Digestibility and energy value of combinations of forage mixtures

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The metabolisable energy (ME) system currently used in the UK assumes the digestibility and energy values of individual forages to be additive. Production experiments have cast doubt as to whether this is consistently the case. Accordingly an experiment was devised to test the effect of mixing forages in different proportions on the digestibility and energy value of the resulting combinations. Forage mixtures were based on grass silage A (GSA) with either untreated (UTS) or sodium hydroxide treated barley straw (TS) which was included at 0, 150, 250 and 400 g kg −1 dry matter (DM); maize silage (MS) with lucerne silage (LS) and maize silage with grass silage B (GSB) were included in the forage mixtures at 0, 250, 500 and 750 g kg −1 DM. Digestibility, digestible energy and ME contents for all 18 forage mixtures were measured in vivo using mature wether sheep. When compared with grass silage only, the inclusion of UTS or TS decreased digestibility coefficients and ME contents. Additions of small amounts of either GSB or LS (250 g kg −1 DM) to MS dramatically reduced the digestible organic matter in the dry matter (714 to 625 g kg −1 and 714 to 651 g kg −1 respectively) compared with MS fed on its own. Cell wall digestibility was also reduced below that calculated from the proportions and the measurements of the forages fed alone. This work has shown that one forage can influence the value of another in terms of digestion and nitrogen utilisation.