This experiment was planned to investigate the efficiency of dietary crude protein (CP) levels (normal, medium [90% of the normal CP level] and low [80% of the normal CP level]) and betaine (Bet; 0 and 1 g/kg of diet) in broilers under heat stress (HS) conditions. A total of 672 one-day-old male broiler chickens were randomly assigned to 7 treatments (4 pens/treatment with 24 birds/pen). All experimental diets were supplemented with synthetic feed-grade lysine, methionine, and threonine at levels sufficient to meet dietary requirements. The positive control (PC) chickens were housed in a thermoneutral chamber and fed with a basal diet (normal diet without Bet supplementation). The other 6 groups were kept in a HS chamber (34°C) for 8 h (9:00–17:00). For the overall period of rearing (0–42 D), the HS control group (normal CP diet with no additive) exhibited lower (P < 0.001 or 0.05) average daily gain (ADG), European performance index (EPI), serum high-density lipoprotein cholesterol level, and primary antibody titers against Newcastle disease virus (NDV), but greater feed conversion ratio (FCR), mortality rate, serum glucose, cholesterol, low-density lipoprotein cholesterol levels, and heterophil to lymphocyte ratio (H/L) than those of the PC group. A decrease in dietary CP levels was accompanied by a decline in growth performance and the H/L ratio. Dietary Bet supplementation improved (P < 0.05) ADG, EPI, FCR, and primary antibody titers against NDV and infectious bronchitis virus in heat-stressed broilers, regardless of the CP levels in the diet. The two-way interaction effects of CP × Bet on all measured parameters were not significant. The present findings indicate that while reducing the CP content of the diet impairs broiler growth performance, dietary supplementation with Bet could not remove the unfavorable impacts of these kinds of diets. However, Bet improved growth performance and humoral immunity in heat-stressed broilers.
