Mobile bag technique for estimation of nutrient digestibility when hay is supplemented with alternative fibrous feedstuffs in horses

Abstract

To evaluate the effect of substituting hay with alternative fibrous feedstuffs, the total collection of faeces was used to measure the apparent total tract digestibility (ATTD). Nutrient disappearance and digestion kinetics were examined with the mobile bag technique (MBT) and marker passage measurements. Four caecally-cannulated horses (body weight (BW) 558 ± 32 kg) were used in a cross-over design experiment with two periods of 14 days adaptation and four days of faecal collection. Horses were fed three times a day with either a hay-only (HAY) diet or a mixture of hay:supplement (MIX) (15.1 and 8.4:6.7 g dry matter (DM)/kg BW/day, respectively). The hay used in both treatments (HAY and MIX) was mainly of Timothy and first cut. The MIX supplement diet consisted of oat hulls, alfalfa-, sugar beet pulp- (SBP), grass- and soya hull pellets, each given in 0.44 g DM/kg BW/meal. On day 15 in each period, 20 bags of either hay or SBP and 6–12 bags (1×2x12 cm; 37 µm pore size; 0.5 g feed) of each feedstuff and ytterbium (Yb, 3 g) were placed in the stomach or caecum, respectively. Bags were harvested from the caecum every hour and faeces were checked for bags every fourth hour, collection time was noted and data from the bags were used to estimate pre-caecal, hindgut and total tract nutrient disappearance. Further, faecal subsamples of 300 g were collected, weighed and stored for Yb analysis and further estimation of feed mean retention time. Rate and extent of feed degradation were estimated from the MBT assuming exponential degradation. The ATTD of DM was similar between the two diets (P > 0.05), but the HAY diet had higher ATTD of crude protein (CP) (P = 0.001), neutral detergent fibre assayed with heat-stable amylase and expressed inclusive of residual ash (aNDF) (P = 0.006), acid detergent fibre (ADF) (P = 0.017), hemicellulose (P = 0.001) and celluloseNDF (P < 0.001). The hindgut mean retention time (MRT) for Yb was longer for the MIX than the HAY diet (P < 0.001). No differences for DM, aNDF or ADF digestibility were measured when comparing the ATTD with nutrient disappearance from bags found in the time interval 20–30 h, indicating the ATTD of these nutrients can be predicted by the MBT. The estimated degradation (Dt), but not effective degradation (ED), is preferred when the MBT is used to predict the ATTD. It can be concluded that hay can be substituted partly by fibrous feedstuffs and that the MBT can predict the ATTD of DM, aNDF and ADF in a mixed ration based on MBT measures on individual feedstuffs.