The effect of allocation frequency in rotational grazing systems on the fatty acid profile in milk fat of dairy cows.

Four Holstein cows were used to evaluate the effect of allocating cows every 4 day (d) to a new 0.5-ha plot of Lolium perenne L. on the profile of fatty acids (FA) in milk. The experiment was run during 2 rotations with 2 measuring periods of 4 d each. During the 4 d period, the proportion (g per 100 g FA) of 18:3n-3 and total FA content (mg per g DM) of grass decreased linearly. Similarly, milk FA composition was largely affected by day within the 4 d period. Proportions of t11-18:1 in milk fat increased on d 2 (4.52 g per 100 g FA) and decreased thereafter (3.77 g per 100 g FA on d 4). Proportions of c9t11-18:2 (2.36 and 1.83 g per 100 g FA), t11c15-18:2 (0.81 and 0.63 g per 100 g FA) and 18:3n-3 (0.92 and 0.88 g per 100 g FA) in milk followed the same pattern. Results from this study suggest short term variation in pasture quality during the 4 d affected milk FA composition, with a greater effect on biohydrogenation intermediates in milk fat compared with its major precursor, 18:3n-3.