Evaluation of the extent of associative effects of two groups of four feeds using an in vitro gas production procedure

A ration formulated for ruminants is often a mixture of individual feeds, and its energetic value is generally calculated by summing the energy value of the individual feeds in it, on the assumption that the energy value of individual feeds will be the same when they are fed in combination with other feeds. In vitro gas production techniques were used to determine whether associative effects of feeds occur. Two sets of four feeds from California (alfalfa hay, AH; barley grain, BG; corn silage, CS1, soybean meal, SM) and four feeds from The Netherlands (grass silage, GS; corn silage, CS2; citrus pulp, CiP; corn gluten meal, CG) were incubated alone, and in various combinations, in buffered rumen fluid using in vitro gas techniques. Gas production (ml/g DM) at 2, 4, 6, 8, 10, 24, 30, 48 and 72h of incubation was measured from all feed combinations. The percent increase in gas production measured on combinations of feeds, versus the gas calculated to have been produced based upon incubation of the individual feeds, was used to determine the extent of the associative effects. One-way associative effects (i.e., substitution of CS1 or CS2 by AH, BG or SM) generally occurred (P<0.05) in the UCD feed set. Although most two-way associative effects of feeds occurred (P<0.05), they dissipated with time of incubation, particularly after 6–8h. Similar to the UCD feed set, one-way associative effects (i.e., substitution of CS1 or CS2 by GS, CiP or CG) generally occurred (P<0.05) in the Dutch ASG feed set. However, there are only a few two-way associative effects (P<0.05), generally at short times of incubations, and they dissipated with time. In order to obtain a practical sense of the importance of these associative effects in rations fed to ruminants (i.e., do they matter), three theoretical diets were formulated from each of the UCD and ASG feed sets. Results demonstrate that associative effects can reach 15–25% at early hours of incubation, and that they dissipate with time of incubation. That associative effects had no impact at 24h gas production is important, as 24h values are commonly used to estimate the ME value of feeds thereby suggesting that ME values of individual feeds calculated from gas data will arithmetically sum to the ME value of the mixed ration fed to the animals.