Impact of feed composition on rearing of broiler chickens without in-feed antimicrobials

In-feed antimicrobials with antibiotic properties have been used for disease-preventive and performance-promoting purposes in rearing of broiler chickens since the mid-twentieth century. The use of antibiotic agents in subtherapeutic doses to promote growth has later been banned in several countries. However, ionophorous polyether antibiotics (‘ionophores’) with anticoccidial and antibacterial properties are still extensively used in broiler production in most countries, except Norway, to control the key production-limiting diseases coccidiosis and Clostridium perfringens-associated necrotic enteritis. Antimicrobial resistance is a serious threat to global public health, and the increasingly higher occurrence of antibiotic resistant pathogens requires urgent action and prudent use of antibiotics. Increased consumer awareness and criticism from other stakeholders, novel therapeutic indications of ionophores and a possible association between ionophores and antimicrobial resistance to antibiotics important in human medicine, may lead to future restrictions of the prophylactic use of ionophores in more countries. The aim of this thesis was to study and assess specific dietary manipulations in broiler chickens reared in ‘antimicrobial-free’ production systems, meaning that no substances with antibiotic properties were added to the feed. Effects of adding non-antibiotic feed additives or changing the dietary ratio of starch to fat on intestinal health and production performance in Ross 308 broilers were investigated by the use of an experimental animal model simulating commercial broiler house conditions. The first hypothesis of this work was that diets supplied with non-antibiotic feed additive products improve production performance and/or reduce intestinal C. perfringens counts compared with a diet with no feed additives. Twenty commercially available feed additives categorized as prebiotics, probiotics, phytogenics and/or organic acid-based products, and four combinations of such products, were evaluated based on their effect on caecal C. perfringens counts and production performance. Probiotic feed additives with Bacillus subtilis strains improved production results in a time interval of the study comprising infection with coccidia, and one of these products concurrently reduced C. perfringens counts. Prebiotic feed additives, all based on non-living components from the yeast Saccharomyces cerevisiae, improved production performance and particularly feed conversion ratio in the rearing interval with coccidia infection. The heterogeneous group of feed additives with phytogenic compounds showed variable results. A product based on oleoresins from turmeric and chilli peppers improved overall feed conversion and reduced C. perfringens counts, and two products containing essential oils improved production performance in the time interval comprising infection with coccidia. Several organic acid-based feed additives containing short- and/or medium-chain fatty acids or derivatives thereof improved overall production performance, and demonstrated superior production performance results during the first two weeks post-hatch. In general, the study of non-antibiotic feed additives provided comparable and unbiased results from testing of a representative selection of commercially available products and product combinations in a uniform experimental model relevant to commercial rearing of broilers. The different non-antibiotic feed additives affected production performance and intestinal C. perfringens counts with varying degrees of success, and some feed additives had beneficial impact only during distinct rearing phases and on specific performance targets. The use of optimized combinations of different active components with synergistic effects, and targeted use of feed additives in specific phases of the production cycle, are promising strategies in antimicrobial-free broiler rearing systems. The second hypothesis was that a high dietary starch to fat ratio impairs starch digestibility, production performance and intestinal health in broilers challenged with coccidia. Two antimicrobial-free diets with similar energy and protein content, but with different inclusion level of starch and fat, were evaluated for their effect on nutrient digestibility, production performance, small intestinal morphology, C. perfringens counts and toxin profile, necrotic enteritis prevalence and caecal short-chain fatty acid (SCFA) abundances in coccidia-challenged broilers. Results showed that the diet with the highest starch inclusion level augmented starch digestibility, led to improved feed conversion and did not have an impact on intestinal C. perfringens counts, toxin profile or necrotic enteritis prevalence. Thus, the hypothesis that a higher dietary starch to fat ratio would impair intestinal health and production performance was not verified. A rejection of the hypothesis needs to take two possibly confounding factors of the high starch to fat ratio diet into consideration: The degree of starch gelatinization and the use of isolated wheat starch. In chickens fed the high starch to fat ratio diet, total caecal SCFA level was greater prior to infection with coccidia and small intestinal villi were longer after infection with coccidia. The study of dietary starch to fat ratios suggests that broiler chickens have the capacity to adapt to different diet compositions by structural remodelling of the small intestine, and that chickens adapted to higher levels of dietary starch might be more resilient to Eimeria spp. infections as a consequence of increased mucosal surface area. Possible future restrictions of the use of ionophores in the global broiler industry call for more knowledge of the impact of feed composition on development of intestinal disease and production performance. Results from this thesis suggest that some non-antibiotic feed additives have disease-preventive and performance-enhancing effects. Further, diets with a high starch to fat ratio do not seem to predispose for intestinal disease if the dietary starch is readily available and easily digested