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Estimation of rumen-undegradable protein (RUP) across high-protein feedstuffs using in situ and in vitro procedures
The objective of the study was to compare the estimate of RUP of the Cornell fermentation procedure to those obtained using the in situ or ammonia release procedures. To do so, 5 independent batch samples of 7 different feedstuffs were collected. The feedstuffs used in this study included: 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). In situ incubations of 16 h were conducted using 2 Holstein cows (days in milk 210 ± 17 and milk yield 27.3 ± 8.00 kg) fitted with flexible ruminal cannulas. Two in vitro procedures were used, Cornell fermentation and ammonia release. Both in vitro procedures required the incubation of the samples in rumen inoculum. Specifically in plastic tubes (50 mL) for the ammonia release and Erlenmeyer flasks (125 mL) for the Cornell fermentation procedures. Measurements of ammonia and total VFA were required in the ammonia release procedure while correction for microbial contamination was done using a sample of neutral detergent residue from corn silage in the Cornell fermentation procedure. The in situ and the ammonia release procedures were conducted at the University of Nebraska-Lincoln while the Cornell fermentation procedure was conducted by a commercial laboratory. Data from the in situ procedure were analyzed as a randomized complete block design and the model included the fixed effect of feedstuff and the random effects of replicate and load within feedstuff and data from the in vitro procedures were analyzed as a complete randomized design and the model included the fixed effect of feedstuff and the random effect of load within feedstuff. For BM1, BM2, BM3, CM, LFDG, SBM, and ESBM, RUP was 14.7, 70.2, 59.3, 24.3, 23.1, 31.2, and 63.0 ± 3.93 % CP based on in situ, 32.5, 67.6, 67.5, 32.1, 48.8, 32.3, and 65.8 ± 3.46 % CP based on ammonia release, and 28.8, 90.1, 93.5, 45.3, 66.8, 36.4, and 62.8 ± 0.87 % CP based on Cornell fermentation. Across feedstuffs, ranking based on RUP was similar for all procedures. Rumen-undegradable estimates obtained using the Cornell fermentation procedure were greater (P < 0.05) for BM2, BM3, CM, and LFDG and similar (P ≥ 0.20) for SBM and ESBM compared to the other procedures. For BM1, RUP estimated using the in situ procedure was lower (P < 0.01) compared to the in vitro procedures. Despite analytical differences, ranking of RUP values from the Cornell fermentation procedure was similar to the other procedures across feedstuffs.
Keywords: rumen-undegradable protein, in situ