Ambient Conditions and Feeding Strategy Influence δ(18)O of Milk Water in Cows (Bos taurus).

There are increasing concerns by consumers regarding agricultural product traceability and authenticity. Oxygen isotope composition (δ(18)O) has been used in this context based on the relationship between δ(18)O of animal products and annual precipitation. However, in dairy products this relationship is affected by the seasonality of δ(18)O in milk water which in turn depends on the feeding system used. We measured 608 milk samples from 28 farms with various feeding strategies in southern Germany throughout the year, investigating the influences of ambient conditions, drinking water source, and feeding strategies on seasonal variation of δ(18)O in milk water (δmilk). The mechanistic Munich-Kohn model reflecting these influences predicted the seasonal and farm-specific variation of δmilk well. The relationship between δ(18)O of precipitation and δmilk varied in different feeding strategies. The interplay of ambient conditions and feeding strategy on δmilk should thus be carefully considered when identifying the origin of milk.