Testing low dietary crude protein and high fat levels as a strategy to mitigate heat stress in broilers.

BACKGROUND: Fast-growing broilers are poorly adapted to heat. Adjusting feed composition may mitigate heat stress (HS) effects in temperate climates, while maintaining performance and health during cooler days. METHODS: One thousand nine hundred and twenty Ross 308 male broilers were housed in 64 pens in 4 climate-controlled rooms, 2 under cyclical HS (d 28-43; 32 ± 2 °C; 60%-70% RH; 09:30-15:30) and 2 under thermoneutral (TN) conditions. In the finisher phase, broilers were allocated to 4 dietary treatments, analyzed values are given except for metabolizable energy (ME): low crude protein (CP) and control fat (LowCP-ConF; 17.0% CP, 5.9% crude fat (CF), 2,925 kcal/kg ME), low CP and high fat (LowCP-HighF; 17.2% CP, 7.9% CF, 3,019 kcal/kg ME), control CP and high fat (ConCP-HighF; 18.1% CP, 8.0% CF, 2,992 kcal/kg ME) and a basal control (ConCP-ConF; 18.7% CP, 6.3% CF, 2,913 kcal/kg ME). LowCP diets contained control levels of digestible amino acids. RESULTS: During the finisher phase, compared to control CP levels, LowCP increased average daily feed intake (ADFI) (+ 2.15%; P = 0.020) and affected average daily gain (ADG) and feed conversion ratio (FCR) negatively under TN (-3.77% and +6.49%; P = 0.003 and P < 0.001, respectively), but not during HS. Compared to control CF, HighF decreased ADFI during TN and HS (-3.16% and -3.17%; P < 0.001 and P = 0.022) and reduced ADG in TN groups (-3.17%; P = 0.010), but not during HS. Mortality was higher in broilers receiving HighF during HS (P = 0.040). Slaughter weights were unaffected. LowCP decreased plasma uric acid and lactate dehydrogenase levels during TN, but increased plasma glucose during HS. LowCP increased breast meat redness (a*) during TN and HS (P < 0.05). HighF decreased fat (-1.68%; P = 0.017), but increased protein levels (+1.53%; P < 0.001) in breast meat of HS-broilers. CONCLUSION: LowCP and HighF impaired performance under TN but not under HS. HighF increased mortality under HS, yet improved breast meat composition. These findings highlight the challenge of designing an optimal diet for both conditions and underscore the need to better understand amino acid needs and energy-to-protein ratios during HS.