Impact of herbage proportion, breed, average days in lactation (ADiL), and season on bulk milk composition of 12 dairy farms were investigated over a year using a mixed effect model approach. A higher proportion of herbage led to higher contents of omega 3 (n-3) fatty acids (FAs), conjugated linoleic acids (CLA), vaccenic acid, and branched chain FAs (+0.08, +0.08, +0.19, +0.05 g 100 g-1 fat, respectively). Breed influenced n-3 FAs and CLA (+0.31, +0.17 g 100 g-1 fat, respectively), κ-casein, lipoprotein lipase, β-lactoglobulin, total whey protein, fatty acid synthase, and glycoprotein-2, resulting in +0.96, –0.019, –0.83, –1.12, +0.009, and +0.01 g 100 g-1 measured proteins, respectively, in Brown Swiss milk. ADiL influenced lactoferrin and crude protein; season affected all FAs and proteins examined. The identification of these quantitative relationships allow predictions that provide new insights for the production, processing and commercialisation of grassland-based dairy products.