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May one eat meat/poultry or drink milk from animals/poultry/fish who were fed Chametz?
It is permitted to eat the meat of an animal that was fed Chametz during Pesach. This applies even if the animal was owned by a Jew and fed Chametz by a Jew. [Thus, one may buy poultry, meat, and fish even if the animal/bird/fish was fed Chametz on Pesach. Likewise, one may drink the milk of such animals if it was owned and fed Chametz by a gentile. Furthermore, some Poskim permit the milk of the animal even if it was owned and fed Chametz by a Jew. However, other Poskim rule the milk may only be drunk if it was milked after 24 hours from when the animal was fed Chametz. Furthermore, some Poskim are stringent and rule one should not eat meat, or drink the milk, of animals who were fed Chametz on Pesach, even if owned by a gentile. Practically, the custom is to be stringent in this matter. The above, however, only applies if the animal was fed Chametz on Pesach, however, if it was fed Chametz before Pesach it is permitted to be eaten according to all. Nonetheless, some Mehadrin Hashgachas are careful several days or weeks before Pesach to not feed Chametz to the animals/poultry in order to diminish the possibility of finding a Chametz kernel inside the organs of the chicken. Likewise, dairy farms begin feeding their cows only Kosher for Pesach fodder beginning several days before Pesach.]
It is permitted to eat meat of animals/poultry/fish who were fed Chametz on Pesach. Many are also lenient to permit their milk. Practically, the custom is to be stringent to avoid eating meat, or drinking the milk of animals who were fed Chametz on Pesach. However, there is no need to abstain from feeding the animals Chametz before Pesach for this reason.
 Admur 450:18 in parentheses based on Michaber Y.D. 60:1; 142:11 and Shach 142: 1; P”M 448 A”A 10 regarding animal of gentile; Shaareiy Teshuvah 448:7 in name of his uncle; Beis Ephraim 35; Igros Moshe 1:258; See Yeshuos Yaakov 448:9; Nishmas Adam 19; Toras Chesed 21; Ashel Avraham Butchach 448; M”B 448/33; Nitei Gavriel 57 footnote 24; Piskeiy Teshuvos 448/33
 The reason: As this is a case of Zeh Vizeh Goreim, as explained in 445:10
 Shaareiy Teshuvah ibid in name of Beis Ephraim ibid; Poskim brought in M”B ibid; Igros Moshe ibid that this applies according to all and there is no need to be stringent even for the meticulous
 Igros Moshe ibid
 P”M ibid
 See Toras Chesed ibid regarding milk and whether one can initially rely on Zeh Vizeh Goreim to buy from a gentile; Ashel Avraham ibid; Kitzur SHU”A 117:13; Teshuvah M’Ahava Y.D. 3:60; Arugas Habosem 138; Maharshag 2:85
 Piskeiy Teshuvos ibid
 Piskeiy Teshuvos ibid footnote 157
 In the past the Eida Hachareidis, under Rav Moshe Bransdofer, was not careful on this issue.