THE EFFECT OF COTTONSEED CONDENSED TANNINS ON THE ILEAL DIGESTIBILITYOF AMINO-ACIDS IN CASEIN AND COTTONSEED KERNEL

The effect of adding cottonseed hulls to casein- and cottonseed-kernel -based diets on the apparent and true ileal digestibility of N and ami no acids, and the proportion of this effect accounted for by condensed tannin (CT), were determined using the growing rat. Sixty rats were a llocated randomly to ten semipurified diets, containing either casein (four diets) or purified unheated solvent-extracted cottonseed kernel (six diets) as the sole protein source, with Cr2O3 added as an indiges tible marker. Two of the casein diets contained no hulls whilst the re maining two diets contained 70 g cottonseed hulls/kg. Two of the cotto nseed-kernel-based diets contained no hulls, with two containing 23 g hulls/kg and the remaining two containing 46 g hulls/kg. For each pair of diets, PEG was either included or excluded. The effect of CT was q uantified by comparing control rats (- PEG; CT acting) with PEG-supple mented rats (+ PEG; CT inactivated) at each level of dietary hulls. Th e rats were given their respective experimental diets for 14 d. Each r at was given the food ad libitam for 10 min hourly from 08.00 to 18.00 hours. On day 14, samples of digesta were collected at death from the terminal 150 mm of ileum at 7 h from the first meal. Apparent and tru e ileal digestibilities were calculated for DM, N and the individual a mino acids. The principal finding was that the inclusion of hulls depr essed the apparent and true ileal digestibilities of N and amino acids , but with the response differing between diets. With the casein-based diet the mean apparent and true ileal amino acid digestibilities were significantly depressed from 0.89 and 0.96 to 0.85 and 0.92 respectiv ely, by the inclusion of 70 g hulls/kg in the diet, and addition of PE G then restored these to 0.89 and 0.95. All of the depression could be explained by the CT content of the hulls. However, with the cottonsee d-kernel-based diet the responses fell into three categories. The appa rent and true ileal digestibilities of the essential amino acids cysti ne and methionine were not affected by hull addition, ileal digestibil ities of leucine, isoleucine, lysine, threonine and valine were marked ly depressed by hull addition with approximately 50% of the depression being explained by CT, whilst the ileal digestibilities of histidine, arginine and phenylalanine were depressed by hull addition but little or none of this effect could be explained by CT. Thus the effect of h ulls on protein digestion clearly differed with source of protein. Wit h the cottonseed-kernel-based diet it seems that components of the hul ls other than CT also depressed the apparent and true ileal digestibil ities of N and amino acids. The identity of these components is unknow n.