Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/6/1996
Publication Date: N/A
Citation: Interpretive Summary: Most of the protein fed to dairy cows is first broken down by the microbes in the rumen (the first compartment of the cow's stomach), then resynthesized into microbial protein prior to being digested and absorbed by the animal. Therefore, when studying the protein nutrition of dairy cows, it is of great value to be able to measure the amount of microbial protein formed in the rumen. However, techniques currently available to do this are both extremely tedious and require extensive surgery that may be harmful to the cow. It has long been known that the excretion of purine derivatives (compounds arising from digestion of the purine components of DNA and RNA in microbial cells) was related to microbial protein supply in sheep. Therefore, we undertook to determine: 1) the precise relationship between purine flow and purine derivative excretion in dairy cows, and 2) whether measuring purine derivative excretion could be used to measure microbial protein supply. Each day incremental amounts of purine were infused into the abomasum (the last compartment of the cow's stomach) and the corresponding excretion of purine derivatives in the milk and urine was measured. It was determined that: total purine derivative excretion = .856 x purine flow + 103; this equation had a high correlation (that is, the relationship was very strong). Therefore, purine flow can be calculated from purine derivative excretion in the milk and urine and, after determining the protein:purine ratio of rumen microbes, microbial protein flow can then be estimated. This means that the supply of microbial protein can be measured from purine excretion in normal dairy cows without resorting to extensive surgery that may actually alter the findings and may even be harmful to the animals.
Technical Abstract: Five multiparous, ruminally cannulated Holstein cows (two lactating, three dry) weighing (mean +/ SD) 667 +/ 35 kg BW were used to study the effect of abomasal purine infusion on purine derivative excretion. Cows were fed corn silage at 90% of ad libitum intake (average = 9.16 kg DM/d), four times daily. Purines were infused into the abomasum as brewer's yeast suspensions sat five incremental levels (0 to 380 mmol/d) during five experimental periods according to a 5 x 5 Latin square design. Periods were 7 d long, with purine infusions during the last 4 d, and urine collections during the last 3 d of each period. Ruminal purine outflow was measured in all cows during an experimental period immediately preceding and following the five infusion periods, as well as in each cow at the 0 level during the infusion portion of the experiment. The relationship between total (milk + urine; mmol/d) purine derivative (allantoin + uric acid) excretion and total purine flow (abomasal infusion + ruminal digesta; mmol/d) was quantified b linear regression analysis and described by the relationship Y = .856X + 103 (P < .001, r**2 = .93). The slope (.856) indicated that 86% of purines reaching the omasum were recovered as purine derivatives. In the two lactating cows, urinary purine derivatives accounted for 98.4% of total purine derivatives excreted. Ruminal microbial CP flow can be estimated from the CP:purine ratio of ruminal microorganisms and purine derivative excretion.