Stable isotope analysis is a fundamental tool in food origin and authenticity testing. Its use in livestock production requires knowledge of isotope discrimination between product and diet. Here, we report (13)C discrimination ((13)Δ) for milk, milk components (fat, casein and lactose) and faeces in eight lactating dairy cows, which grazed pasture or were fed fresh pasture herbage in the stall. Cows were supplemented with grain maize at 1.72 kg d(-1) (dry matter). Feed components were collected daily, and faeces, milk fat, casein, lactose and whole milk 4 times per week during an 8-week-long sampling period. Carbon isotope composition (δ(13)C) of each sample was analysed. δ(13)C was lowest in milk fat (-29.8‰) and highest in casein (-26.4‰). Compared to the diet, whole milk was depleted in (13)C ((13)Δ = 0.4‰) due to a strong (13)C-depletion of fat ((13)Δ = 2.2‰), which was not fully compensated by the (13)C-enrichment of casein ((13)Δ = -1.1‰) and lactose ((13)Δ = -0.7‰). Faeces were also depleted in (13)C ((13)Δ =1.7‰). Influences of feeding environment (stall vs. pasture) and herbage quality were minor (<0.4‰). A review of literature data shows large variation between studies. We consider that the present results are superior, as they are based on a much larger data set regarding the number of cows and milkings (total n = 256) with greater detail in analyses of diet and milk products. Also, the study covered both stall- and pasture-feeding scenarios in realistic settings with long periods of equilibration. This is the first comprehensive analysis of (13)C discrimination between diet and all main milk components (as well as faeces). Thus, the results will improve the use of stable isotope analyses in regard to authenticity testing and proof of origin.
Keywords: animals; carbon-13; grazing; isotope ecology; isotope fractionation; isotope shift; milk constituents; trophic shift.