Sources of variation in yields and concentrations of milk solids in dairy cows

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Abstract
Eighteen multiparous Holstein-Friesian cows were offered grass silage ad libitum and 3,6 or 9 kg dry matter per day of concentrates. Triplicate milk samples were taken at each of two daily milkings over a 28-day period and analysed for fat protein and lactose concentration. Milk yield and fat concentration were more variable than protein and lactose concentrations both between animals and between days. Sampling variation was small for all component concentrations. The main source of variation for all traits was time of milking (morning or afternoon) followed by variation between cows and by day-to-day variation. There was little linear trend with time in the data due to cows being close to peak yield. There was no evidence of cyclical variation over time for any trait. Mean weekly or monthly yields and concentrations were better predicted using samples from four rather than from two consecutive milkings but there was no further advantage from using six samples.