Effect of phenological stage and proportion of fresh herbage in cow diets on milk fatty acid composition

• An overall effect of herbage phenology on milk FA composition was observed.
• High fresh herbage proportions in cow diets increased the phenology effect on FA.
• Early herbage phenology increased milk concentrations of healthier FA.
• The phenology effect was greater in controlled conditions than on-farm.
• Only CLAc9t11 and C18:1t11 decreased with mature herbage on-farm.

The aim of this work was to test the effect of herbage phenology on the fatty acid (FA) composition of milk and its interaction with fresh herbage proportion in cow diets in controlled conditions, as well as to validate the results in on-farm conditions. Three experiments were conducted in controlled conditions, and were designed to be representative of dairy systems with increasing fresh herbage proportions in cow diets: low (25% of diet dry matter (DM)); high (75% of diet DM), and full grazing (100% of diet DM). The fourth experiment was a survey conducted in commercial farms that adopt feeding systems similar to those of controlled conditions. An overall effect of herbage phenology on milk FA composition was observed. The number of FA that were affected by herbage phenology was higher as the proportion of fresh herbage in the cow diets increased. The C16:0, total even chain saturated FA, odd chain FA, branched-chain FA, C18:1c9, and monounsaturated FA concentrations in milk increased with increasing herbage phenology, whereas the de novo synthesized FA, C18:1t11, CLAc9t11, C18:3n − 3 and total polyunsaturated FA concentrations decreased. When low proportions of fresh mature herbage was fed to cows, the FA profile of the derived milk differed from the control diet (without fresh herbage) only for the higher concentrations of C18:1t11, CLAc9t11, and C18:3n − 3, whereas the differences were greater and significant for several FA for similar proportions of fresh herbage at an early phenological stage. The results obtained in controlled conditions were confirmed on-farm: the C18:1t11 and CLAc9t11 were the most sensitive FA to the effect of fresh herbage phenology. However, when low fresh herbage proportion were used in cow diets, the effect of herbage phenology on milk FA concentration was negligible.