Nutritional status and intake regulation in sheep. II. The influence of sustained duodenal infusions of casein or urea upon voluntary intake of low-protein roughages by sheep

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Abstract
In each of two experiments, casein dissolved in 1% disodium hydrogen orthophosphate given daily to sheep per duodenum increased the intake of the roughages offered. Results in each experiment were compared with those of infusions of equivalent volumes of phosphate solution alone. In experiment 1, casein (4.5 g nitrogen/day) infused per duodenum resulted in a 42% increase in intake of a chaffed oaten hay (0.56% nitrogen) diet. Mean retention time of food in the alimentary tract was not altered, but the digestibility of the dry matter and the rate of cotton thread digestion in the rumen were depressed. Urea (4.5 g nitrogen/day) given per duodenum gave a 12% increase in intake, associated with a greater rate of cotton thread digestion in the rumen, and a consistently shorter mean retention time of residues in the alimentary tract. Evidence suggested a greater amount of dry matter in the gut when casein was given but not when urea was given. Nitrogen balance was improved from negative values to positive values by both casein and urea infusions. In experiment 2, a diet of wheaten straw supplemented with 3% urea was offered. Under these conditions, casein (6 g nitrogen/day) improved nitrogen balance and produced an average increase of 11% in daily dry matter intake. Rates of cotton thread digestion, digestibility of dry matter, mean retention time, and number of rumination chews were all unaltered by casein infusion. Results from both experiments are discussed in relation to current hypotheses of the mechanisms for regulating roughage intake in ruminants. A possible role of protein status of the animal in influencing the amount of dry matter which is held in the alimentary tract under ad libitum feeding conditions is suggested.

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