Fractions of ruminant feeds: kinetics of degradation in vitro

A widely adopted procedure to characterise the degradation in the rumen and its dynamics is the in situ incubation technique that assumes the washable fraction of feeds (W) to be equal to the soluble (S) fraction and that both are rapidly and completely degraded which may not be the case. Because W fraction is washed out of nylon bags, their behaviour cannot directly be measured. This thesis, therefore, aimed to characterise the degradative behaviour of some important unprocessed and processed concentrate (barley, maize, milo, peas, lupins and faba beans) ingredients and their fractions, using a methodology that fractionates feed samples into its inherent constituents (non-washable, NWF; insoluble washable, ISWF and soluble washable fraction, SWF) in combination with an in vitro gas production technique. The results show that the size of the W fraction obtained in nylon bag studies differs from that of the soluble fraction. Except in lupins, ISWF of the concentrate ingredients was very rich in starch. SWF was relatively rich in ash, crude protein, soluble sugars, anda residualfraction(chemically not determined)but contained only a negligible quantity of starch. The degradative behaviour of this difference (ISWF), measured with in vitro gas production,isvery similar to that of the non-washable fraction. The nature of the VFA profile resulting from the fermentation of the different fractions differs between fractions and changes with time of fermentation. Except in lupins, expander processing decreases the contribution of the truly soluble fraction (SWF) in concentrate ingredients. In maize, milo, peas and faba beans, gelatinised starch serves as a binding agent reducing the size of ISWF. However, the pelleting after expander processing disturbs the binding effect of gelatinised starch to some extent, and increases the size of ISWF. In all cereal grains, faba beans and lupins, thermo-mechanical processing (expander processing and ensuing pelleting) shows a tendency to increase the fractional rate of substrate degradation and rate of gas production at the early stage of fermentation, thereby shifting the pattern of fermentation towards a more glucogenic fermentation. Moreover, it appears that the expander processing and the ensuing pelleting process provides a certain level of protection to dietary protein as represented by a lowered NH ₃ -N production. Our data show that grinding the samples of technological processing changes the particle size distribution in the samples of processed material. As a consequence, the kinetics of gas productionchange. When monitoring the effects of feed processing by using an in vitro gas production technique, no grinding prior to in vitro incubation should be applied.