Journal of the Science of Food and Agriculture J Sci Food Agric 87:1345–1355 (2007) In vitro gas production profile and the formation of end products from non- washable, insoluble washable and soluble washable fractions in some concentrate ingredients Arash Azarfar, 1,2∗ Barbara A Williams, 1 Huug Boer 1 and Seerp Tamminga 1 1 Wageningen Institute of Animal Science, Animal Nutrition Group, Department of Animal Science, Wageningen University, Marijkeweg 40, P.O. Box 338, 6700 AH, Wageningen, The Netherlands 2 University of Lorestan, P.O. Box 465, Khorramabad, Iran Abstract: A procedure that mimics washing in the in situ incubation technique, combined with an in vitro gas and volatile fatty acids (VFAs) production technique, was used to verify the assumption that rumen degradation behaviour of material washed out of nylon bags is instantaneous and complete. In a 6 × 4 factorial arrangement of treatments with three replicates, fractions of maize, barley, milo, yellow peas, lupins (a mixture of white and spotted lupins) and round-seeded brown faba beans were subjected to an in vitro incubation technique. Fractions were whole (WHO), non-washable (NWF), insoluble washable (ISWF) and soluble washable (SWF). In a manually operated in vitro fermentation system, another 24 samples of the same substrates were fermented for VFA and ammonia analysis. Except in lupins, ISWF in the concentrate ingredients was very rich in starch. SWF was relatively rich in ash, crude protein, soluble sugars, and a residual unknown fraction but contained only a negligible quantity of starch. Thus, the fermentation characteristics of ISWF were more like WHO and NWF than SWF. Total gas production of SWF was considerably lower than the other fractions. A very rapidly degradable fraction was seen in the first phase of degradation of SWF. The pattern of fermentation end-product formation for SWF differed from that of the other fractions.  2007 Society of Chemical Industry Keywords: concentrate ingredients; feed fractionation; gas production technique; washable fraction INTRODUCTION High-producing dairy cows have a high requirement for energy supplied by nutrients that differ in nature. Nowadays, it has become common practice to divide energy for dairy cows into ketogenic, glucogenic and aminogenic energy. The ratio in which such nutrients are supplied to organs and tissues is highly influenced by microbial fermentation in the rumen. Although dynamic models to describe rumen fermentation and to predict the quantity and quality of the nutrients resulting from it have been developed, 1,2 so far there is insufficient information regarding the conversions and type of nutrients supplied by different feed fractions that differ in degradative behaviour in the rumen. The information available is mainly based on in situ incubations in the rumen, which measure the disappearance of feed fractions rather than the appearance of nutrients. This method divides major feed fractions like starch, proteins and cell walls into washable (W), non-washable but degradable (D) and undegradable (U) fractions. Much information is available on the degradative behaviour in the rumen of starches, proteins and cell walls present in the D fraction, but this is not the case for these components in the W fraction. It has become apparent that part of the W fraction may in fact consist of small insoluble particles and their degradative behaviour is likely to be more similar to the D fraction than to the W fraction. 3,4 The combination of feed fractionation that separates the W fraction into an insoluble washable (ISWF) and a soluble washable fraction (SWF), and in vitro gas and VFA production techniques is considered a promising method to characterise the degradative behaviour of these fractions in feedstuffs. 5–7 Therefore, the current study aimed to characterise the degradative behaviour and the end products of the non-washable fraction (NWF), ISWF and SWF in some concentrate ingredients: maize, barley, milo, yellow peas, lupins (a mixture of white and spotted lupins) and round-seeded brown faba beans. ∗ Correspondence to: Arash Azarfar, Department of Animal science, Wageningen University, Marijkeweg 40, PO Box, 338, 6707AH, Wageningen, The Netherlands E-mail: arash.azarfar@wur.nl Contract/grant sponsor: Ministry of Science, Research and Technology of Iran (Received 1 December 2005; revised version received 30 May 2006; accepted 8 September 2006) Published online 2 April 2007; DOI: 10.1002/jsfa.2856  2007 Society of Chemical Industry. J Sci Food Agric 0022–5142/2007/$30.00