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Abstract
Decreasing heat increment through lowering crude protein (CP) level via supplemental amino acids (AA) have long been suggested as a nutritional practice to alleviate negative heat stress effects on broiler performance. However, there is a considerable body of inconclusive reports on optimisation of such practices, whereby bird performance remains unchanged. The exact mechanism underlying the impaired growth performance in birds fed with reduced-CP diets is not clear yet. Furthermore, adding the environmental temperature factor to the situation may complicate the solution. To date, there is no agreement on the extent of reducing CP level via AA supplementation so that growth performance remains unaffected. Evidence suggests that dietary CP could be reduced safely by 2.3% via essential AA supplementation during later ages, when birds are exposed to an average daily ambient temperature of ≤27.3°C. When Gly was added (a non-essential AA source), the margin of CP reduction could be increased to 5.1% without compromising the growth of broilers subjected to cyclic heat stress. Nonetheless, feeding broilers with a similar Gly-fortified, reduced protein diet failed to support optimal performance under hot and humid tropical climates in 1-21-d-old broilers and had a major impact on growth in broilers reared at 34°C. Regardless of supplemental AA composition or the level of CP reduction, the performance of broilers was negatively affected when birds were subjected to chronic heat stress conditions (≥30°C). These discrepancies can be attributed to a wide range of confounding factors, such as the extent of lowering CP level, types of AA used, age and environmental conditions. Accordingly, the addition of Gly may represent a good approach for reducing dietary CP levels for broilers raised under elevated ambient temperature. Reducing dietary CP is recommended when birds are exposed to moderate but not chronic heat stress conditions.