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Methionine and lysine have been identified most frequently as first-limiting essential amino acids in the protein nutrition of dairy cattle (NRC, 2001). According to Patton (1996), 50- 75% of the methionine requirement of a high producing dairy cow is covered by the microbial protein synthesized in the rumen, while the remaining amount should arise from the contribution of the ruminally undegraded feed protein. Soybean proteins are not a good source of by-pass methionine in comparison to animal-derived proteins such as meat or fish meal (Cozzi et al., 1995). Therefore, the ban of the use of animal protein sources for ruminant feeding due to the BSE crisis has made more difficult the fulfillment of the methionine requirement in the lactating cow,........