We understand that it is totally prohibited within the same animal species and not only for protein, but for all slaughterhouse by-products. Therefore, tallow cannot be used in ruminants, lard cannot be used in pigs, etc. According to the European Commission

http://europa.eu/rapid/press-release_MEMO-04-107_en.pdf

MEMO/04/107
Brussels, May 6, 2004
QUESTIONS And Answers on animal by-products

Intra-species recycling (cannibalism) ban The Regulation extends the current ruminant intra-species recycling (cannibalism) ban to other species. Porcine animal by-products cannot be fed to pigs and poultry animal by-products cannot be fed to poultry. However, derogation is provided for in the case of fish and fur animals subject to strict controls by the competent authority.

"The regulation extends to other species the prohibition, already applicable to ruminants, of recycling (by-products) within the same species..."

Clearer water!