1. Dipartimento di Chimica “G. Ciamician”, University of Bologna, via Selmi 2, I-40126 Bologna, Italy, Nutrient Conservation and Metabolism Laboratory, Livestock and Poultry Sciences Institute, United States Department of Agriculture, Beltsville, Maryland 20705, and Centro di Studio per la Conservazione dei Foraggi, National Council of Research, via F. Re 8, I-40126 Bologna, Italy