I. Previous publications by the author and his colleagues have provided extensive records of the eating and ruminating behaviour of cows receiving a wide range of diets. These records, and additional unpublished records, have been used to justify a proposed index of the physical property of fibrousness, or roughage, in diets for ruminants.2. I t is proposed that the total time spent by ruminants in chewing their food, during eating and during ruminating, should be the basis of the roughage index.3. Expression of the roughage index as total time spent chewing per kg dietary dry matter largely eliminates differences resulting from variation in the amount of food consumed, and differences resulting from the time of access to the food.
I. Six adult, non-pregnant, non-lactating, Friesian cows were used when fat and when thin to measure differences in the voluntary intakes of straw, hay and hay plus concentrates caused by the fatness of the animals. Measurements of digestibility, time of retention of food in the digestive tract, rate of breakdown of cotton threads in the ventral sac of the rumen and amounts of digesta in the reticule-rumen were included.2. The mean voluntary intakes of straw were similar for fat and thin cows. In absolute terms, thin cows consumed 31 % more hay and 23 yo more hay and concentrates than fat cows; in relation to metabolic body size ( W 0 9 , these differences were 76% and 52% respectively.3. Small decreases in digestibility of these diets by the thin cows, reflected in slight reductions in the rate of loss of weight of cotton threads placed in the rumen, did not alter the significance of the differences in intake between fat and thin cows.4. Small changes in time of retention of food in the digestive tract suggested that the capacity of the tract may have been greatest in the thin cows.5 . The presence of a greater amount of digesta in the reticulo-rumen of thin cows than in that of fat cows after eating hay supports the suggestion of a greater gut capacity in these animals. In both fat and thin cows, the capacity of thereticulo-rumen didnot appearto havelimited the intake of the hay and concentrate diet. In both groups the lowest levels of rumen fill were observed after straw was given.6. The results are discussed in terms of possible mechanisms which may operate to reduce the voluntary intake of medium-and good-quality diets as cows become fatter. When poorquality roughages are given, other factors appear to conceal any differences in intake which may exist between fat and thin cows.It is well known that in adult man and adult animals body-weight is usually maintained at more or less constant levels, sometimes for very long periods of time. Thus a long-term regulation of food intake must have an important and precise role. As a possible mechanism bringing about such control, [Kennedy, 1953) postulated a ' lipostatic' theory of regulation of voluntary food intake. This theory proposes that some compound, released from the fat depots in proportion to their total size, inhibits food intake. Thus, over a period of time, food intake is gradually reduced as the animal becomes fatter, until eventually only the maintenance requirement of the animal will be met, and the animal will thus gain no further weight.The quantitative effects of such a mechanism in ruminants are not clear. Schinckel (1960) observed a progressive decline in the ad lib. intake of grazing sheep, which was associated with increasing bodily fatness. However, this was confounded with an increase in environmental temperature during the last part of the experiment. He refers also to a syndrome in extremely fat sheep in which some unknown factor precipitates a complete refusal to eat. The animals lose weight and usually die in Japan.
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