Hexachlorobenzene retention and excretion by dairy cows.

Two groups of three cows each were fed either 5 or 25 mg of hexachlorobenzene per day for 60 days. A reference compound, DDE (1,1-dichloro-2,2-bis(p-chlorophenyl)ethylene), was fed to the cows at the same rate. Residues were determined in milk at 5-day intervals during the 60-day dosing period and for 60 days after dosing was stopped. The concentration of hexachlorobenzene in milk fat increased more slowly than did that of the reference compound during feeding. The ratio of the two was 1:2 at 10 days, but approached 1:1 after 40 days of feeding. The average concentrations in milk fat for the 40th to 60th days of feeding were 9.0 and 10.4 ppm with the 25-mg/day intake and 2.1 ppm for each compound with the 5-mg/day intake. Corresponding values in subcutaneous body fat were 8.8, 8.0, 1.9, and 1.4 ppm at 60 days. Milk fat concentrations of hexachlorobenzene declined 32% and those of the reference 51% within 15 days after feeding stopped. Thereafter, the decline was slower but similar for both compounds. Biological half-lives ranged from 29 to 64 days for individual cows. The milk fat:body fat concentration ratio was .87:1 for hexachlorobenzene and .71:1 for the reference during the 60 days when cows were not fed the compounds.