L'azote non protéique du lait : facteurs de variationM. JOURNETa, R. VERITEa and B. VIGNONb
a Laboratoire de la Production laitière C.R.Z.V. de Theix, 63110 Beaumont
b E.N.S.A. - 54000 Nancy
(Reçu pour publication en février 1975.)
Abstract - The authors have reviewed the main factors affecting the NPN content of cow's milk.
The NPN content of milk generally lies in the range 25-30 mg N / 100 g (equivalent to 1,6-2,1 g crude protein (N X 6,39 per kg). It represents 5 to 6 p. 100 of total milk nitrogen. However extreme values of 18 and 50 mg N / 100 g have been observed (table 1). These variations arise mainly from variations in the urea content of milk since the content in the other NPN components varies only very little (between 12 to 15 mg N / 100 g) (table 1).
Urea content of milk is closely correlated and very similar to urea content of blood. It varies mainly with the total nitrogen intake of the cow : when the crude protein content of the diet increases by 1 point (p. 100), the NPN content of milk increases by 1,5 mg N / 100 g. Urea content of milk decreases when energy to nitrogen ratio of diet increases. With normal energy and protein intakes, increasing the porportion of NPN (with urea) in the total N intake, has only a small effect on the NPN content of milk.
The variation of NPN content of milk due to the diet, season, physiological state of the cow and between cows, is generally 10 times lower than that of the protein N content of milk.
The NPN content of milk has very little influence upon the measurement of the protein content of milk when the Amido Black method is used. However, it is more convenient to calibrate the method to true protein content (total N minus NPN)