Methodical considerations when estimating nutrient digestibility in horses using the mobile bag technique

Total collection of faeces is considered the golden standard for estimating apparent total tract digestibility (ATTD) in horses. However, the evaluation of individual feedstuffs is limited and determination of nutrient digestibility in different segments of the gastrointestinal tract (GIT) is excluded. The rationale for performing this study was that the mobile bag technique (MBT) can provide information on individual feedstuffs' degradation, and the use of fistulated animals does provide additionally information regarding degradation in individual segments of the GIT. Recommendations for using the MBT in ruminants are well established, but limited methodical studies have been published with horses. The objective of this study was to evaluate the MBT by comparing the ATTD with the nutrient disappearance and degradation kinetics of hay in horses. It was hypothesised that DM degradation as estimated by the MBT is equal to the ATTD of the DM. Furthermore, we hypothesised that bag size has no effect on nutrient disappearance but increasing the feed to surface area (FSA) decreases the DM disappearance. Five caecum cannulated horses were fed a hay-only diet (6.7 kg DM/day) with 14 days of adaptation followed by four consecutive days of total faeces collection. Three bag sizes (height × length × side, cm; 1.2 × 10 × 2, 3 × 4 × 2, 1 × 6 × 2) and three FSAs (10.4, 20.8 and 41.7 mg/cm²) were administrated at each meal (3 meals/day) on days 1 and 2 of the collection. Faeces were checked for bags every 6th h, the collection time was noted and the DM disappearance together with the transit time (TT) for each bag type was estimated. Dry matter disappearance from the individual bags was fitted to degradation profiles, and the effective degradability (ED) and degradation (D_t) were determined. The results of the study showed that the ATTD of DM, organic matter (OM), NDF and ADF can be predicted based on their disappearance from the mobile bags, but that ash and CP are overestimated in comparison to the ATTD. The TT for the bags was 29.2 h, and when using a mean retention time of 30 h to predict ED and D_t, it was clear that ED was underestimated, whereas D_t reflected the ATTD of DM. In conclusion, the MBT can be used to estimate the degradability of DM, OM and fibre as these nutrients resemble the ATTD. The bag size did not affect the DM disappearance, but the FSA should be kept below 20 mg/cm² as higher levels might limit the degradation kinetics.