Sensitivity of ideal protein nutrition to gut health in broiler chickens

Broilers have high dietary requirements for protein as a major constituent of the biologically active compounds in the body and a vital nutrient for growth. In addition, protein and essential amino acids are essential for antibody formation and a well-functioning immune system, however protein is one of the most expensive nutrients in poultry diets. Therefore, considerable attention is being given to optimise (minimise) crude protein (CP) levels in poultry diets, including by applying the ideal protein concept (IP), which represents essential amino acid (AA) requirements as a ratio to lysine leading to an ideal dietary AA profile. However, since AA supplementation has been shown to improve resilience to sub-clinical pathogen exposure, the assumed generality of the IP concept may be challenged. The aim of this project was to investigate the effect of AA supplementation on resilience of broiler chickens to sub-clinical health challenges using IP-based rationing as the starting point. The first objective was to assess the sensitivity of broiler resilience to reused litter exposure and to supplementation of IP-based control diets with individual or a combination of threonine, arginine and glutamine (TAG) on performance, plasma biochemistry and organs weights. In the first study, TAG was included at the expense of corn starch to maintain iso-energetic conditions. Host responses to specific AA supplementation may be sensitive to microbiota exposure from reused litter, as immune and pathological responses could result in increased whole bird AA requirements that deviate from the ideal AA profile. Since the control diet was formulated on an IP-basis, it was hypothesised that AA supplementation improves performance in the presence but not in the absence of exposure to reused litter. Our results showed that reused litter exposure resulted in greater weight gain, better feed conversion ratio and heavier bursal weights compared to birds on clean litter. However, TAG birds reduced weight gain and feed intake relative to control birds. Since reused litter benefitted performance and the control diet was IP formulated, the absence of positive responses to AA supplementation would be consistent with AA tested being in excess and resulting in a cost rather than a benefit on growth performance and feed efficiency. To test this hypothesis, we assessed whether TAG supplementation leads to excess protein associated changes in caecal microbiome profiling and metabolites and litter characteristics of broilers exposed to reused litter. Our results confirmed that the impaired growth of birds fed TAG diets compared to control diets may have arisen from enhanced proteolytic fermentation, supported by elevated caecal iso-butyric acids levels and lowered abundance of bacteria known to dominate in well-performing birds. Overall, exposure to reused litter, particularly when it has no detectable levels of pathogenic bacteria, might be a source of recycled nutrients and exposure to direct fed microbials, which can benefit bird performance. The second objective was to investigate effects of TAG supplementation to IP-formulated low CP diets at the expense of glycine to maintain both iso energetic and iso-nitrogenous diets on resilience of broilers to a combined exposure to reused litter at placement and challenge with E. maxima at day 14. Growth performance at appropriate stages over the whole course of the trial was measured. In addition, organ weights and plasma biochemistry were measured at three time points, i.e. when birds had been exposed to reused litter only (10 days of age), at the acute phase following E. maxima challenge (20 days of age) and at the recovery phase (27 days of age). The low CP diets reduced bird performance compared to birds fed relatively high CP control diet (Chapter 4 and Chapter 5), which might have resulted from dietary electrolyte imbalance and limiting non-essential AA other than glycine. This further indicated a reduced ability to successfully apply the IP concept, particularly during the starter phase. In agreement with the first trial, placement on reused litter increased body weight. Though superimposing birds with sub-clinical coccidiosis reduced bird performance during the acute phase of the challenge, TAG supplementation to the low CP diets did not improve resilience. We further investigated the effect of TAG supplementation to low CP diets on apparent ileal nutrient digestibility, digestible nutrient intake and litter pH and moisture content. Our results showed that exposing birds to reused litter and sub-clinical coccidiosis did not impact digestibility measured at 10 and 27 days of age. On the other hand, TAG supplementation to low CP diets reduced apparent ileal CP digestibility, which might indicate that this specific AA supplementation caused AA imbalance, reduced AA availability and excess protein. Litter pH and moisture content for birds fed low CP diets were lower than those fed control diets, which confirms that lowering dietary CP could be a tool to reduce nitrogen excretion and ammonia emissions from poultry houses. The third objective was to test the effects of storage duration on microbial and physicochemical properties of reused litter to provide a possible basis to the perhaps unexpected beneficial growth response observed upon reused litter exposure, which deviated from previous studies. Litter samples, weekly collected over 28 days, showed a reduced litter pH and moisture content on day 28 compared to day 0. In addition, the major pathogen of concern in poultry, such as Salmonella spp, Clostridium perfringens, E. tenella and E. maxima, were not detected in our litter samples. The average copy number of 16S rRNA gene of total bacteria and FimH gene copy numbers of avian pathogenic E. coli reduced from day 0 to day 21 though then slightly increased at day 28. The average copy number of 18S rRNA of total fungi decreased from day 0 to day 28. These results confirmed that reused poultry litter showed variation in physicochemical properties and pathogen load over time, and may in part explain the variability in growth performance observed between studies when broilers are exposed to reused litter. In conclusion, TAG supplementation to IP-formulated standard or lowered protein diets did not improve resilience to sub-clinical enteric challenge but reduced bird performance. This reduced performance may have been due to reduced ileal digestibility of AA, resulting in increased excess protein and thus enhanced proteolytic fermentation. Lowering dietary CP while maintaining the IP concept and glycine requirements reduced bird performance, which could indicate the inability to apply the IP concept, particularly during the starter phase. Exposing birds at placement to reused litter in which Salmonella spp., C. perfringens and Eimeria spp. were below detection level, improved bird performance, indicating that reused litter might be a source of recycled nutrients and direct-fed microbiome rather than a source of pathogen challenge. Therefore, before employing reused litter exposure, detailed litter characterisation is required to optimise study design, including origin description, duration and storage condition between collection and usage.