Animal Science Department


Date of this Version



Published in Journal of Animal Science 79(E. Suppl.):E208–E217. Copyright © 2001 American Society of Animal Science. Used by permission.


Forage proteins are degraded rapidly by rumen microorganisms and therefore supply relatively small quantities of undegraded intake protein (UIP). Growing cattle with high metabolizable protein requirements and lactating beef and dairy cows respond to UIP supplementation when fed high-forage diets, even though degradable intake protein (DIP) is adequate. This observation suggests that an accurate estimate of forage UIP is needed to establish optimal supplementation conditions. Microbial protein must be quantitated in duodenal or in situ residue samples to accurately measure forage UIP. Purines commonly are used as a microbial protein marker. Recent reports suggested that the original purine procedure generates interfering compounds that reduce estimates of microbial protein. Reanalysis of samples with a modified purine procedure yielded three to four times more purines in both duodenal samples and NDF residue incubated in situ. An alternative in situ procedure removes the microorganisms by refluxing with neutral detergent after ruminal incubation. This alternative correlates highly to the purine-corrected in situ procedure, and it is less variable and simpler to perform. Differential centrifugation may be inappropriate for obtaining clean samples of rumen microbes for determination of purine-to- Nratio because it does not represent particle-associated microorganisms. We suggest an alternative method of measuring that ratio in which NDF is incubated in situ and the particle-associated microorganisms can be measured. Rate of passage is used along with in situ rate of degradation to calculate UIP. We propose that a lag time associated with passage should be added to that calculation. Degradation with no passage (applying a lag) reduced UIP values of forage with high potentially available UIP pools. Enzyme analysis and near infrared reflectance spectroscopy show promise for measuring UIP when fistulated cattle are unavailable. Both are useful predictors of UIP and need more validation research to firmly establish their efficacy. Methods suggested in this paper may improve the accuracy and precision of UIP estimation. The in situ NDIN procedure appears to be a simple and acceptable method of UIP determination. When measurement of purines is needed, the modified assay described in this paper should be considered.