Abstract :
[en] Body weight (BW) variation within broiler flocks is a persistent challenge in poultry production, with implications for economic efficiency, animal welfare, and sustainability. Despite genetic uniformity and standardized management practices, significant growth differences often emerge, and the biological factors driving these disparities remain insufficiently understood. This thesis aimed to identify the gut-related biological factors underlying BW divergence and to evaluate hatching and nutritional strategies to improve the performance of underperforming birds and reduce BW variability. We hypothesized that BW differences are driven by distinct gut microbiota and host physiological profiles, and that early-life interventions could enhance gut health and narrow performance gaps.
In the first part of this thesis, the role of gut microbiota in BW divergence was investigated by comparing low BW (LBW) and high BW (HBW) male Ross 308 chicks, classified on day 7 and followed until day 38. Cecal microbiota composition and predicted function, along with volatile fatty acid (VFA) profiles, were assessed on days 7, 14, and 38 using 16S rRNA sequencing, PICRUSt2 functional prediction, and gas chromatography. Microbial diversity and composition were strongly influenced by BW category. HBW broilers were enriched with VFA-producing taxa, including unclassified Lachnospiraceae, Alistipes, and Faecalibacterium, while LBW birds showed greater abundances of Lactobacillus, Akkermansia, and Escherichia-Shigella. HBW birds had higher acetate concentrations at day 14, whereas LBW birds showed higher isocaproate and isobutyrate levels at earlier and later stages. Predicted functional potential was greater in HBW microbiota, suggesting a more metabolically active microbial community. These results indicate that BW divergence is closely associated with differences in microbiota composition, metabolic potential, and VFA production patterns.
The second study in Part 1 built upon the first by shifting the focus from microbial factors to host-related mechanisms underlying BW divergence. Male Ross 308 chicks (n = 908) were ranked at day 7 into LBW and HBW groups and monitored for growth, visceral organ development, intestinal permeability, histomorphology, and ileal gene expression profiles at days 7, 14, and 38. A panel of 79 genes related to gut barrier integrity, immune function, nutrient transport, hormones, metabolism, and oxidation was quantified using high-throughput qPCR. HBW broilers remained heavier throughout the production cycle, primarily due to higher feed intake. They had shorter relative small intestine length but greater villus height and villus-to-crypt ratios, indicating superior absorptive capacity. LBW birds displayed increased intestinal permeability on day 38 and upregulation of immune-related genes such as TNF-α on day 7 and CYP450 on day 38, reflecting a potentially more inflammatory gut environment. In contrast, HBW birds upregulated genes associated with barrier function, nutrient transport, and oxidative metabolism, suggesting a more efficient intestinal physiology. Multivariate modelling (PLSR) identified sets of key genes at each age that accurately discriminated BW phenotypes, providing potential molecular biomarkers for early prediction of growth potential.
In the second part of the thesis, three targeted interventions were evaluated to improve the performance of broilers (underperforming) and reduce BW variability. The first intervention assessed the impact of on-farm hatching (HOF) on growth performance, intestinal development, barrier function, immunity, and gene expression. Male Ross 308 chicks hatched either in a hatchery or on-farm were monitored until day 38. HOF chicks had higher day 1 BW, but this advantage disappeared within the first week. Nonetheless, HOF birds exhibited enhanced intestinal morphology; wider duodenal villi, deeper ileal crypts, and greater submucosal thickness and higher relative bursal weight, suggesting improved immune organ development. Gene expression analysis revealed that HOF chicks upregulated immune-related genes (e.g., IL-8, IL-6, IFN-γ, AVBD9) and oxidative stress response genes (e.g., HIF1A), whereas HH chicks upregulated certain barrier and nutrient transporter genes. Although performance benefits were transient, HOF improved mucosal morphology and immune modulation, indicating potential long-term health advantages.
The second intervention tested whether in ovo injection of sodium butyrate (SB) could improve growth and gut health, particularly in chicks with low hatch weight (LHW). Ross 308 eggs were injected on incubation day 12 with saline or SB at 0.1%, 0.3%, or 0.5%. Post-hatch, chicks were classified as high or low hatch weight, creating a 4 × 2 factorial design. SB supplementation did not affect hatchability but significantly modulated growth, intestinal morphology, gene expression, and cecal microbiota, with effects varying by SB dose and hatch weight category. The 0.3% SB dose produced the most consistent benefits in LHW birds, leading to the highest final BW, upregulation of gut barrier genes (CLDN1, TJP1), anti-inflammatory cytokines (IL-10), and mucin (MUC6), along with improved microbiota diversity and enrichment of beneficial taxa. High HW birds generally performed better than low HW birds on control dose, but SB narrowed the performance gap between low and high HW broilers, particularly at the optimal dose.
The third intervention examined whether dietary structural components could improve the performance of LBW broilers and reduce BW disparity with HBW birds. At day 7, 1400 Ross 308 males were classified into LBW or HBW groups, with LBW birds receiving one of four diets: control (fine corn), coarse corn, oat hulls, or a combination of both. HBW birds received the control diet. By day 38, oat hull supplementation (3%) led to the greatest improvement in BW among LBW birds, significantly reducing the gap with HBW controls. Structural components improved gizzard development, intestinal morphology, and gene expression related to barrier integrity, nutrient transport, and immunity, while reducing cecal concentrations of certain branched-chain VFAs associated with protein fermentation. Microbiota shifts in LBW birds fed structural diets included increased beneficial taxa and reduced potential pathogens.
Collectively, this thesis shows that BW divergence in broilers is associated with distinct microbiota and host physiological profiles established early in life. Interventions such as optimal-dose in ovo sodium butyrate application for low hatch weight chicks and dietary oat hull inclusion for LBW broilers can improve gut health, narrow performance gaps, and enhance flock uniformity. These insights contribute to the development of precision nutrition and management strategies aimed at improving both flock performance and economic efficiency in commercial broiler production.
