Submitted to: Canadian Journal of Animal Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 2/3/2005
Publication Date: 6/15/2005
Citation: Hristov, A.N., Mcallister, T.A., Ouellet, D.R., Broderick, G.A. 2005. Comparison of purines and nitrogen-15 as microbial flow markers in beef heifers fed barley- or corn-based diets. Canadian Journal of Animal Science. 85:211-222. Interpretive Summary: Dairy cows obtain much of their protein needs from synthesis by the microbes living in the rumen, the first compartment of the cow's stomach. This microbial protein is of high quality, providing amino acids in about the correct proportions required by the cow to build its own proteins. Moreover, optimizing protein production in the rumen will improve nitrogen efficiency and reduce the negative environmental impact of dairy farming. It very important to know how altering the diet influences microbial protein formation because of its importance to the health and productivity of the cow. The purpose of this research was to test the reliability of two common methods used by dairy nutritionists to measure protein synthesis by rumen microbes. Both techniques use the purines in the microbial DNA and RNA as indicators of the amount of microbial protein. However, purines are also found in varying amounts in dietary ingredients. In this experiment, two different diets, formulated from barley, barley silage and soybean meal or from corn, corn silage and corn gluten meal, were fed to young cows that had rumen cannulas (holes put into their rumens by veterinarians using surgery). An inorganic ammonia salt, labeled with heavy nitrogen, was infused into the rumen. The amount of heavy nitrogen incorporated into the rumen microbes is the standard way to measure the amount of microbial protein formed in the rumen. Samples of digesta flowing out of the rumen were collected through the cannulas to measure the microbial protein using heavy nitrogen (the standard method), from purine flow out of the rumen, and from excretion of purine derivatives in the urine. This research showed that measurement of microbial protein based on purine outflow, an approach that is used widely by researchers, was unreliable. Our work also showed that the measurement based on urinary excretion of purine derivatives, which only requires urine collection from normal dairy cows, was nearly as accurate as the standard approach. This research indicates that the purine derivative procedure will yield reliable results in dairy feeding studies on how diet influences the supply of microbial protein to the cow.
Technical Abstract: The objective of this study was to estimate the contribution of microbial purine bases to duodenal purines and to purine derivatives [allantoin and uric acid (PD)] excreted in the urine. Additionally, microbial protein (MCP) flow estimated using duodenal flow of purine bases was compared to estimates using 15N as a microbial marker. Four beef heifers were fed two diets, barley silage/barley grain/soybean meal (diet B) or corn silage/corn grain/corn gluten meal (diet C), in a cross-over design study. (15NH4)2SO4 was infused in the rumen for 8 d to label ruminal microorganisms and their purine bases. Rumen contents, duodenal digesta, urine, and feces were sampled during the last 2 d of tracer infusion and for 48 h after the infusion ceased. The animals consumed more (P < 0.01) dry matter (DM), organic matter (OM), N, and neutral detergent fiber (NDF) with diet B than with diet C. Total tract digestibilities of DM, OM, and NDF were also higher (P < 0.01) with diet B. Ruminal ammonia (P < 0.01), volatile fatty acids (P < 0.05), and acetate (P < 0.01) concentrations and xylanase activity (P < 0.05) were higher with diet B compared with diet C. Flow of MCP to the duodenum was estimated from duodenal samples using purines or 15N as microbial markers, or from urinary PD excretion. The effects of diet or method of measurement on MCP flow were not significant. However, when the urinary PD method was excluded from the analysis, MCP flow was greater (by 26%; P = 0.01) when estimated using 15N vs. the purine-based method. The difference was mainly due to underestimation of the proportion of microbial N in the liquid duodenal digesta with the purine method. Feed purines contributed from 3.5 (liquid digesta phase) to 19.7% (solid digesta phase) of the total purine flow at the duodenum. 15N-enrichment of urinary PD was 1.08 of the enrichment of duodenal purines, suggesting that feed purines contributed little N to urinary allantoin and uric acid in cattle.