The significance of the degradation products of rapeseed meal proteins in the rumen according to different meal processing techniques

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 (Nt), ammonia N (NH3-N), non-ammonia nitrogen (NAN), true protein); and 3) electrophoretical characteristics of the protein in feeds, bag residues and ruminal fluids. Measurements were made on four sheep fed successively with five diets: hay alone (basal) or hay plus one of four rapeseed oilmeals (60% hay, 40% meals). Oilmeal batches originated from different heating and solvent extraction processes: control at 60 degrees C (T60 degrees), commercial (TC), and experimental at 90 degrees C (T90 degrees) or 130 degrees C(T130 degrees). They differed in their effective protein degradability as assessed by an in situ method which gave results of 0.83, 0.69, 0.39, 0.42 respectively for T60 degrees, TC, T90 degrees and T130 degrees. In the rumen fluid, the Nt and NAN contents peaked 1 h after feeding and then decreased rapidly (for the 7 h post-feeding). The NAN peak level was generally higher for highly degradable oilmeals (0.56 mg/g and 0.36 mg/g respectively in diets containing T60 degrees and TC) than for low-degradable oilmeals (0.15 mg/g and 0.23 mg/g) in diets containing T90 degrees and T130 degrees. NHS-N was fairly high (0.3 mg/g) whatever the oilmeal, due to the high CP content of the diet (21%) and showed only small variations in the post-feeding hours. At peak time, the NAN/Nt ratios in rumen fluid were 0.61, 0.52, 0.30 and 0.32 mg/g respectively in diets containing T60 degrees, TC, T90 degrees and T130 degrees. The true protein-N was roughly the same and low whatever the time, for the diets containing T90 degrees and T130 degrees, but true-protein-N was 0.30 mg/g 1 h after feeding for T60 degrees and TC. Using electrophoresis (SDS-PAGE), feed proteins had similar characteristics among oilmeals, though extraction yields differed widely: 70%, 52%, 21% and 19% respectively for T60 degrees, TC, T90 degrees and T130 degrees. The main proteins contained in the oilmeals are 2S and 12S, which were degraded in nylon bags in the rumen at various degradation rates. Similar proteins were found in the bag residues for various times apparently related to degradability: up to 8 h, 16 h, 24 h and 24 h respectively for T60 degrees, TC, T90 degrees and T130 degrees. They were also observed in rumen fluid for 1 h and 2 h post-feeding for T60 degrees and TC respectively. Therefore, it appeared that there could be some rumen outflow of solubilized protein. Here, whatever the oilmeal, this outflow would represent 5 to 7% of the amount of degraded protein (assuming the passage rate of rumen fluid in sheep to be 0.07 h(-1)). New protein systems could therefore underestimate the potential protein by-pass unless their calibration is based on the duodenal protein flow (such as the Inra system). ((C) Elsevier/Inra).