Sheikh `Abd Allah b. Bayyah, professor at `Abd al-`Azîz University in Jeddah
The ruling on the meat and milk of animals that are fed on impure substances – referred to in Islamic Law as jallâlah – is a matter of disagreement among scholars.
Some scholars consider the flesh of such animals and their milk to be unlawful for human consumption if the majority of their fodder is impure. According to this opinion, if more than 50% of an animal’s fodder is from dead meat, blood, or other impure substances, then its meat and milk are prohibited. If 50% or less of its feed comes from impure substances, then its meat and milk will be lawful.
Other scholars hold the view that the meat and milk of such animals will not be prohibited unless the impurity noticeably affects the taste or smell of the animal’s meat or its milk.
I prefer the second opinion. If there is any bad taste or smell in the meat or the milk, or if consuming it may cause any kind of sickness, then it is prohibited. Otherwise, its meat and milk are permissible because the impurity that the animal had eaten has been transformed into something else.
And Allah knows best.
Source: Islam Today