The traces of dioxins and PCBs (polychlorinated biphenyls) were found during tests last week at the farm in Statbroek, north of Antwerp near the Dutch border, the federal food safety agency AFSCA said in a statement.
Tests at four neighbouring farms revealed that two of them also had levels of dioxins and PCBs that slightly exceeded norms, AFSCA said, adding that livestock has been banned from leaving the farms pending further test results.
AFSCA could not pinpoint the origins of the contamination, but it recalled that dioxins and PCBs had been used extensively in the 1970s to manufacture electrical transformers.
Dioxin, a by-product of garbage burning and industrial activities, can cause a range of illnesses in humans such as cancer, metabolism malfunction and miscarriages.
In 1999 Belgium dealt with a major dioxin crisis in 1999 -- dubbed "chicken a la dioxin" -- which contributed to the downfall of then prime minister Jean-Luc Dehaene.