RUMEN-UNDEGRADABLE DRY-MATTER AND NEUTRAL DETERGENT FIBER AS RATIO INDICATORS OF DIGESTIBILITY IN SHEEP GIVEN CEREAL STRAW-BASED DIETS

Studies were conducted in 1989 at the University of Zaragoza, Spain, t o determine the value of undegradable dry matter and neutral detergent fibre (uDM and uNDF) for estimating 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 o f 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, respect ively. Faecal uDM excretion was overestimated, especially with TS as s ole feed and in B-supplemented diets (116 %), whereas underestimation of uNDF excretion was observed mainly in unsupplemented US (92 %), sup plemented TS (92 %) and in diets including H (91 %). Digestibility est imations from uDM or uNDF concentration in feeds and faeces (DuDM or D uNDF) were affected in the same way as DMD by the dietary sources of v ariation. Correlation coefficients between in vivo digestibility and i ts estimates with both internal markers were 0.80, and differences bet ween DMD and DuDM and DuNDF were -2.5 +/- 0.14 and 2.2 +/- 0.13 percen tage units, respectively. The suitability of both markers for predicti ng apparent digestibility, and the choice between them, would depend o n diet characteristics.