Thus, as milk yield increases we would expect protein content to decrease. The benefit of changing milk composition by management or genetic selection depends on the value of the product and how the milk will be used. Casein is the primary protein product of value in milk for cheese manufacturing. As milk fat and casein decrease, cheese yield will decrease. Nonprotein nitrogen is composed of milk urea, amino acids, uric acid, creatine and creatinine.
Most DHIA laboratories use infrared analyzers to measure true protein in milk.
NPN in milk is measured by precipitating true protein in milk and measuring the N content of the supernatant. Infrared analyzer is then adjusted to total protein based on the NPN to CP content of the calibration samples. The adjusted value is reported on the DHIA record. This creates a problem because the NPN to CP ratio is not constant and will vary as a function of the urea content in milk. Since the NPN level in this milk is higher than in the calibration set, the adjustment is not sufficient to represent the total N content in the milk.