The management of the transition from hay-to pasture-based diets affects milk fatty acid kinetics

As fatty acids (FA) may become included among the quality parameters setting milk prices, their variation during diet changes needs to be investigated. The aim of this work was to study the kinetics of milk FA during rapid or progressive transition from hay- to pasture-based diets. Two farms in the southwestern Alps were selected. The milk of five Valdostana Red Pie cows from each farm was analysed on days 0, 1, 2, 3, 5 and 7 (at day 1, pasture was introduced into the diet), and there after twice a week. Changes in the milk FA became significant after days 3 or 5. The FA from C4:0 to C18:0, C18:2n -6, C18:3 n -3 and C20:0 became stable after day 5 for both transition types. The cis9-C18:1, trans10 + trans11-C18:1 and cis9 trans11-conjugated linoleic acid (CLA) became stable 3, 16 and 16 days after the maximum pasture intake, respectively. The C4:0 peaked at 3 days after pasture introduction or increase in the diet, probably reflecting high butyrate production in the rumen due to the high carbohydrate concentration in fresh herbage. The kinetics of all FAs fitted a log-normal cumulate distribution (except for C4:0), but model coefficients did not differ between the rapid and the progressive transition. After diet transition, only cis9-C18, cis9 trans11-CLA, C18:3n-3, the mono-unsaturated and poly-unsaturated FA, and the sum of CLA isomers increased faster in the rapid transition than in the progressive transition diet (higher a coefficients), because of the higher initial pasture intake. Knowledge of the kinetics of milk FA when pasture is introduced into the diet could help farmers to improve milk nutritional quality.