Alleviation effects of niacin supplementation on beef cattle subjected to heat stress: A metagenomic insight

The objective of this study was to investigate the alleviation effects of niacin supplementation on beef cattle subjected to heat stress and to provide a theoretical basis for exploring the alleviation methods of heat stress environmental factors on the rumen of beef cattle. In the experiment, 36 Jinjiang bull cattle with a body weight of about 400 ± 20.0 kg were randomly divided into three treatments, each treatment contains four replicates, with three cattle in each replicate. Treatments included thermoneutral treatment (TN; temperature: 24–25°C, humidity: 45–55%), heat stress treatment, exposure to environmental temperature (HS; average THI: 82.74), and heat stress supplemented with niacin treatment (HN; high temperature + 800 mg/kg NA). Measured indicators were body temperature, respiratory rate, production performances, rumen fermentations, and microbial diversity. Results showed that adding niacin reduced the body temperature and respiratory rate (P < 0.05) but had no significant effect on the production performances compared with heat-stressed beef cattle. HS treatment significantly increased body temperature and respiratory rate (P < 0.01), while decreasing the content of acetic acid, butyric acid, and total volatile fatty acids (P < 0.05) compared with the TN treatment. Supplement of niacin did not affect ruminal fermentation parameters (P > 0.05) but had a decreased tendency on A/P (P < 0.1). Microbial diversity results showed that, at the phylum level, the relative abundance of Desulfobacterota in the HS treatment was increased compared with TN and HN treatment (P < 0.05). At the genus level, the relative abundance of Succiniclasticum and Family_XIII_AD3011 group in the HN treatment significantly proliferated compared with the HS treatment (P < 0.05). In conclusion, niacin supplementation may alleviate heat stress by proliferating those bacteria belonging to the phylum Succiniclasticum, which may further contribute to the digestion of cellulose and the improvement of the metabolic function of Jinjiang cattle under heat-stress conditions.