The herbs and some compound in them could act similar to antibiotics. This compound reduced the growth
of some harmful bacteria in the gastrointestinal tract of broilers. This can results in a higher efficiency in the
feed utilization, and it can leads to a higher weight gain and better feed efficiency (Bedford, 2000).The effect
of onion on broiler performance probably is due to antibacterial and antifungal effects originated from some
of its compounds. These compounds by decreasing harmful microbial population in gut improve healthy level and performance (Lee et al, 2003). There are suggestions that essential oil can improve digestion. It may
be reason that spices and herbs will positively affect food digestion. Body weight and other organs can
increase by improving the nutrient absorption. Also, as it mentioned the using of onion extract in drink
water resulted in increasing of feed intake. This in turn can increase daily weight gain. The using onion in diet
can reduce the blood glucose. Hypoglycemia can stimulate nervous system for higher feed intake (Goodarzi et
al. 2013). Onion contains sulfur organic compounds including S-Methylcysteine sulfoxide and Sallylcysteine
suiloxide. These compounds are related to decreasing of blood lipid, liver protein and
glucose. So in our trial it is possible onion could improve growth performance of chicks due to content of
organosulphur compounds. Like to our results Aji et al. (2011) and Goodarzi et al. (2013) reported the
positive influence in BW, FCR and ADFI of broilers fed diets containing fresh onion compared with broilers
fed diet without any onion and antibiotics. Unfortunately, there are few reports on the effects of onion on
broiler performance.