PCDD and PCDF depletion in milk from dairy cows according to the herd metabolic scenario

High level of PCDD + PCDF contamination in bulk milk (9.7 pgWHO-TE g−1 fat) from 1604 Holstein Fresian lactacting cows was observed just four weeks after the beginning of their exposure to a feed supplement contaminated at 10.4 ngWHO-TE kg−1 dry matter. In-farm produced hay and silage showed levels not exceeding 0.2 ng WHO-TE kg−1 dry matter. After the supplement withdrawal, it was possible to monitor the depletion phase for a following 75-day period in milk, until the levels dropped well below 3.0 pgWHO-TE g−1 fat, the EU regulatory Maximum Residue Level for PCDD + PCDF. During this phase, the half-life was calculated as 17 ± 3 days, on WHO-TEQ basis. The full availability of farm data on both cow nutrition and milk production allowed the calculation of the carry-over rate (COR) (PCDD + PCDF milk excretion vs. feed), which was 46% at the end of the exposure. This COR value is justified from the main TE contribution of Penta-CDD and -CDF congeners (63%), and the half-life is among the shortest of all those described in the literature both for experimental and naturally-exposed dairy cows. A fugacity-based model predicts a bulk milk contamination of 5 pgWHO-TE g−1 fat, compared to the 10 pgWHO-TE g−1 fat level observed. Such findings are discussed in light of the lactation and metabolic status of the herd for which the transition period, characterised by a negative metabolic energy balance and a consequent adipose tissue mobilization, could play a relevant role.