Markers of rumen bacterial protein supply for cattle

Four metabolism trials and an in situ trial were conducted to investigate markers of rumen bacterial protein supply for cattle. In Trials I, II, and III, ruminally and duodenally cannulated cattle were used to compare total urinary allantoin excretion and duodenal purine flow as estimates of bacterial crude protein. Forages fed were alfalfa hay (Trials I and II) and switchgrass hay (Trial III). In Trial I, both urinary allantoin excretion and duodenal purine flow increased as alfalfa hay intake increased. A positive lnear relationship also was observed between urinary allantoin excretion and duodenal purine flow. Equations for predicting urinary allantoin excretion and duodenal purine flow from intake were calculated. In Trial II, no effect of volatile fatty acid infusion was observed for allantoin excretion; therefore, any increase in allantoin excretion can be attributed solely to the flow of purines from the rumen. In Trial III, cattle were fed a constant amount of switchgrass hay and treatments of supplemental wet corn gluten feed were applied. Duodenal purine flow and urinary allantoin excretion increased linearly as supplementation increased. Data from Trials I and III were combined in an equation for predicting duodenal purine flow from urinary allantoin excretion. It was concluded that urinary allantoin excretion is an effective bacterial crude protein marker. In Trial IV, the effect of corn wet milling byproducts on urinary allantoin excretion was investigated. Diets were a dry rolled corn finishing diet or incremental portions of corn replaced with the byproducts, steep liquor or distillers' solubles. The inclusion of byproducts decreased urinary allantoin excretion. Although ruminal pH was not significantly lowered by the inclusion of byproducts, allantoin excretion by individual animals and average daily ruminal pH were correlated. Therefore, ruminal pH appears related to urinary allantoin excretion. In the in situ trial, a comparison was made between purines and phospholipid phosphorus as bacterial crude protein markers. It was concluded that phospholipid phosphorus was an ineffective marker because estimates were influenced by forage type and analytical run.