Rumen-undegradable dry matter and neutral detergent fibre as ratio indicators of digestibility in sheep given cereal straw-based diets

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Abstract
SUMMARY: Studies were conducted in 1989 at the University of Zaragoza, Spain, to determine the value of undegradable dry matter and neutral detergent fibre (uDM and uNDF) forestimating apparent dry matter digestibility (DMD), with sheep diets based on untreated or ammonia-treated barley straw (US or TS) offered alone or supplemented with 300 or 600 g/day of grass hay (H), barley (B) or sugarbeet pulp (P). Marker recovery in faeces was 107·9±1·90 and 96·4±1.43% for uDM and uNDF, respectively. Faecal uDM excretion was overestimated, especially with TS as sole feed and in B-supplemented diets (116%), whereas underestimation of uNDF excretion was observed mainly in unsupplemented US (92%), supplemented TS (92%) and in diets including H (91 %). Digestibility estimations from uDM or uNDF concentration in feeds and faeces (DuDM or DuNDF) were affected in the same way as DMD by the dietary sources of variation. Correlation coefficients between in vivo digestibility and its estimates with both internal markers were 0·80, and differences between DMD and DuDM and DuNDF were –2·5±0·14 and 2·2±0·13 percentage units, respectively. The suitability of both markers for predicting apparent digestibility, and the choice between them, would depend on diet characteristics.