Degradation in the rumen of lupin ( Lupinus albus L.) and pea ( Pisum sativum L.) seed proteins : Effect of heat treatment

Abstract

The extent to which rumen soluble nitrogen can contribute to the intestinal protein flow is unknown. Therefore, a study was carried out to assess simultaneously the kinetics of: (1) protein disappearance from rumen bags; (2) the amount of various N products in the rumen fluid contents: total nitrogen (N), ammonia nitrogen (NH 3-N), non-ammonia nitrogen (NAN), true protein (protein-N); (3) the electrophoretical characteristics of the protein in feeds, bag residues and ruminal fluids. Measurements were made on four sheep fed successively with hay alone (basal) or 60% hay and 40% of (white lupin seeds, ‘Arès’ variety untreated or extruded) or 65% hay and 35% pea (‘Baccara’ variety, unextruded or extruded). These seeds differed in their effective nitrogen degradability (DegN) assessed by an in situ method which gave results of, respectively, 0.954, 0.682 for control and extruded lupin (clupin and extlupin) and 0.657, 0.697 for control and extruded pea (cpea and extpea). In the ruminal fluid, the various nitrogen forms (N, NH 3-N, NAN, protein-N) evolved differently depending whether the diet contained lupin or pea. The N contents were higher in the ruminal fluid of the diets containing clupin or extlupin than for those containing cpea or extpea while the NH 3-N contents were close. For extruded seeds, the contents for the various nitrogen fractions were lower than for the corresponding control seeds following a slower degradation in the rumen, which was confirmed by the DegN parameters. For the clupin, part of the nitrogen was transiently solubilised as protein (42% N at 1 h and 33% N at 2 h) while for pea, whether extruded or not and for the extlupin, the solubilised N was mainly in the form of peptides, amino-acids and NH 3-N. Using sodium dodecylsulfate-polyacrylamide gel electrophoresis (SDS-PAGE), separation of pea seeds protein indicated that globulin, the most important fraction in legumes, comprised of 7S and 11S proteins were found in lupin and pea seeds. The 7S proteins (β conglutin) in lupin disappeared rapidly from the nylon bags while 7S proteins (vicilin and convicilin) from the pea variety used in this study disappeared less rapidly from the bags. The 11S proteins (α conglutin in lupin and legumin in pea) were resistant to degradation in the rumen. Proteins could also be found in ruminal fluid 1 and 2 h after feeding for the clupin. These results confirm that solubilised proteins from ruminal bags can escape degradation in the rumen. The proportion of NAN able to escape degradation in the rumen would represent 10–11% of the crude protein degraded in the rumen (calculated from the DegN in nylon bags) for lupin and 4–8% for pea.