Alternatives to In-Feed Antibiotics: Effects on Broiler Performance and Gut Health

Abstract

<p>This thesis examined the efficacy of several alternative feed additives in enhancing performance and enteric health in broilers in the absence of antibiotics. Chapter 1 provides background information, leading to the objectives of conducting the experimental studies. This is followed by a review of literature in Chapter 2, covering the general characteristics of the gastrointestinal tract, gut microflora and the factors affecting gut health. The review also covers enteric diseases, namely necrotic enteritis, factors predisposing birds to necrotic enteritis and methods of controlling it.</p> <p>Chapter 3 evaluates the efficacy of yeast cell wall extract derived from Saccharomyces cerevisiae as a replacement for zinc bacitracin and the anticoccidial ionophore salinomycin, using an established necrotic enteritis challenge model. The results showed that weight gain, feed intake and livability of challenged birds were lower than for unchallenged birds on d 24 and 35 (P < 0.05). All of the additives had a greater positive impact on weight gain, feed intake and livability in challenged compared to unchallenged birds. Birds given zinc bacitracin, yeast cell wall extract or salinomycin significantly improved in weight gain and livability when compared to control birds given no additives. Challenged birds fed yeast cell wall extract exhibited decreased crypt depth, increased villus height and increased villus to crypt ratio.</p> <p>Chapter 4 investigates whether acetylated high amylose maize starch or butyralated high amylose maize starch play a positive role in digestion in broiler chickens and particularly in ameliorating the severity of necrotic enteritis in broilers under experimental disease challenge. The results showed that on d 24 and 35, all challenged birds had lower (P < 0.001) livability, weight gain and feed intake compared to unchallenged birds. Birds fed acetylated high amylose maize starch and butyralated high amylose maize starch diets had higher (P < 0.001) weight gain and feed intake compared with those fed the control diet. Birds fed acetylated or butyralated high amylose maize starch diets had poorer (P < 0.002) feed conversion ratio at d 35. Butyralated high amylose maize starch increased jejunal villus to crypt ratios, ileal and caecal butyrate levels and decreased caecal pH. Acetylated high amylose maize starch significantly improved ileal acetate content and decreased caecal pH.</p> <p>Chapter 5 determines the efficacy of acylated starches on performance, heat production and energy efficiency of broiler chickens during an induced outbreak of necrotic enteritis. The data demonstrate that Eimeria sp. and Clostridium perfringens challenge reduces growth performance, heat production, respiratory quotient, heat increment, metabolisable energy and metabolisable energy intake of birds fed the control, acetylated or butyralated high amylose maize starch. However, birds fed antibiotics exhibited nearly total resistance to the challenge with necrotic enteritis.</p> <p>Chapter 6 describes the effect of encapsulated sodium butyrate on growth performance and gut health in broilers fed wheat or corn based diets, at normal or high protein levels and normal or low energy levels. The inclusion of encapsulated sodium butyrate at 1 g/kg had no effect on growth performance, ileal and caecal pH and levels of SCFAs, but increasing the inclusion rate to 2 g/kg improved bird performance.</p> <p>Chapter 7 discusses the major findings on the effects of some feed additives, namely microencapsulated sodium butyrate, yeast cell wall extract and acylated starches, on broiler chickens production and gut health, and the role of yeast cell wall extract and acylated starches in ameliorating the severity of necrotic enteritis in broilers under experimental disease challenge. In conclusion, necrotic enteritis is characterized by necrosis and inflammation of the gastrointestinal tract with a significant decline in growth performance. However, yeast cell wall extract and acylated starches can be used as tools for reducing the severity of necrotic enteritis outbreaks.</p> <p>The series of experiments reported in this thesis suggests that the yeast cell wall extract consists mainly of mannano-oligosaccharides, butyralated starch and acylated starch products are able to partially ameliorate the impact of necrotic enteritis in broiler chickens by improving gut health, but are unable to replace antibiotics in terms of effectiveness against severe challenges. Further studies are required to investigate the modes of action of these feed additives in improving gut health and reducing the severity of necrotic enteritis.</p>