Variation in the composition of milk protein from pasture-fed dairy cows in late lactation and the effect of grain and silage supplementation

Sources of variation in milk protein composition were determined after approximately 203 days-in-milk for spring-calving Friesian cows grazing ryegrass-white clover pastures in New Zealand. Three groups each of 15 multiparous cows were managed as separate herds. Each herd was subjected to one of three management strategies: pasture only, pasture supplemented with 2-3 kg cow(-1) d(-1) maize grain, and pasture plus 5-6 kg cow(-1) d(-1) maize grain and 3-4 kg cow(-1) d(-1) pasture silage. During a 3-week period beginning in February 1994, milk samples were collected twice-weekly from individual cows and the detailed protein composition determined. Variation was observed in milk yield and the concentrations of all protein components measured, with between-cow variation being greater than within-cow variation for all parameters except non-protein N and urea. Supplementing pasture with maize grain and/or silage increased milk yield but had only minor effects on protein composition. These data will enable increased efficiency of experimental design in this research area, and provide details of the natural variation in milk protein composition. This variation gives an indication of the extent that milk protein composition could be manipulated by on-farm practices, or through the selection of cows with defined milk protein profiles.