A series of in vitro incubations were performed in which a constant substrate (antelope cube) was used and only the inoculum source was varied. With an inoculum obtained from 7 different wild ruminants, gas production rate varied considerably and was influenced by season. This was further investigated by testing fermentation rate on the same substrate with rumen fluid obtained from free-ranging kudu on a monthly basis over the year. It was found that fermentative ability declined during the winter. Thus in vitro digestibility of a given substrate is a function of inoculum source, time of the year and feed selection by the donor animal