Quick changes in milk fat composition from cows after transition from fresh grass to a silage diet

Two experiments were conducted to evaluate the effects on milk composition of transition from a fresh grass diet on pasture to a winter diet of mixed grass/maize silage. In trial 1, the change in milk fatty acid profile for six dairy cows in mid-lactation was followed via analysis of milk samples collected from individual cows on days 0, 2, 6 and 14 after the transition of diet. In trial 2, milk of 12 cows was analysed on days 0 and 4 after transition. In trial 1, dry matter intake and milk production did not change during the experiment. After transition, the average milk fat content increased from 43.7 g/kg on day 0 to 54.9 g/kg on day 14 and the milk fatty acid composition altered within two days. The largest changes occurred in the proportions of C14:0 (from 89 g/kg milk fat on day 0 to 117 g/kg on day 14), C16:0 (226–348 g/kg), C18:0 (110–88 g/kg) and C18:1 cis-9 (238–170 g/kg). The average content of conjugated linoleic acid (CLA) decreased from 24.3 g/kg milk fat on day 0 to 4.4 g/kg on day 14, while the proportion of the beneficial n-7 fatty acid (FA) rumenic acid declined from 0.95 to 0.85 of total CLA. Vaccenic acid contents decreased linearly with CLA contents. Two days after transition large changes had occurred, whereas the changes between day 6 and day 14 were relatively small. These results were confirmed by the results of trial 2, which also indicated that most changes actually took place within four days after transition. The milk fatty acid profile of grazing cows was more favourable from a consumer health perspective than that of silage-fed cows.