Praise be to Allah.
With regard to feeding chickens or other animals that may be eaten with impure substances (najasah) such as blood and the like, if it is a small amount, that does not matter and it does not make them haraam or make them jallaalah (animals that feed on filth and dung); rather the jallaalah should be detained until it becomes pure and clean, if most of its food is impure (najis).
But if only a small amount of the animal’s food is impure (najis), then it does not have any impact and it does not make the animal haraam, so long as the ratio is 20% or 30% and so on. This does not matter. Rather what makes it haraam is if the ratio is more than that - 60% or 70%. If this is the ration, then the animal is regarded as jallaalah, so it should be detained so that it can be given good and pure food and water. If it has been detained for a suitable number of days, then it becomes pure and is halaal. This applies if the impure material is the majority of what it eats. The time for detaining animals varies. Chickens should be detained for three days, and that is enough for them to be fed good food and good water. Other animals, such as sheep, cattle and the like should be detained for longer than that, such as seven days or more, so that they can be given good food and water, and their flesh becomes good (halaal) after that. We have asked many of those who have knowledge of this matter, and they said that what is fed to chickens of dried blood is a small amount in comparison to other, pure food. End quote.
Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allah have mercy on him).