8.The results of previous studies o

8.
The results of previous studies of the effect of essential oils or enzymes on growth performance in piglets have been inconsistent, showing either improvement (Fan et al., 2009; Maenner et al., 2011) or no effect (Muhl and Liebert, 2007; Jiang et al., 2015). These contradictory results may be explained by the compositions of the additives, the dietary doses being tested, the route of administration, or the different experimental approaches used to test the effects of the substances (Windisch et al., 2008). In our study, XB or EOXB piglets under an E. coli challenge did show an improvement in feeding efficiency during the last week although no effect of diet on the gain to feed ratio was observed during the overall study period. The results suggest that XB or EOXB may allow piglets to better respond to stress in the post-weaning period when performance is most likely to be impaired. However, no significant improvement in growth performance with EO and XB alone or in combination was observed in this study, which may be due to the low doses of EO and XB used. Fan et al. (2009) observed improved growth performance in piglets using a 1.8 g/kg enzyme complex containing 10,000 U/g