In vitro methods to determine rate and extent of ruminal protein degradation

Rates of ruminal protein degradation are required in most of the current nutritional models; however, protein degradation in the rumen is very complex and difficult to measure reliably using simple in vitro and in situ methods. In vitro assays based on using mixed ruminal organisms in which microbial N incorporation is quantified or in which microbial growth is inhibited have proven to be the most robust for estimating rate and extent of protein degradation. Use of gas production to estimate N incorporation or release has become widely adopted. Recent developments have allowed study of the time course of protein breakdown so degradation rates can be determined. The inhibitor in vitro procedure has been useful in a number of applications but is limited to short term incubations of 4 to 6 h. Also, this assay has not been widely adapted beyond the laboratory where it was developed, possibly because it requires large numbers of assays for ammonia and total AA. Under some conditions, extents of protein hydrolysis obtained using several different cell-free proteases have been correlated to extents of in situ degradation but, thus far, cell-free proteases have not proven completely reliable for predicting rates of ruminal protein degradation. Future research might be directed toward identifying how protein degradation activities of cell-free proteases differ from that of the proteases elaborated by mixed ruminal organisms.