A 42 d-trial was conducted to investigate the effects of mannan-oligosaccharide (MOS) and betaine (BT), on the hemagglutinin antibody response against sheep red blood cells (SRBC) and Newcastle disease virus (NDV), as well as a cell-mediated response to phytohemagglutinin-P (PHA-P) in broiler chickens reared under heat stress conditions. A total of 288 one-day-old male broiler chickens were randomly divided into 24 experimental units. Birds were offered either a corn-soybean meal basal diet (control) or the basal diet supplemented with 1 g/kg MOS (MOS diet), 1 g/kg BT (BT diet), or 1 g/kg MOS and 1 g/kg BT (MOS+BT). From day 21, the birds were exposed to heat stress (33±1 °C) until the end of the study (day 42). The relative humidity was kept at 55%in all groups throughout the trial. The results showed that at 35 d, the toe web swelling in response to PHA-P injection in the birds fed diets BT or MOS+BT was significantly higher (P<0.05) than in the birds fed the control diet. The secondary antibody titers against SRBC and NDV in the birds fed diets BT and MOS+BT were also significantly higher (P<0.05) than in birds fed the control diet, where the birds receiving the MOS+BT diet had the highest secondary antibody titers. However, primary antibody responses against SRBC and NDV were not affected by dietary treatments (P>0.05). In conclusion, the addition of BT, particularly in combination with MOS, has the potential to enhance immune function in broilers exposed to heat stress.
