Differences between Tasmanian forage genera in the relation between in vitro and in vivo digestibility

Abstract

In vivo and in vitro organic matter digestibilities (OMD's) were determined on cuts of Lolium perenne (perennial ryegrass cv. Tasmanian No. I ) , Lolium perenne (long rotation ryegrass cv. Grasslands ariki), Lolium perenne x L. multiflorum (short rotation ryegrass cv. Grasslands Manawa), Lolium multiflorum (tama ryegrass cv. Grasslands Tama) Dactylis glomerata (cocksfoot cv. Grasslands Apanui), Dactylis glomerata (cocksfoot cv. Currie), Festuca arundinacea (tall fescue cv. Demeter), Avena sativa (oats cv., Blythe), Phalaris tuberosa (phalaris cv. Australian), Trifoleum repens (white clover cv. Grasslands Huia) Medicago sativa (lucerne cv. Hunter River), Brassica napus (rape cv. Rangi) and Brassica oleracea (kale cv. Chou moellier). Linear regressions relating in vitro and in vivo OMD were calculated for the individual generic groups. There were no significant differences between the regressions of ryegrass, cocksfoot, phalaris, oats and lucerne (P > 0.05) (group A) or between clover and brassicas (P > 0.05) (group 6). However, the combined regression for group A (y = 0.78 x + 13.0) was significantly different from that of group B (y = 0.76 x + 19.1 ) (P < 0.01 ) and both were significantly different from the fescue regression (y = 1.23 x - 24.4) (P < 0.01 ). The in vitro technique underestimated the in vivo OMD of white clover and the brassicas by about 4.5 units and over-estimated that of fescue by about 3 units, relative to the other species. These differences were not caused by a modification of the in vitro method in which final residues were collected by filtering through sintered glass crucibles rather than by centrifuging. However, the filtering method gave in vitro results averaging 7.2 per cent higher than the centrifuging method.