Effects of heat stress and depressed feed intake on the cecal and ileal microbiota of broiler chickens.

Chronic heat stress (HS) is known to depress feed intake (FI) and growth in broiler chickens, and may alter the gut microbiota and health, however, the amplitude and duration of each (HS and depressed FI) effect on gut microbiota is still unclear. This study aimed to determine the effects of chronic and acute HS on the cecal and ileal microbiota profiles. Broilers were subjected to thermoneutral (TN, 23°C) or chronic cyclic HS (CHS, 8 h/d, 35°C) condition from d29-42, and one group to acute HS (AHS, 2 h, 35°C) at d42. To distinguish between the effect of HS and FI, one pair-fed (PF) group (maintained at TN condition but had similar FI as CHS birds) was used from d29-42. Chronic HS decreased richness in the cecal microbiota compared to TN, while PF did not, and AHS led to higher richness compared to CHS. Acute HS and PF altered cecal microbiota profiles similarly to CHS under weighted UniFrac. In the ileum, PF altered microbiota under unweighted UniFrac, and CHS further altered microbiota. Decreased Lactobacillus and increased Ligilactobacillus were observed with CHS, AHS, and PF in the cecum. Chronic HS may influence different predicted metabolic pathways compared to PF, while AHS shared some similarities with CHS. Understanding both the direct effects of CHS and indirect effects through depressed FI, along with the short-term effects from AHS, will be of importance in devising nutritional strategies to maintain gut microbial balance and limit negative effects to broiler chickens' performance and health.