N-15 LABELING OF THE NITROGENOUS FRACTIONS OF MILK USING ORAL-ADMINISTRATION OF LABELED AMMONIUM-SULFATE IN THE DIET

In an attempt to obtain N-15-labelling milk proteins far digestive stu dies in humans, labelled ammonium sulphate [((NH4)-N-15)(2)SO4] was in corporated in the diet of dairy cattle. Cows were fed a total mixed ra tion including 300 g (NH4)(2)SO4. In trial 1, three cows received a si ngle dose of 50, 150 or 300 g ((NH4)-N-15)(2)SO4 in substitution for a n equal dose of non-labelled product. In trial 2, three cows received 300 g ((NH4)-N-15)(2)SO4 the first day plus 150 g 24 h and 150 g 48 h after the initial supply. Feed intake, individual milk yields and nitr ogen (N) content of the milk were recorded. Kinetic evolution of the N -15 enrichment of total (TN) soluble (SN) and casein (CN) nitrogen was followed. In trial 1, N-15 enrichment of protein reached maximum leve ls 36 h after the supply of labelled product. In trial 2, the administ ration of repeated doses of N-15 ammonium sulphate resulted in an enri chment plateau from 36 h to 84 h after beginning labelled product admi nistration The respective levels in N-15 enrichment of TN, SN and CN h ave been discussed.