Ambient Conditions and Feeding Strategy Influence δ¹⁸O of Milk Water in Cows (Bos taurus)

There are increasing concerns by consumers regarding agricultural product traceability and authenticity. Oxygen isotope composition (δ¹⁸O) has been used in this context based on the relationship between δ¹⁸O of animal products and annual precipitation. However, in dairy products this relationship is affected by the seasonality of δ¹⁸O 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 δ¹⁸O in milk water (δ_milk). The mechanistic Munich-Kohn model reflecting these influences predicted the seasonal and farm-specific variation of δ_milk well. The relationship between δ¹⁸O of precipitation and δ_milk varied in different feeding strategies. The interplay of ambient conditions and feeding strategy on δ_milk should thus be carefully considered when identifying the origin of milk.