BACKGROUND: Current feed evaluation systems for dairy cattle assume that nutritive values of feed components can be added, but do not take into account possible interactions among feed components. The main objective of this study was to investigate associative effects on in vitro gas production from total mixed ration (TMR), based on grass silage and cereal concentrate, and separate TMR components. TMR was incubated with inocula of free rumen liquid (FRL) and particle-associated rumen liquid (PAL) for 48 h and donor cows were fed TMRs with three different particle sizes to evaluate their influence on associative effects. RESULTS: Associative effects on gas production largely occurred at 2 and 4 h of incubation and dissipated with time of incubation. Incubation of TMR with PAL increased (P < 0.01) gas production compared to FRL at all incubation times; however, associative effects were not different between rumen inocula. Dietary particle size affected associative effects on gas production when TMR was incubated with FRL (P < 0.05) compared to PAL, particularly at early hours of incubation. CONCLUSIONS: Associative effects at early incubation hours emphasise that summing the nutritive values of feedstuffs may underestimate fermentation intensity of TMR which may have implications for prediction of ruminal fermentation yield.
BACKGROUND: Current feed evaluation systems for dairy cattle assume that nutritive values of feed components can be added, but do not take into account possible interactions among feed components. The main objective of this study was to investigate associative effects on in vitro gas production from total mixed ration (TMR), based on grass silage and cereal concentrate, and separate TMR components. TMR was incubated with inocula of free rumen liquid (FRL) and particle-associated rumen liquid (PAL) for 48 h and donorcows were fed TMRs with three different particle sizes to evaluate their influence on associative effects. RESULTS: Associative effects on gas production largely occurred at 2 and 4 h of incubation and dissipated with time of incubation. Incubation of TMR with PAL increased (P < 0.01) gas production compared to FRL at all incubation times; however, associative effects were not different between rumen inocula. Dietary particle size affected associative effects on gas production when TMR was incubated with FRL (P < 0.05) compared to PAL, particularly at early hours of incubation. CONCLUSIONS: Associative effects at early incubation hours emphasise that summing the nutritive values of feedstuffs may underestimate fermentation intensity of TMR which may have implications for prediction of ruminal fermentation yield.