A mixture simplex design to study associative effects with an in vitro gas production technique

A mixture simplex design (special cubic version of Scheffé’s canonical polynomial) was used to investigate associative effects of feed mixtures incubated using an in vitro gas production technique. The model included the principal effects (single feeds), double and multiple interaction (mixtures of two and three feeds). Two forage trees (Leucaena leucocephala and Lysiloma latisiliquum) were evaluated in two separate incubation batches. Commercial concentrate feed, grass hay and a fodder tree were used to prepare the mixtures in all possible combinations resulting from 0.0, 0.25, 0.50, 0.75 and 1.0 of grass, concentrate and one forage tree per batch. Pressure and gas volumes were monitored up to 168 h. After incubation the residues were used to measure DM and OM digestibility (in vitro dry matter (IVDMD) and in vitro organic matter (IVOMD)). Cumulative gas production (CGP (ml)) profiles were fitted to the equation CGP=a+b(1−e−kt). A significant associative response in total gas production (P<0.01) was found when the forage trees were mixed with the concentrate but not with the grass. Grass and concentrate mixtures increased gas production but the interaction was significant only in the L. latisiliquum batch (P<0.05). IVDMD and IVOMD were reduced by the inclusion of leucaena in the mixture (P<0.01). Rate of gas production, k (% h−1) was significantly depressed when L. latisiliquum was a component of the mixture (P<0.01) but not in all remaining mixtures, including those that contained leucaena. It was concluded that mixture simplex designs are useful tools to identify and study in vitro associative effects of feed mixtures.