Efficiency of utilization of absorbed amino acids in growing lambs given forage and forage: barley diets

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Abstract
The efficiency of utilization of absorbed essential amino acids (AA) was studied in wether lambs (35 to 40 kg live weight) given dried grass and dried grass: barley pelleted diets over an intake range from maintenance (M) to 2·5 M energy intake. Each animal was prepared with a duodenal and Heal simple (T-shaped) cannulafor the collection of digesta entering and leaving the small intestine and with a catheter into the abomasum for the infusion of digesta phase markers (103Ru phenanthroline and51Cr ethylene diamine tetra acetic acid). The efficiencies of utilization of total AA and individual essential AA (EAA) were calculated from the ratios of the regressions describing AA retention per unit nitrogen (N) intake (assessed using the N retention data obtained in the present study and the AA composition of N retention derived during an accompanying comparative slaughter experiment) and AA absorption per unit N intake. These ratios for total EAA were 0·5 for the grass diet and 0·59 for the grass plus barley diet. Values for individual EAA ranged from 0·32 for threonine in sheep given the grass diet to 0·88 for arginine in sheep given the grass: barley diet. Whilst the ratios for total and individual EAA were generally higher for the grass: barley diet the very wide 95% confidence limits associated with these derived values make any between-diet or between-EAA comparisons equivocal. The data appear to support the introduction by the Agricultural and Food Research Council (1992), of a scaling factor to reduce the high efficiency of utilization of AA used previously.