Comparison of in sacco and in vitro techniques for estimating the rate and extent of rumen fermentation of a range of dietary ingredients

Fifteen plant-derived ingredients (crude protein (CP) 28–390 g kg−1 DM (dry matter) and neutral detergent fibre (NDF) 55–902 g kg−1 DM), were incubated in sacco (2, 4, 8, 12, 18, 24, 48 and 72 h) and in vitro (2, 4, 8, 24 and 48 h). Fermentabilities (F) were fitted to the asymptotic equation F = a + b(1 − e−ct) where a represents material disappearing immediately, b the fraction which does not disappear immediately but which does so eventually, and c the rate of disappearance of the b fraction, where t is time. Curve parameters from the porous synthetic fibre (psf) bag technique were not readily predicted from feed chemistry or in vitro estimates. The only useful prediction was that for the completely unfermentable fraction (1 − a − b) from Christian lignin content (r2 = 0.70; n = 15; P < 0.001). This relationship was particularly strong within the Gramineae (r2 = 0.98; n = 8; P < 0.001), where lignin protects around three times its mass from fermentation. Comparison of asymptotes (a + b) estimated from the two techniques showed that completely unfermentable material was leaving psf bags and that this was a particular problem with soya hulls and maize germ. At shorter incubation times (2–8 h), unfermented material that leaves bags (particularly with wheat, sweet potato and maize germ) is likely to be fermented by 24 h in any case. The overestimation of fermentability by the in sacco technique, relative to in vitro, was greatest with feeds of low acid detergent fibre (ADF) content, being particularly marked at less than 250 g ADF kg−1 DM. The discrepancy accelerated from 2 to 8 h as the feed structure opened up and thereafter declined to a mean overestimation of around 0.1 g g−1 organic matter at the asymptote. The study casts serious doubt on the applicability of the psf bag technique for studies of concentrate ingredients with a high content of either soluble constituents or unfermentable ‘fines’ which will pass out of bags.