Studies on Milk Analyses of Individual Cow Milk Samples

In a collaborative study between the University laboratory and central laboratory, accuracy and precision of milk analyses for fat and protein content were studied. Each week during a 2-year period, milk from individual cows of four breeds (SRB, SLB, SJB and SRB×SLB crosses) was analysed by reference methods in use at the University laboratory (situated near the herd) and by fully automated indirect methods (Milko-Scan 300 and Milko-Tester) at a central laboratory. Systematic inter-laboratory errors were determined for both fat and protein analyses. The central laboratory gave lower (0.12 percentage units) fat values (p<0.001) and higher (0.07 percentage units) protein values (p<0.001). The overall accuracy (standard deviation of difference between laboratories) of fat and protein determinations was 0.21 percentage units (C.V.=4.4%) and 0.14 percentage units (C.V.=4.1%), respectively. The overall inter-laboratory variation in analysis differences for both fat and protein was explained by biological factors (breed, cow, stage of lactation), level variations (calibration) and a small proportion of extremely large deviating results (outliers). In addition, protein determinations were affected by somatic cell count level due to low lactose content.