Ruminal degradability of 15N labelled ribonucleic acid in grass.
The ruminal degradation of RNA in rye grass (Lolium perenne) was studied using the bag method. A non-lactating cow (BW 550 kg) fitted with a rumen cannula was used and fed twice daily at maintenance level with a chopped grass hay-based ration containing 30% ground barley. Rye grass, labelled during growth by fertilization with 15N2-urea (9.5 atom% 15N, 20 g N/m2), was cut at seven stages of growth and maturity and freeze-dried. RNA-N represented 6 to 17% of total N. Labelled grass samples (milled to 5.0 mm screen, 5.0+/-0.1 g DM) were incubated in polyester bags (100 x 200 mm, pore size 50 microm) in the rumen for periods of 1, 3, 6, 9, 12, 24, and 48 h. Data of N and RNA disappearances from the bags were fitted to an exponential equation to estimate parameters of degradation. The effective degradability of RNA in the rumen averaged 90+/-4%, for N it was 11% units lower (P < 0.001). Degradability of RNA was correlated to that of N (R2 = 0.92). Degradability of RNA (R2 = 0.96) and N (R2 = 0.93) decreased with increasing fibre content of grass. Increasing the fibre content by 1% diminished the degradability of RNA and N by 1.1% units and 2.4% units, respectively (P < 0.001). Assuming a microbial protein synthesis in the rumen of 150 g/kg DOM, a N: RNA ratio of 1:1.35 in rumen microbes and a rumen outflow rate of 0.06 h(-1), a model calculation indicates that about 9 to 19% of duodenal RNA are of dietary origin in animals fed grass. This should be taken into account for the calculation of microbial N on the basis of RNA as marker.
Schönhusen
U
U
Research Institute for the Biology of Farm Animals, Oskar Kellner, Dummerstorf, Germany. schoenhu@fbn-dummerstorf.de
Voigt
J
J
D000818:Animals; D001673:Biodegradation, Environmental; D002417:Cattle; D004043:Dietary Fiber / Q000378:metabolism*; D004063:Digestion; D005260:Female; D005285:Fermentation; D008129:Lolium / Q000235:genetics; D009587:Nitrogen Isotopes; D010940:Plant Proteins / Q000032:analysis / Q000378:metabolism*; D018749:RNA, Plant / Q000032:analysis / Q000378:metabolism*; D012417:Rumen / Q000378:metabolism* / Q000382:microbiology
D004043:Dietary Fiber; D009587:Nitrogen Isotopes; D010940:Plant Proteins; D018749:RNA, Plant