Comparison of In Vitro Rumen Fermentation Methods

An intra-laboratory comparison of different in vitro rumen fermentation methods for measuring rate of cellulose digestion was made. The study consisted of two experiments ; the first involved four in vitro methods and three substrates (alfalfa hay, alfalfa meal and bromegrass hay) fermented over four time periods (6-, 12-, 18- and 24-hr.). Cellulose digestibilities for the in vitro methods could be categorized into two groups, with differences between groups but not within groups. Differences were found between fermentation periods, and an interaction between periods and methods occurred. This interaction was primarily the result of changing rates of cellulose digestion between time periods. Differences were found between bromegrass hay and alfalfa substrates but not between the two alfalfa samples. The second experiment involved three in vitro methods utilizing an alfalfa meal substrate studied over five fermentation periods (6-, 12-, 18-, 24- and 48-hr.). Differences between methods were obtained after 6- and 12-hr. fermentation periods but not after 18-, 24-, and 48-hr. periods. The primary factors which could have contributed to differences between methods were size of substrate, nutrient media, preparation of inoculum, maintenance of anaerobiosis and type of fermentation vessel. The variability between duplicate samples decreased as the length of the fermentation period increased. Differences also existed between methods in the magnitude of the within-trial variability.