EXCRETION OF PURINE DERIVATIVES BY HOLSTEIN COWS ABOMASALLY INFUSED WITH INCREMENTAL AMOUNTS OF PURINES

Five multiparous, ruminally cannulated Holstein cows (two lactating an d three dry) weighing((X) over bar +/- SD) 667 +/- 35 kg were used to study the effect of abomasal purine infusion on the excretion of purin e derivatives. Cows were fed corn silage four times daily at 90% of ad libitum intake ((X) over bar = 9.16 kg of dry matter/d). Purines were infused into the abomasum as brewer's yeast suspensions in five incre mental amounts (0 to 380 mmol/d) during five experimental periods acco rding to a 5 x 5 Latin square design. Periods were 7 d; purine infusio ns were conducted during the last 4 d, and urine was collected during the last 3 d of each period. Ruminal purine outflow in all cows was me asured during an experimental period immediately preceding and immedia tely following the five infusion periods and in each cow during the 0- mmol/d infusion period of the experiment. The relationship between tot al (milk plus urine) daily excretion of purine derivatives (allantoin plus uric acid) and total (abomasal infusion plus ruminal outflow) dai ly purine flow was quantified by linear regression analysis and was de scribed by the relationship: Y = 0.856X + 103 (r(2) = 0.93). The slope (0.856) indicated that 86% of purines that reached the omasum were ex creted as purine derivatives. In the two lactating cows, urinary purin e derivatives accounted for 98.4% of the total purine derivatives that were excreted. Ruminal flow of microbial CP can be estimated from the CP:purine ratio of ruminal microorganisms and the excretion of purine derivatives.