Contribution of dietary nitrogen and purine bases to the duodenal digesta: comparison of duodenal and polyester-bag measurements

Abstract

Four ewes fitted with ruminal and duodenalT-piece cannulas were given fourdietsin a 4 × 4 factorial design. Diets consisted of 700 (HF) or 400 (LF) g/day of ammonia-treated barley straw supplemented respectively with 150 or 600 g/day of concentrate made up with barley plus either soya-bean meal (SBM) or fishmeal (FM) as the protein source, offered at 2-h intervals. Duodenal flowsof digestawere estimated by the dual-phasetechniqueusing CoEDTAand Yb-acetate as markers and (¹⁵NH₄)₂SO₄ was infusedinto the rumento label microbial N. Bacteria were isolated from the liquid (LAB) or solid (SAB) rumendigesta. Purinebases (PB) were isolated by precipitationin an acid solution of AgN0₃, and microbial contribution either to the duodenalnitrogen(N) or PB were determinedby ¹⁵N measurements induodenaldigesta and bacteria.Simultaneously, therumen degradation of Nand PB contained in SBM and FM was studiedby incubating supplements in polyesterbags in the rumen.PBcontent (mmol/g dry matter)and guanine: adenine(G/A) ratio of barley strawwas 2·89 and 5·23; barley grain,7·91 and 111;SBM, 18·8 and 1·26; and FM, 58·9 and 6·96, respectively. Duodenal flow ofPB(mmol/day)was significantly higher than PB intake on all diets and G/A ratio showed a meanvalue of 0·97, similarto the ratios determined in SAB(0·80) and LAB (1·04) and muchlower than diets(1·31 to 4·32). Microbial contribution to duodenal Nflow ranged from43·3% to 61·0%, beinghigherin SBM(59·0%)thanin FM(46·7%)diets. However, microbial contribution to duodenal PB was not affected by the experimentaltreatment, accounting for proportionately 0·77 of total PB at the duodenum. Rumen degradability of PB was much higher than that of total N and in both cases degradability was higher in SBM than FM. Direct measurements of non-microbialN were significantly higher than values determined by the polyester-bagmeasurements. However, once corrected forthe endogenousN (52 mgN per kg live weight)contribution, results show edan acceptable agreement. Duodenal flow of PB non-attributable to microbes (unlabelled PB) showed a mean value of 3·25 mmol/daywithouta significanteffect of dietary treatment. However, undegradablePBsupply determinedfor0·02, 0·05 and 0·08 per h fractional out flow rates were proportionately lower than 0·025 with SBM and 0·100 with FM diets of the estimated duodenalPB flow. Despite the magnitudeof the unlabelledduodenalPB, the close agreement between G/A ratios in duodenaldigesta and bacteria suggests thatthe contribution of dietary PB to the duodenalflow was low and seemsto confirm the reliability of values obtained from polyester-bag measurements.