Dietary supplementation with natto fermented by Bacillus subtilis var. natto strain TTCC903 alleviates heat stress and improves egg production in aged laying hens

Heat stress is a major constraint on poultry production, particularly in aged laying hens with diminished thermoregulatory and metabolic capacity. Probiotics, including Bacillus species, have been proposed as dietary supplements to mitigate these effects. This study investigated whether supplementation with natto fermented by Bacillus subtilis var. natto strain TTCC903 alleviates heat stress and improves productivity in aged laying hens. Ambient conditions indicated mild to moderate heat stress throughout the trial. In controls, blood malondialdehyde (MDA) increased by 26 %, whereas natto-supplemented groups maintained stable or reduced levels, with the 0.05 % supplementation group showing a 29 % reduction. The hen-housed egg production rate (HHEP) in controls averaged 87.4 %, while the 0.05 % and 0.5 % supplementation groups averaged 89.5 % and 90.8 %, respectively (p < 0.05), with the 0.5 % group exceeding 92.8 % during weeks of high THI. Egg production expressed as g/hen/day tended to increase in natto-fed hens, and the feed conversion ratio improved significantly in the 0.5 % group (1.60 vs. 1.69 in controls). The defective egg rate was ≤0.5 % across all groups but showed a relative 50 % reduction in the 0.5 % group. Eggshell strength was preserved in natto-fed hens but declined in controls. Natto supplementation also reduced diarrhea incidence (4 % vs 9 %) and restored the ileal villus-to-crypt ratio (5.00 vs. 3.76 in controls). In conclusion, supplementation with natto fermented by B. subtilis var. natto strain TTCC903 mitigated oxidative and physiological responses to heat stress, improved egg production efficiency, and enhanced intestinal health in aged hens. These findings extend previously reported benefits of this strain in fish and ruminants to poultry, supporting its potential as a probiotic technology in the livestock industry.