ISO 5983-1:2005规定了通过凯氏定氮法测定动物饲料中氮含量的方法，以及计算粗蛋白质含量的方法。 该方法不测量氮或杂环氮化合物的氧化形式。 该方法不区分蛋白质氮和非蛋白质氮。如果确定非蛋白氮含量很重要，则应使用适当的方法。

ISO 5983-1:2005 specifies a method for the determination of the nitrogen content of animal feeding stuffs by the Kjeldahl process, and a method for the calculation of the crude protein content. The method does not measure oxidized forms of nitrogen or heterocyclic nitrogen compounds. This method does not distinguish between protein nitrogen and non-protein nitrogen. If it is important to determine the content of non-protein nitrogen, an appropriate method should be used.