Four castrated horses were utilized in randomized blocks experimental design. The objective was to study the "in vitro" acquirement of indigestible fiber, neutral detergent fiber (NDFi) and acid detergent fiber (ADFi), using as inoculum source rumen fluid (RF) or horse feces (HF), to estimate the nutrient digestibility (ND) in horses. Treatments consisted of direct method by total feces collection (TC) and indirect method either by the use of NDFi or ADF by the use of LR or HF as inocula source. The first essay was done with horses fed coast-cross hay exclusively, while for the essay two, the horses were fed of 70% of coast-cross hay and 30% corn grain. For the essay 1, ADFi-HF presented the best marker recuperation rate (RR) (103.67%), being similar to control (TC), while NDFi-RF and ADFi-RF resulted in lower RR (P<0.05), 83.43 and 88.28%, respectively. The ND was adequately estimated by NDFi-HF and ADFi-HF. On the second essay there were no significant effects of the marker type, as well as for the indigestible marker acquirement method, for the marker RR (average value of 101%). The ND was adequately predicted by the ADFi obtained for both innocula source and NDFi obtained by the use of HF as inoculum, for horses fed a mixed diet. It can be concluded that horse feces may be used as inoculum source for the acquirement of NDFi and ADFi "in vitro" for digestibility determination on horses fed coastcross hay exclusively, as well as for horses fed a mixed diet.
horse; indirect method; internal marker and nutrient