The relationship between microbial N synthesis and urinary excretion of purine derivatives in Dorper ×thin-tailed Han crossbred sheep

This study aimed to investigate the relationship between microbial N (MN) yield and urinary excretion of purine derivatives (PD) from the rumen in Dorper × thin-tailed Han crossbred sheep. Twelve Dorper × thin-tailed Han crossbred, non-castrated male lambs (41.3 ± 2.8 kg body weight) fitted with ruminal and duodenal cannulae were randomly assigned to one of three levels of DM intake: ad libitum intake, 70%, or 50% of the ad libitum intake, with four lambs at each level. The MN yield from the rumen was measured using 15N or purine bases (PB) as a microbial marker, and urinary PD were determined by colorimetric methods. Microbial components (OM, N, PB and ratio of PB to N) were not affected (P > 0.05) by dietary treatment. The MN yield measured using PB was slightly lesser and more variable than using 15N. No difference in MN yield was found between markers. The efficiency of MN synthesis (g N/kg organic matter apparently digested in the rumen) was not affected (P > 0.05) by feed intake regardless of the marker used. Urinary PD decreased significantly (P < 0.05) with decreasing level of feed intake and the molar recovery of duodenal PB measured with digesta markers as urinary PD was 0.83 (r2 = 0.89, P < 0.001). Urinary PD excretion was linearly correlated with DOMI (r2 = 0.94, P < 0.001), and MN yield estimated by using either 15N or PB (r2 = 0.91 and 0.80, respectively, P < 0.001). These results suggest that urinary PD excretion is an accurate indicator of MN synthesized in the rumen, and an equation, MN (g/d) = 0.030 + 0.741 × PD (mmol/d), is proposed to predict the MN yield from the rumen of Chinese sheep.