Model explanation of the seasonal variation of δ(18)O in cow (Bos taurus) hair under temperate conditions

Oxygen isotopes (δ(18)O) in animal and human tissues are expected to be good recorders of geographical origin and migration histories. However, seasonal variation of δ(18)O may diminish the origin information in the tissues. Here the seasonality of δ(18)O in tail hair was investigated in a domestic suckler cow (Bos taurus) that underwent different ambient conditions, physiological states, keeping and feeding during five years. A detailed mechanistic model was built to explain this variation. The measured δ(18)O in hair significantly related (p < 0.05) to the δ(18)O in meteoric water in a regression analysis. Modelling suggested that this relation was only partly derived from the direct influence of feed moisture. Ambient conditions (temperature, moisture) also affected the animal itself (drinking water demand, transcutaneous vapor etc.). The clear temporal variation thus resulted from complex interactions with multiple influences. The twofold influence of ambient conditions via the feed and via the animal itself is advantageous for tracing the geographic origin because δ(18)O is then less influenced by variations in moisture uptake; however, it is unfavorable for indicating the production system, e.g. to distinguish between milk produced from fresh grass or from silage. The model is versatile but needs testing under a wider range of conditions.