The present study showed that the addition of xylanase to wheat-based diets significantly
improved body weight gain and feed conversion efficiency of broilers from 7 to 49 days
in accordance with previous results (Brenes et al., 1993; Choct et al., 1999; Wang et al.,
2005). Responses to enzyme supplementation depend on the bird’s age, which is apparently
related to both the type of gut microflora present and the physiology of the bird. Old birds,
because of the enhanced fermentation capacity of the microflora in their intestines, have a
greater capacity to deal with the effects of high viscosity (Choct et al., 1996; Vukic-Vranjes
andWenk, 1995). The improvement of enzymes on growth was more pronounced from 7 to
21 days than from 22 to 49 days (13% vs. 5%). However, chicken grew less than expected
for modern lines and feed conversion ratio was higher than expected. This may be caused
mainly by the lower body weight day-old chickens that is one of the most important factors
affecting chickens’ growth, but it is regretful that we have not weighed them (the trial started