Increased stocking densities are frequently reported to depress chicken growth performance, but the mechanisms behind this are not fully understood. This study was conducted to investigate the effects of stocking density on growth performance and digestive microbiota, known to be sensitive to environmental factors. Chickens were reared at 2 stocking densities, 12 or 17 birds/m(2). Growth performance was recorded between d 1 and 39, and litter was scored for quality on d 25, 31, and 37. Digestive microbiota was analyzed along the digestive tract (crop, ileum, ceca) of 3- and 6-wk-old chickens by using 2 molecular approaches: a qualitative method (fingerprinting by temporal temperature gradient gel electrophoresis) and a quantitative method (real-time PCR). An increase in stocking density was found to negatively affect the feed conversion ratio (+3.1%) and depress the daily BW gain of broilers (-5.5%) during the period from d 32 to 39 (P ≤ 0.05). Litter quality was reduced with the high stocking density as early as d 25. At 3 wk of age, stocking density strongly affected the fingerprint profiles of the bacterial community, with the highest modifications observed in the crop and ceca (R analysis of similarity = 0.77 and 0.69, respectively, P ≤ 0.05). At 6 wk of age, significant differences in the fingerprint profiles between the stocking densities appeared in the crop and ceca (R analysis of similarity = 0.52 and 0.27, respectively, P ≤ 0.05). The abundance of bacterial groups targeted by real-time PCR was affected by stocking density, but only to a limited extent. Because digestive microbiota may have consequences on the physiology of the digestive tract, its modification by an increase in stocking density may be involved in the reduced growth performance of the bird.

Abstract. Among the terrestrial production animals, chickens are the most efficient users of energy. Apparent metabolisable energy (AME) is a measure of energy utilisation efficiency representing the difference between energy consumed and energy lost via the excreta. There are significant differences in the energy utilisation capability of individual birds that have a similar genetic background and are raised under identical conditions. It would be of benefit to poultry producers if the basis of these differences could be understood and the differences minimised. We analysed duodenal gene expression and microbiota differences in birds with different energy utilisation efficiencies. Using microarray analysis, significant differences were found in duodenal gene expression between high-and low-AME birds, indicating that level of cell turnover may distinguish different groups of birds. High-throughput sequencing of bacterial 16S rRNA genes indicated that duodenal microbiota was dominated by Lactobacillus species and two operational taxonomic units, identified as lactobacilli species, were found to be more abundant (P < 0.05) in low-AME birds. The present study has identified gene expression and microbiota properties that correlate with differences in AME; further studies will be required to investigate the causal relationships.

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