Ruminal degradation and intestinal digestibility of crude protein and amino acids and correction for microbial contamination in rumen-undegradable protein

Abstract Text: Two Holstein cows fitted with ruminal and proximal duodenal cannulas were used to determine crude protein (CP) and AA ruminal degradation using an in situ incubation of 16 h and intestinal digestibility using the mobile bag technique (pore size 50 µm). Bacterial contamination of the rumen-undegradable protein (RUP) was corrected using purines or DNA as bacterial markers. The feedstuffs evaluated were: 3 sources of blood meal (BM1, BM2, and BM3), canola meal (CM), low-fat distillers dried grains with solubles (LFDG), soybean meal (SBM), and expeller soybean meal (ESBM). Data were analyzed as a randomized complete block. Ruminal degradation of CP varied (P < 0.001) across feedstuffs, 85.3, 29.8, 40.7, 75.7, 76.9, 68.8, and 37.0 ± 3.93% for BM1, BM2, BM3, CM, LFDG, SBM, and ESBM, respectively. Ruminal degradation of both total essential AA and nonessential AA followed a similar pattern to that of CP. Based on the ratios of AA concentration in the RUP to AA concentration in the original feed, ruminal incubation decreased (ratio < 1; P < 0.001) the concentrations of His, Lys, and Trp and increased (ratio > 1; P > 0.001) the concentrations of Ile and Met across feedstuffs. Estimations of BCP contamination using purines were 0.75 ± 0.86, 0.65 ± 0.88, 0.55 ± 0.91, 2.50 ± 0.88, 6.45 ± 0.91, 2.61 ± 0.88, 10.8 ± 0.91% CP and using DNA were 0.68 ± 0.86, 0.18 ± 0.88, 0.63 ± 0.91, 4.52 ± 0.88, 2.58 ± 0.91, 1.36 ± 0.88, and 2.49 ± 0.91% CP for BM1, BM2, BM3, CM, LFDG, SBM, and ESBM, respectively. Intestinal digestibility of RUP could not be estimated for BM1, BM3, and SBM due to insufficient recovery of residue. For the remaining feedstuffs, intestinal digestibility of RUP was highest (P < 0.001) for ESBM, followed by BM2 and LFDG, and lowest for CM, 98.8, 87.9, 89.7, 72.4 ± 1.40%, respectively. Intestinal absorbable dietary protein was higher (P< 0.001) for BM2 compared to CM and LFDG, 61.7, 17.9, and 20.7 ± 2.73% CP, respectively. Ruminal degradation and intestinal digestibility of AA determine the supply of intestinal absorbable AA across feedstuffs. These factors are not constant across AA within feedstuffs and nutrition models need to account for them in order to increase the accuracy to predict the AA supply to the animal.

Keywords: rumen degradation, intestinal digestibility, amino acids, bacterial CP contamination