In current feed evaluation systems for dairy cattle the energy value of a feed is calculated by summing the energy value of the individual feed ingredients, although multiple studies have revealed that feeding a mixture of different forages can result in digestive interactions between individual substrates. Those interactions called associative effects might influence metabolic processes in a positive or negative way and therefore might change nutritive values. The aim of this thesis was the evaluation of associative effects of grass-silage-based total mixed ration (TMR) and its separate components using in vitro gas production for up to 48 hours and rumen inoculum of dairy cows in early lactation. In this study two different types inocula (free rumen liquid (FRL), particle associated liquid (PAL)) were used in parallel in order to investigate whether the origin of rumen inoculum affected in vitro gas production and the magnitude of the associative effects between TMR and single ingredients. Furthermore the dairy cows in this study were fed TMR with three different particle sizes, hypothesizing that this influences in vitro gas production and the amount of associative effects. In this experiment positive associative effects between observed and calculated gas production of grass silage-cereal concentrate-based TMR occurred up to a percentage of 19 per cent at early hours of incubation (2 and 4 h), declined with the passing of incubation time and mostly disappeared after 8 h of incubation. When incubated with PAL, gas production of TMR increased (P<0.01) compared to FRL, but the amount of associative effects was not influenced by varying rumen inocula. Dietary particle size influenced associative effects during incubation with FRL (P<0.05), particularly at early hours of incubation. However, the deviation between observed and calculated values in this experiment emphasize that summing the nutritive value of feedstuffs might result in an underestimation of fermentability of this type of TMR at early hours of incubation with consequences for ruminal physiology.