Validation of use of purine bases as a microbial marker by 15N labelling in growing lambs given high-concentrate diets:: effects of grain processing, animal age and digesta sampling site

The origin of post-ruminal purine bases (PB) was studied in 24 growing lambs that were given a pelleted concentrate plus barley straw (C) or whole barley grain plus protein supplement (WB). Six lambs from each treatment were slaughtered at 10 and 30 days post weaning after N-15 labelling of microbial nitrogen (N) and PB. Microbial contribution to digesta non-ammonia N (NAN) and PB was lower (P < 0.01) when estimated from duodenal rather than abomasal samples (0.36 v. 0.52 (s.e.d. 0.021) for NAN and 0.47 v. 0.77 (s.e.d. 0.029) for PB) as a result of endogenous contamination. In comparison with 15N, total PB/N led to higher estimates (P < 0.01) of microbial contribution to abomasal NAN in WB treatment (0.62 v. 0.46 s.e.d. 0.049). The difference was removed after correcting for microbial PB, while this effect was not observed with < the C diet, resulting in a marker by diet interaction (P < 0.05). Abomasal PB flow increased (P < 0.1) from 10 to 30 days after weaning mainly due to the higher proportion of microbial PB (0.70 v. 0.81 (s.e.d. 0.047)). Rumen apparent PB degradation did not differ between diets in older lambs, but it was proportionally 0.39 lower for WB treatment (P < 0.05) in younger lambs. When the microbial PB flow was estimated indirectly from labelled microbial N and the PB/N ratio of bacterial extracts the estimates were in agreement with those derived from PB-N-15 in the WB treatment but resulted in unrealistic values in lambs on diet C. Results suggest that significant proportions of dietary PB can escape rumen degradation which may lead to overestimation of microbial contribution to abomasal NAN when the PB/N ratio is used as marker. The extent of the overestimation is affected by the lamb age and grain processing.