Impact of pasture feeding cows and seasonality on the concentration of selected fat-soluble compounds in milk and their ability to be used as biomarkers of “Grass-Fed” dairy

• Experimental herds were established to examine the impact of high, medium or no pasture in the diet on a selection of fat soluble nutrients including β-carotene, lutein, retinol, α-tocopherol and zeaxanthin across an entire lactation. • High proportions of pasture resulted in significantly higher levels of target compounds, including > 2 fold increase in β-carotene on average across lactation. • These result offer significant insight into changes in these compounds throughout lactation in a spring calving pasture based dairy system, which can affect characteristics, such as colour, of final high fat dairy products. • A biomarker model was developed and successfully differentiated milk from each system, highlighting the potential of these products for future authentication purposes. This study investigated the impact of proportion of pasture in the cow diet on the concentration of selected fat-soluble compounds in milk including β-carotene, lutein, retinol (vitamin A), α-tocopherol (vitamin E) and zeaxanthin. Bulk tank milk samples ( n = 102) were collected weekly ( n = 34) across an entire lactation from three herds operating either a high (GRS), medium (PMR) or no pasture allowance (TMR) feeding system. Concentrations of β-carotene and lutein increased significantly with increasing pasture allowance, with a > 2 fold increase of β-carotene in milk, between high and no pasture allowance treatments. GRS feeding system resulted in milks with higher concentrations of retinol, α-tocopherol, and zeaxanthin, compared to both PMR and TMR systems. A biomarker model based on the fat-soluble compounds concentration in milk, demonstrated excellent differentiation of high pasture allowance derived milks from that of medium and no pasture milks, with area under the curve scores of 0.955 and 0.971 respectively. Strong correlations were observed between the concentrations of lutein and zeaxanthin in milk and the milks corresponding yellow colour, while a significant correlation was exhibited between β-carotene and b*-value. Such correlations with milk colour may have a significant impact on the consumer preference of milk and dairy products produced therefrom.