinal microflora.It has been reported that rabbits excrete both hard and soft faeces with virtually complete consumption of the soft type (Morot, 1882; Madsen, 1939;Taylor, 1939Taylor, , 1940Taylor, , 1941; Eden 1 9 4 0~; Southern, 1940Southern, , 1942 Olsen & Madsen, 1944; Harder, 1949; Thacker & Brandt, 1955;Kandatsu, Yoshihara & Yoshida, 1959 al. 1965) found that a diet (L-477) containing mostly inorganic iron produced anaemia in G F rabbits although it was adequate for normal haemoglobin formation in conventional (CV) rabbits. Except for this one mineral, no comparison has been made of the digestibility of nutrients in G F and CV rabbits. Only a few studies have been reported on the digestibility of the major nutrients in other G F species (Luckey, 1963;Evrard, Hoet, Eyssen, Charlier & Sacquet, 1964). Stern, HukoviC & Fukarek (1955) reported that in CV rabbits treated orally with antibiotic the rise in blood sugar after a meal of oats was slower than normal. This suggested that the rabbit as a species might have become relatively deficient in carbohydrases after it had evolved an elaborate interrelationship with its intestinal microbiota. A comparison of the digestibility of the major nutrients in G F and CV rabbits could help to explain the role of micro-organisms in the nutritional economy of this species.The present study was therefore undertaken (I) to determine if the G F rabbit excretes two kinds of faeces, and whether it re-ingests any of its faeces; (2) to determine by analysis of faeces the appare...