Estimating forage protein degradation in the rumen

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