. 2005. Comparison of purines and nitrogen-15 as microbial flow markers in beef heifers fed barley-or corn-based diets. Can. J. Anim. Sci. 85: 211-222. 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 15 N 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. ( 15 NH 4 ) 2 SO 4 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 15 N 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 15 N 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. 15 Nenrichment 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. a été infusé dans le rumen pendant huit jours afin de marquer les microorganismes du rumen et leurs bases puriques. Les auteurs ont prélevé des échantillons du contenu du rumen, des digests du duodénum, de l'urine et des fèces au cours des deux derniers jours de l'infusion et 48 heures après celle-ci. Les animaux ont consommé plus (P < 0,01) de matière sèche (MS), de matière organique (MO), d'azote (N) et de fibres au détergent neutre (FDN) avec la ration B qu'avec la ration C. La digestibilité de la MS, de la MO, du N et des FDN dans l'ensemble du système digestif était aussi plus élevée (P < 0,01) avec la ration B. La concentration d'ammoniaque (P < 0,01), d'acides gras volatils (P < 0,05) et d'acétate (P < 0,01) dans le rumen ainsi que l'activité de la xylanase (P < 0,05) dans le fluide du rumen étaient plus élevées avec la ration B qu'avec la ration C. On a estimé le MCP dans le duodénum à partir d'échantillons d...