The Digestion of Protein from Forage Diets in the Small Intestine of the Sheep

The digestion of nitrogen other than ammonia (NAN) in the small intestine of sheep fed on forage diets has been estimated by developing a structural relationship y = a+bx, where y = NAN (g/d) leaving the ilium and x = NAN leaving the abomasum. In this relationship the intercept a represents endogenous NAN passing from the ileum and the slope b the indigestibility of abomasal NAN; hence ( 1-b ) gives true digestibility. The values obtained (mean &SE) were: a = 1.04¦0.32 and b = 0.34 ¦ 0.02 , which indicated that the true digestibility of NAN leaving the abomasum was 66%. The amino acid composition of endogenous NAN leaving the ileum was estimated in ileal fluid collected from sheep whose rumens had been emptied and which were maintained by infusion of volatile fatty acids, minerals, vitamins and small amounts of amino acids into the stomach. The values obtained did not differ substantially from mean values obtained for abomasal and ileal digesta from sheep fed on four forage diets. Total amino acids (g/16 g NAN) in abomasal and ileal digesta and endogenous ileal fluid were 74.4, 62.9 and 82.5 respectively. From information on the true digestibility of abomasal NAN and the amino acid composition of digesta, estimates were made of the true absorption coefficients of individual amino acids from the small intestine. Values were mostly greater than 70% of the amount leaving the abomasum but for cystine the value was only 52%.