Interactions between non-starch polysaccharide (NSP) level, dietary lipid type, exogenous carbohydrase enzymes and irradiation were investigated. Ten treatment diets were fed to broilers in a performance and digestibility trial. Eight of the diets contained high levels of NSP, achieved by a high barley inclusion of 55% of the diet. Four of the treatments made use of non-irradiated barley, whereas the barley included in the other four diets was irradiated. By adding either 10% soya oil or yellow grease (fat) as the lipid source, sub groups were created which differed in fatty acid profile. Lastly, these treatments were further subdivided by supplementing one of the two diets from each subgroup with a commercially available combination of exogenous carbohydrase enzymes consisting of cellulases, xylanases and â - glucanases (Roxazyme G at 150 g/ton). The two control diets were based on maize (low NSP diets) with either soy oil or yellow grease. The high NSP diets had significantly lower (P<0.05) apparent metabolisable energy (AME) and lipid digestibility values than the low NSP diets. Lipid digestibility and AME values were also significantly lower (P<0.05) for diets containing yellow grease compared to soya oil. The birds that received yellow grease performed worse in terms of growth, feed intake and feed conversion ratio (FCR) than the oil-containing diets. These trends were evident throughout all treatments, although not always significant. The addition of carbohydrase to diets based on barley improved the dietary lipid digestibility and AME values. Significant improvements (P<0.05) in bird performance were noted for the barley diets with the yellow grease. Pre-irradiation of barley significantly increased (P<0.05) the AME value of diets, and improved lipid digestibility of the fat-containing treatment. The simultaneous combination of carbohydrase supplementation and barley irradiation proved to have an additive positive effect on feed quality and bird performance. For all treatments this combination improved the barley based diets to such an extent that it performed equally or significantly better (P<0.05) than its maize based counterpart. The irradiated barley-yellow grease based diets showed a more pronounced benefit with the addition of carbohydrase enzymes to the feed (P<0.05). / Dissertation (MSc(Agric))--University of Pretoria, 2008. / Animal and Wildlife Sciences / unrestricted