Microbial nitrogen production in growing heifers: direct measurement of duodenal flow of purine bases versus urinary excretion of purine derivatives as estimation procedures

The present study compares estimates of rumen microbial nitrogen (MN) production derived from duodenal flow measurements with those obtained from urinary purine derivatives (PD) excretion. Four crossbred Holstein–Friesian heifers (initial weight 306±6.0 kg) fitted with rumen and duodenal cannulae were randomly allocated in a double 2×2-crossover design to one of two experimental diets. Both diets were composed of (g/kg as fed) 250 barley straw and 750 concentrate, given together twice daily at a fixed rate close to ad libitum intake. The concentrate consisted of (g/kg as fed) either 655 corn and 225 barley (diet C) and 225 corn and 655 barley (diet B), respectively. Heifers receiving every experimental diet were infused with four levels of effective rumen degradable protein (ERDP) as urea:casein (1:1 on N basis) equivalent to 0, 25, 50 and 75 g/kg of concentrate. Duodenal flow of purine bases (PB) was determined either by using urinary PD excretion or by the dual-phase marker technique during continuous intra-ruminal infusion of Cr-EDTA and Yb-acetate. Microbial contribution to duodenal non-ammonia N (NAN) flow was estimated by the PB/N ratio in bacterial fractions extracted from the liquid (LAB) and the solid (SAB) fraction of rumen digesta. Digestible organic matter intake was higher in animals fed barley than corn diets (4.86 versus 4.25 kg/day) and increased with the ERDP supplementation (4.36–4.70; P<0.05) although most of the response was observed at the first level (25 g/kg) of supplementation. The microbial PB/N ratio was different between LAB (1.16±0.196) and SAB (0.62±0.226) leading to different estimates of MN yield from direct measurements of duodenal PB (93.3±2.68 versus 175.3±3.67 g/day) or urinary excretion of PD (62.9±1.44 versus 118.3±1.98 g/day). Urinary PD closely reflected relative differences among treatments obtained in digesta flows measurements, although urinary PD rendered consistently lower absolute values.