Welsh-cross pony geldings (about 300 kg live weight) were used in a 4£4 Latin square experiment to determine the rate of passage and apparent digestibility of unchopped big-bale grass silage (BBL), chopped big-bale grass silage (BBS), unchopped grass hay (HL) and chopped grass hay (HS) offered at approximately 15 g/kg live weight per d. On day 1 of collection weeks, ponies were fed 85 g ytterbium chloride hexahydrate-marked feed 1·5 h after the morning meal. Total faecal collections commenced 8 h later and continued for 168 h. Apparent digestibilities of feed DM, organic matter (OM), crude protein (CP, N£6·25), acid-detergent fibre (ADF) and neutral-detergent fibre (NDF) were also determined. Faecal excretion data were subjected to the models of Pond et al. (1988) and digesta mean retention time (MRT) calculated from these models and using the algebraic method of Thielmans et al. (1978). Silage had significantly (P, 0·05) higher digestibilities of DM, OM, CP, ADF and NDF than hay; however, chop length had no effect. All the models of Pond et al. (1988) accurately described (R 2 .0·8) the pattern of faecal marker excretion. MRT of BBL (29·0 h). BBS(27 h). HS and HL (26 h).Compartmental analysis using the G3 model of Pond et al. (1988) showed BBL and HS diets had longer MRT in the time-dependent compartment, whereas BBS and HL had longer MRT in the time-independent compartment. Results from this experiment indicate that BBL and BBS are readily accepted and digested by ponies. While Yb is a successful external marker for determining total tract MRT and for modelling faecal excretion curves in horses, the results did not allow any definite conclusions to be drawn on digesta MRT within the different compartments of the equid gut. Digesta passage rate: Mean retention time: Apparent digestibility: Horse

A wide range of roughage foodstuffs is available for feeding to horses. Considerable variation exists in the quality of these feeds in terms of nutrient composition and freedom from dust, fungal and bacterial contamination and infestation by mites. Many horse-owners are now feeding forages such as haylage and baled silage to horses as opposed to hay. However, limited information is available regarding the effect of chop length on digestibility and thus the feeding value of chopped or long-cut silage to horses. More information on fodder type and preparation could help to provide the basis for improvements in the practical guidelines and recommendations for those involved with the production and feeding of forages to horses.