Percentages of protein and nonprotein nitrogen with varying fat and somatic cells in bovine milk.

Over 15 mo, 28,390 individual milk samples from 3,600 cows in 62 Quebec Holstein herds were analyzed for crude protein, true protein, nonprotein nitrogen, fat, and somatic cell counts. Unadjusted means with standard errors were 3.51% +/- .002, 3.31% +/- .002, 31.70 mg/100 ml +/- .12, 3.67% +/- .004, and 297,230 cells/ml +/- 4002. Nonprotein nitrogen as a percentage of total nitrogen was 5.57% +/- .02. Least squares analyses showed significant effects of herd, age of cow, month of test, stage of lactation, somatic cell count, and fat percentage on contents of crude protein, true protein, and ratio of nonprotein nitrogen to total nitrogen. Highly variable nonprotein nitrogen fraction content during different months of the year and various stages of lactation is responsible for changes of crude protein content whereas changes of crude protein for different ages of cows, fat, and somatic cells in milk are due to changes of true protein content. Automatic infrared instrument calibrated against crude protein standards can be used satisfactorily to measure crude protein in milk if variations are due to age of cow, fat, and somatic cell counts. However, the instrument becomes inaccurate for measuring large variation of crude protein caused by variability of nonprotein nitrogen due to season and stage of lactation.