In horses, forages account for the greatest proportion of the diet (Micol and Martin-Rosset, 1995). As a result, the energy value of forages has to be determined very accurately to meet the horse requirements. In the UFC system (Horse Feed Unit) proposed by INRA (Martin-Rosset et al., 1994), the energy value of forages is closely related to organic matter digestibility (OMD).
In ruminants, OMD has been related to the in vitro gas production after 24 h of fermentation of foodstuffs with rumen fluid (Menke et al., 1979). The prediction of OMD with the gas test method is less accurate than with the nylon bag method but better than prediction performed with in vitro method or from chemical composition of foods (Khazaal et al., 1993 and 1995).
In the UFC system, three routine methods have been performed by our laboratory and proposed by INRA to predict routinely OMD of foodstuffs. OMD can be predicted either from chemical composition (Martin-Rosset et al., 1996a), by a pepsin cellulase method (Aufrère, 1982; Martin-Rosset et al., 1996b) or by near infra-red spectrometry (NIRS) (Andrieu et al., 1996).
The gas test method was subjected to be adapted in horses for predicting OMD. The accuracy of the prediction with the gas test method was compared with the other routine methods.