Gut function and related parameters in densely stocked broiler chickens : responses to a novel Bacillus probiotic mixture.

Abstract

Poultry meat is one of the most affordable sources of animal protein and other micronutrients, whose demand is driven by the increasing human population. In response, producers have intensified poultry production systems to meet this demand. Antibiotic growth promoters (AGP) have been extensively used to control disease outbreaks and increase productivity in the poultry industry. However, their use has raised concerns regarding human and environmental health. Therefore, it is important to investigate appropriate dosages and the effectiveness of alternative nutritional strategies, such as probiotics, to replace AGP in poultry feed formulations. Accordingly, this study evaluated the impact of incremental dosages of a Bacillus subtilis probiotic mixture as a replacement for AGP in densely stocked broiler chickens. The probiotic mixture contained B. subtilis strains CPB 003, CPB 029, HP 1.6, and D 014 in equal proportions, resulting in a final concentration of 1 × 10⁹ cfu/g of product. The study was split into two experimental chapters, 3 and 4. Chapter 3 reports the effects of B. subtilis probiotic mixture on growth performance, haemato-clinical biochemistry, carcass traits, gastrointestinal tract (GIT) and accessory GIT visceral morphometry, and meat quality. In Chapter 4, the effects of these probiotics on gut histomorphology and the phylogenetic composition of the gut microbiota of broiler chickens are reported. The feeding trial was conducted at Rooigrond farm in the North West province, South Africa. In a completely randomized design, 900 one-day old Ross 308 broiler chicks (43.21±0.372 g live weights) were distributed evenly into 36 pens (0.75 m² each; 13 birds per pen), to which six experimental diets were randomly assigned. The experimental diets, provided from the starter through to the finisher phase, comprised: 1. a basal broiler diet without AGP or B. subtilis probiotic mixture (PRB0); 2. the basal diet supplemented with 0.5% (w/w) zinc bacitracin (POSC); and four versions of the basal diet supplemented with B. subtilis probiotic mixture at 0.3 (PRB3), 0.4 (PRB4), 0.5 (PRB5), or 0.6% (PRB6). Feed intake and body weight gain were measured weekly, blood was collected two days prior to slaughter for haemato-biochemical analyses, and meat quality attributes and nutrient digestibility were measured post-slaughter. A quadratic effect was noted in the feed intake during the starter phase. The 0.5% inclusion rate improved body weight gain and feed efficiency, whereas the 0.3% increased feed intake and reduced body weight gain. Diets supplemented with B. subtilis mixture significantly improved (p < 0.05) digestibility of organic matter, neutral detergent fibre, and gross energy, while a negative quadratic effect was noted on crude protein digestibility. Changes in haemato-biochemical parameters, including increased neutrophils and white blood cells, indicate reduced stress and inflammation in broiler chickens fed diets containing 0.3%, 0.4%, and 0.5% of B. subtilis probiotic blend. Carcass traits were statistically unaffected (p > 0.05). The relative duodenal weight and jejunal length were significantly different (p < 0.05), with PRB0 and POSC promoting the highest weight, and PRB6 promoting the longest jejunum. In Chapter 4, significant differences (p < 0.05) were observed in jejunal villi absorption surface area and PERMANOVA on microbial diversity among the dietary treatments. However, no dietary effects were noted on the alpha diversity of caecal microbiota. Taxonomic profiling of the caecal digesta revealed a microbial community structure dominated at the phylum taxonomic rank by Verrucomicrobiota, Bacillota, and Asgardarchaeota. At the genus rank, Akkermansia, SOKP01, Exiguobacterium A, and Rhizomicrobium emerged as the most abundant across dietary treatments. Beneficial microbes were most dominant in caecal digesta from chickens fed PRB5, indicating a healthy gut, which may explain the improved performance metrics reported in Chapter 3. In contrast, broiler chickens on the PRB6 diet carried more unclassified and scientifically unknown gut microbial populations not found in other diets, which may have contributed to the poor performance reported in Chapter 3. The results indicate that the B. subtilis probiotic mixture enhanced growth performance and nutrient digestibility, alleviated stress and inflammation, improved jejunal absorptive surface area, and promoted beneficial gut microbes in broiler chickens reared under high stocking density conditions. Therefore, it is recommended that the B. subtilis probiotic mixture be supplemented in broiler diets at a concentration of 0.5% (w/w).