Traits associated with protein composition and colour of bovine milk

The aim of this study was to determine the associations of parity, stage of lactation and milking time with protein composition and colour of bovine milk. Mid-infrared spectral (MIRS) data from 2013 to 2015, inclusive were obtained from the milk samples of 3 research herds. Spectral outliers were detected using mahalanobis distance and removed. Data were further edited to remove records from cows which were greater than 305 days in milk or were greater than tenth parity. Equations developed recently to predict individual protein fractions and milk colour directly from the MIR spectrum were applied to the data yielding 64,606 records from 1,102 Holstein-Friesian cow lactations of alpha s1, beta, and kappa casein as well as the yellowness of milk. Factors associated with each of the milk quality traits were quantified using linear mixed models. All models were adjusted for milking time, parity (1, 2, 3, 4 and 5+) and stage of lactation (5 equal groups of 60); and the random effects of within and across lactation permanent environmental effects and herd_year_season. Older animals had yellower milk with a higher level of kappa casein and lower level of beta casein (P<0.001). As lactation progressed, milk got yellower in colour (P<0.001) and had higher levels of kappa, beta and alpha s1 casein (P<0.001). Futhermore, although evening milk had a yellower colour than morning milk and higher levels of alpha s1 casein, there was no difference in beta casein across milking time, and the difference in kappa casein was small.