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Abstract
High level of PCDD+PCDF contamination in bulk milk (9.7 pg WHO-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 ng WHO-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
pg WHO-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 pg WHO-TE
g(-1) fat, compared to the 10 pg WHO-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.