Estimating ruminal crude protein degradation of forages using in situ and in vitro techniques

Rumen insoluble feed crude protein (CP) escaping degradation (RUP) was estimated from a variety of forages using two in vitro procedures: the Cornell protein fractionation procedure and the enzymatic degradation procedure by Streptomyces griseus protease. Some recent improvements to both in vitro procedures were applied. The in situ technique served as a reference method and a novel combination of methods was used to correct for microbial colonisation of residues. Twenty-five forages, varying in conservation type (unconserved, ensiled, dried) were analysed. Assumed passage rates (Kp) of 0.02, 0.04 and 0.06/h were applied to estimates of RUP using in situ and chemical fractionation. Results from both in vitro procedures correlated linearly with in situ values (P<0.05). In terms of variation around the regression line, distance of the slope from zero and root mean square error, enzymatic degradation was the more accurate method in estimating in situ RUP (r2 = 0.71, P<0.0001; Kp4). In both in vitro procedures, silage and dried forage were more accurately estimated than unconserved forage, which may be explained by the higher and more variable concentration of intermediately degraded CP fraction B2 in the latter. Estimation of in situ RUP from CP fractions needs further improvement. Results imply that in vitro procedures may be used to predict in situ estimates of RUP with a higher level of confidence.