**2.1. Bird growth performance and dietary available energy**

It has been hypothesised that PE additives may stabilise overall digestive functions in the gastrointestinal tract of poultry, however, the available literature does not provide a consistent picture. Numerous feeding trials involving dietary supplementation with various PE have been reviewed [1, 6]. Regarding growth performance (assessed primarily as feed efficiency) and nutrient digestibility, the effect of added PE was beneficial in 11 of the studies; no effects were observed in 17 and there was a detrimental effect in 18 of the reviewed studies.

The information on the effect of PE on dietary metabolisable energy (ME) is also inconsistent. Some authors found an increase in dietary ME in response to PE [8, 9], others [10, 11] did not. There is also a discrepancy in the published data in the differences in ME and growth performance of birds fed dietary PE. Recent studies [10, 11, 8] found an improvement in bird growth performance but not in dietary ME when various PE were fed to poultry. It has been reported [9] that there is a parallel improvement in dietary ME and feed efficiency when feeding a standardised commercial mix of carvacrol, cinnamaldehyde and capsaicin to broiler chickens.
