Identification of biological factors that can be consistently linked to performance variation in modern commercial broiler flocks

Abstract

The intensification of the chicken meat industry over the past 50 years has resulted in a 400% increase in the growth rate of meat birds and a 50% reduction in feed conversion ratio, maintaining poultry as a cost-effective source of protein. Improvements have been a direct result of genetic selection for growth and feed efficiency (85-90%), advances in poultry nutrition and improved management practices. Despite production gains, performance variation remains both within and between broilers strains, which is a negative economic trait resulting in losses to producers and the industry alike. We therefore aimed to elucidate biological factors contributing to variations in growth and performance, particularly in meat birds. As growth has been repeatedly shown to be an immunological trade-off, the first study investigated whether functional changes in intestinal barrier function and innate immunity could be consistently linked to the phenotypic expression of feed conversion ratio (FCR) in meat birds. Genes in the small intestine were investigated between high- and low-performing phenotypes (selected on individual FCR), collected from three separate trials. There was no evidence linking flock performance variation with basal parameters of innate intestinal immunity in the ileum in this study. Higher variation in the expression levels of two genes, Toll-like Receptor 2 (TLR2) and membrane protein CD36 were of interest however, as both exhibit numerous overlapping and individual functions contributing to both innate immunity and fatty acid metabolism. A second study was conducted to investigate whether links between innate immunity and fatty acid metabolism could be contributing to variations in growth and performance. Total carcass fat %, carcass and blood lipid composition, key genes involved in fatty acid metabolism and selected innate immune parameters were investigated in meat birds, layer birds and F1 layer x meat bird crosses at d14 post hatch. The results indicated a total upregulation of fatty acid metabolism in meat birds when compared to the F1 cross and layer birds, for both fatty synthesis as well as β-oxidation in the liver, suggestive of altered metabolism. There was no evidence to suggest that any birds were exhibiting cellular hepatic stress or that fatty acid metabolism was interacting with parameters of innate immunity in this study. A third study used RNA-Seq to compare liver transcriptomes of meat birds, layer birds and their F1 cross. The objective was to identify differentially expressed (DE) genes between differing growth phenotypes to identify genes and biological pathways contributing to growth variations. Of the total genes identified, 155 were DE between all three groups. Transcriptional differences between the groups were large, particularly between meat birds and layers. Of the genes analysed, 19% were DE between meat birds and layers; 9.6% of genes DE between meat birds and cross; and 1.6% of genes DE between cross and layer birds. The most significant finding was the repeated enrichment of the FoxO signalling pathway, particularly genes related to cell cycle regulation and the insulin receptor. There was also a high correlation between FoxO pathway genes and bodyweight, as well as genes related glycolysis and bodyweight. In summary, this thesis explores several biological factors associated with growth and performance variation in commercial meat birds. The results indicated that intestinal barrier/innate immune function was not associated with the phenotypic expression of FCR nor was altered immune function detected with differential fatty acid metabolism between birds differing in growth potential. There was however significant evidence implicating the FoxO signalling pathway (via cell cycle regulation and altered metabolism) as an active driver of growth variations in chicken. We recommend further functional characterisation and analysis of this pathway, in meat birds in particular, to further characterise variations in growth and performance.