Four experiments were conducted to examine the growth promoting effect of different levels of autolyzed whole yeast (WY), yeast cell wall (YCW) and enzymatically hydrolyzed cell wall components, yeast α-mannan (YM) and yeast β-glucan (YG), in broiler chickens. The mechanisms underlying these effects in broilers were determined. The potential of these yeast products as possible alternatives to in-feed sub-therapeutic antibiotics was determined by measuring their effects on broiler performance, meat yield and immune-regulation under mild stress. In the first experiment, lower levels of supplementation of autolyzed WY and YCW (0.5 and 1.0 g/kg) did not have a significant effect on performance, visceral organ weight, digestibility and intestinal enzyme activities in experiments one. However, WY and YCW at 1.5 or 2.0 g/kg had marked improvement in bird performance response for both whole yeast and yeast cell walls.
In experiment two, feeding lower levels of YM (0.05 and 0.10 g/kg diet) to unchallenged did not have significant effect on the parameters considered. However, YM supplemented at 0.15 and 0.20 g/kg diet were superior to the control group in the majority of the parameters recorded (gross performance, protein digestibility and meat yield). Birds fed diets containing YG did not have in effect in most of the parameters considered.
In experiment 3, the effect of autolyzed WY (2.0 g/kg diet), YCW (2.0 g/kg diet), yeast mannan (0.20 g/kg diet), YG (0.20 g/kg diet) and zinc bacitracin (50 ppm diet), Salinomycin (60 ppm diet) was compared with a control group (without supplementation) in broilers under mild stress of Salmonella lipopolysaccharide challenge (LPS). The LPS challenge increased bird rectal temperature and immune response (haematological and serum metabolite), with a resultant decrease in performance, meat yield, flock uniformity and spleen weight. However, supplementation with autolyzed WY, YCW, YM, YG or antibiotics (Salinomycin and zinc bacitracin) improved performance, flock uniformity and meat yield in challenged birds. These improvements were associated to ability of these additives to reduce the level of LPS-induced immune responses. These additives, possibly through different mechanisms, improved most parameters (performance, flock uniformity, haematological indices, plasma metabolites and meat yield) considered.
In a further trial, YCW and zinc bacitracin when supplemented to the diets of birds unchallenged or challenged with Eimeria and Clostridium perfringens showed the growth enhancing and challenge-ameliorating effects. In unchallenged broilers, supplementation of yeast cell wall and bacitracin tended to improve performance relative to the control group. Eimeria and C. perfringens resulted in poorer feed intake, body weight gain, FCR, meat yield and flock uniformity. The challenge also resulted in a higher number of intestinal lesions. There was no effect of the challenge or dietary treatments on the pH of duodenal, ileal, jejunal and caecal digesta. Challenge resulted to an increase in C. perfringens count. The Lactobacillus and the Bifidobacteria caeca count decreased in CP challenged birds. Both YCW and zinc bacitracin supplementation decreased C. perfringens counts and increased Lactobacillus and Bifidobacteria counts. This may be associated to the better performance, meat yield, flock uniformity and lesion score observed in the study. Challenge or supplementation did not significantly influence the mortality of birds.
The results of these studies provided evidence that autolyzed WY (at 1.5-2.0 g/kg diet), YCW (at 1.5-2.0 g/kg diet) and YM (at 0.15-0.20 g/kg diet) exhibited some growth enhancing effects. Furthermore, WY and YCW (both included at 2.0 g/kg diet) as well as YM and YG (both included at 0.20 g/kg diet) showed immunomodulatory controlling abilities which led to improved growth performance in both mildly stressed and disease challenged broiler chickens.