INVITRO FIBER DIGESTION - ASSOCIATIVE EFFECTS IN TROPICAL GRASS-LEGUME MIXTURES

This research evaluated the effects of source of ruminal fluid inoculum and grass: legume proportion on the kinetics of in vitro neutral detergent fibre (NDF) digestion in grass-legume mixtures. Species used were the grasses, bahiagrass (Paspalum notatum) and limpograss (Hemarthria altissima), and the legumes, aeschynomene (Aeschynomene americana) and hairy indigo (Indigofera hirsuta). Sources of ruminal fluid were from steers fed limpograss hay (6.0 g/kg N) alone, or limpograss hay supplemented with urea or cottenseed meal. In vitro NDF digestion of 100% grass was greater when ruminal fluid from steers fed limpograss hay plus supplemental N was used than ruminal fluid from steers fed limpograss hay only. This suggested that activity of ruminal microorganisms from steers fed limpograss hay only was limited in N and/or other factors necessary for optimal in vitro fibre digestion. When ruminal fluid from steers fed limpograss hay only was used, mixtures of aeschynomene with either grass resulted in in vitro NDF digestion values that were greater than the weighted average of in vitro NDF digestion values of the individual forages. The magnitude of these increases was similar to the improvement in in vitro NDF digestion of pure grass obtained using ruminal fluid from steers fed limpograss hay plus supplemental N, suggesting that aeschynomene provided soluble and/or degradable N to the in vitro fermentation. The likely mode of action of the positive associative effects on in vitro NDF digestion was a reduced lag time for the initiation of fibre digestion.