Ruminal use of undegraded-feed soluble protein and accuracy of the estimations of the nutrient content in ruminal bacteria

BACKGROUND: The possible escape from the rumen of undegraded-feed soluble proteins (SP) was investigated using nitrogen-15 (¹⁵N) infusions (25 mg) in three rumen and duodenum cannulated wethers. Animals were fed three isoproteic diets differing in SP content and including protein concentrates either untreated (control) or treated with solutions of either malic acid (MHT) or orthophosphoric acid (OHT) and heat. RESULTS: Compared with control, MHT and OHT diets reduced ruminal concentrations of ammonia-nitrogen (NH₃-N) by 35.9% (P = 0.007), non-ammonia nitrogen (NAN) by 36.8% (P = 0.007), and SP-nitrogen (SP-N) by 45.2% (P = 0.072) over the post-feeding period. Both NAN and SP-N were lower (P ≤ 0.040) for OHT than for MHT diet. The ¹⁵N enrichment of NAN and SP-N did not vary either among diets or with time, and both values were closely related (R² = 0.965; P < 0.001). Estimations, either using solid-associated bacteria (SAB) or liquid-associated bacteria (LAB) as a reference, indicated that 0.983 and 0.894 of SP-N (values averaged across diets) was of microbial origin, respectively, which would indicate a practically negligible ruminal escape of feed SP-N. Values of ¹⁵N-enrichment in SAB and LAB fitted well to previously published SAB–LAB relationships, indicating a 22% underevaluation of the N supply from SAB when only LAB is used as a reference. CONCLUSION: Both the negligible ruminal escape of feed SP and the underevaluation of the bacterial nutrient supply as a consequence of the use of LAB as the only bacterial reference should be considered to improve ruminant protein feeding systems.