Mathematical modelling of digesta passage rate, mean retention time and in vivo apparent digestibility of two different lengths of hay and big-bale grass silage in ponies

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.