Evulation of an in vitro batch culture technique for estimating the in vivo digestibility and digestible energy content of equine feeds using equine faeces as the source of microbial inoculum

Abstract

The suitability of an in vitro batch culture technique for estimating both the dry matter digestibility in vivo (DMIV) and the digestible energy (DE) content of equine feeds was examined. Sixteen feedstuffs of known DMIV and DE contents in equines were incubated at 39°C with an inoculum prepared from fresh pony faeces. Gas production was recorded throughout using the manual pressure transducer technique. Volatile fatty acid (VFA) concentrations along with dry matter loss (DML) were determined at the end of the incubation period. Prediction equations were developed using cumulative gas production volumes, modelled gas production parameters, values for VFA molar proportions and DML. The best equation for predicting DMIV (g kg −1) was obtained using the total VFA production from the feed (TVFA) and the acetic acid (Ac) molar proportion, i.e. DMIV=15431−53.58 Ac+0.04426 Ac 2+368.1 log TVFA ( R 2 = 0.859; RSD = 51.2). Using only gas production data as predictors yielded the equation DMIV=234.2+4.907 g44−8.15 g23+13.63 g11 ( R 2 = 0.836; RSD = 55.3) where g44, g23 and g11 are cumulative gas volumes measured at 44.5, 23 and 11 h of incubation. TVFA and Ac were also good predictors of the DE (MJ kg −1 DM) content of the feedstuffs: DE=251.4−0.857 Ac+0.000692 Ac 2+6.55 log TVFA ( R 2 = 0.865; RSD = 1.04). However, this equation tended to underpredict DE relative to the actual values with a bias of −0.36 MJ kg −1 DM. Using only the modelled gas production parameters Z and T along with DML resulted in a better equation with negligible bias; DE=−0.68+0.01087 DML+6.82Z−2.297 log T ( R 2 = 0.878; RSD = 0.99). The results demonstrate that equine faeces is a suitable source of microbial inoculum for in vitro gas production studies and that the in vitro batch culture technique evaluated has considerable potential as a routine predictor of the nutritive value of a wide range of equine feedstuffs.