Frozen rumen fluid as microbial inoculum in the two-stage in vitro digestibility assay of ruminant feeds

This study was carried out to investigate the possibility of using frozen rumen fluid (RF) as an inoculum source in the determination the in vitro dry matter digestibility (IVDMD) of ruminant feeds. In the first experiment six roughages (barley straw, wheat straw, lentil straw, wheat silage, maize silage and lucerne hay) and six different compound feeds (two dairy cattle, two beef cattle, one ram lamb and one lamb finishing compound feeds) were chosen as test material. Two different types of cryoprotectants (5% glycerol vs. 5% dimethyl sulfoxide, DMSO) and two types of freezing methods (liquid nitrogen vs. deep freezing) were used to preserve the RF. Fresh RF was used as the control. In response to the results of the first experiment, a second experiment was conducted to establish if microbial digestibility could be improved by extending the duration of incubation of the roughages from 48 to 60, 72 or 96 h. When using the RF inoculum treated with 5% DMSO and frozen in liquid nitrogen, the IVDMD of the compound feeds were similar to those of the control. However, when the 5% DMSO treated inoculum was frozen in the deep freezer, some of the compound feeds had similar IVDMD values to the control but not all. Lower IVDMD values were recorded when glycerol was used as a cryoprotectant. In the case of the roughages the use of both freezing methods and cryoprotectants resulted in lower IVDMD values compared to the control. In the second experiment the increasing of the duration of incubation of roughages from 48 to 72 h improved their IVDMD to values similar to that of the control. This suggested that the length of the incubation of roughages should be 72 h instead of 48 h when using inocula treated with 5% DMSO as a cryoprotectant and frozen in a deep freezer. However, further research is required before frozen RF can be recommended as source of inoculum.