The percentage of skimmed-milk powder in the compound feedingstuff is calculated by the following formula % MMP= (Nx6,38/27,5 x 100)-2,81/0,908 where N is the percentage of para-casein nitrogen; 27,5 is the factor for converting determined casein into the percentage of skimmed-milk powder; 2,81 and 0,908 are correction factors obtained from regression analysis.