Dairy cow grazing selection on upland pasture affects milk fatty acid concentrations

This work aimed to explore variations in milk FA concentrations according to the rate of pasture utilisation and cows grazing selection. Two groups of nine dairy cows were assigned to a long duration (LD) of paddock utilisation (17 days) on heterogeneous pasture, or to a medium duration (MD) of paddock utilisation (7-10 days) on a more intensively managed pasture. Grazing selection was characterized at the beginning and at the end of each MD paddock utilisation and at the same dates for LD. Individual milk samples were collected the first three days and the last two days of grazing on each MD paddock for both systems. A principal component analysis (PCA) was performed on milk fatty acid (FA) concentrations and grazing selection data, aiming to highlight their relationships. The LD samples were poorly separated by the PCA according to the day of paddock utilisation, because of small variations in LD milk FA composition. The free-grazing selection allowed LD cows to ingest herbage with a quite constant nutritional value throughout the experiment. In contrast, the PCA showed a marked separation between samples from the beginning and from the end of each MD paddock utilisation. The MD cows grazed “by layers” inducing a rapid decrease in the nutritive quality of ingested patches during paddock utilisation, with consequently important variations in milk FA composition