Developing an in vitro Method for Determining Feed Soluble Protein Degradation Rate by Mixed Ruminal Microorganisms

The objective of this work was to describe a novel in vitro system based on the subtraction of ammonia pools obtained with and without rumen fluid inoculum to determine the soluble protein fraction of feeds and their degradability, with adjustments for microbial contributions and bacterial contamination. Four corn-milling coproducts were used in this study as random factors. The feeds (Fd) were dried distillers grain (DDG), one high protein (HP-DDG), one containing added solubles (BPX-DDGS), and the corn coproducts BRAN and GERM, concentrated corn kernel components derived during the processing of HP-DDG. Three treatments were investigated: Fd was fermented in vitro with rumen fluid (Rf) and buffered media (Md) (TRT1) or with Md alone (TRT2). Two controls were used without the inclusion of feed: Rf + Md (C1) and Md alone (C2). The third treatment (TRT3) was calculated as TRT1 – (TRT2 – C2) – (C1 – C2) – C2 to account for bacteria protein contamination. Feeds were incubated in duplicates for 0, 1, 3, 6, 12, 24, and 48 h and subsamples of TRT1, TRT2, C1, and C2 were taken to determine ammonia and bacterial protein determination. The fractional rate of disappearance of soluble protein for BPX-DDGS (0.06 h-1) was less than half of HP-DDG (0.13 h-1), BRAN (0.13 h-1), and GERM (0.15 h-1). These results suggest that this method may be used to determine the degradability of the soluble protein fraction of ruminant feeds.