The ban on antibiotic growth promoters in livestock feeding has encouraged the utilization of phytogenic feed additives. These phytogenics recently attracted much attention and are generally recognized as residue-free ideal feed additives in animal Production. The current study was conducted to investigate the effects of the phytogenic herbs on feed intake, feed conversion ratio, and immune responses associated with the infectious bursal disease (IBD) vaccine in broiler chickens. For this study, 360 day-old broiler chicks were randomly assigned to six feeding trials, each with three replicates containing 20 chicks. The control group (T1) was only fed a basal diet alone, while the treatment groups were given the basal diet supplemented with 1% of basil (T2), lemongrass (T3), peppermint (T4), rosemary (T5), and thyme (T6) leaves powder, respectively. Body weight, feed intake, and feed conversion ratio were recorded. All chicks were vaccinated against IBD on days 7 and 19. A serology test was conducted to check the antibody titer against the IBD vaccine. The findings of this study showed that chickens in group T2 had significantly consumed more feed, followed by T1 and T6. During the overall study period, chickens in group T4 had significantly better feed conversion efficiency, followed by T3 and T6. Chickens in groups T5 and T6 showed a more pronounced antibody titer against the IBD vaccine at days 21 and 42 of the experiment. Therefore, these findings indicated that supplementation of basil leaf powder improved feed intake. Moreover, peppermint and lemongrass leaf powder improved the feed conversion ratio. In addition, supplementation of rosemary and thyme enhances the immune status of broiler chickens and could be considered a natural growth promoter feed additive. Therefore, further studies should be done to discover their beneficial effects to use as alternative feed additives in broiler chickens. Keywords: Body weight, Broiler chicken, Feed conversion, Feed intake, Immune response, Infectious bursa disease

Phytogenic feed additives (PFAs) have promising importance in chicken production as antibiotic alternatives to balance chicken gut microorganisms and improve productivity. The objectives of this study were to evaluate the body weight gain (BWG) and gut bacterial load of broiler chicks fed on selected herbs. For this experiment, 360 unsexed one-day-old broiler chicks of Cobb 500 with an average weight of 40.74 g were randomly allocated into six treatment groups with three replicates of 20 chicks in each pen. The treatment 1 (T1) group was fed by a basal diet alone. Chickens of T2, T3, T4, T5, and T6 were fed the basal diet containing 1% of basil, lemongrass, peppermint, rosemary, and thyme leaves powder, respectively for 49 days. Body weight (BW), BWG, and average daily weight gain (ADWG) data were recorded every week and at the end of every phase. On days 21 and 42, three chickens from each replicate were slaughtered for microbiological analysis (pathogenic and normal flora) of cecum contents aseptically. The obtained result showed that chickens kept on T3 had significantly higher BW, BWG, and ADWG during the starter and grower phases. Significantly highest final BW was recorded during the finisher phase on T3 and T6. Chickens that consumed T3 and T6 had significantly higher overall BWG and ADWG. The lowest Escherichia coli counts were seen in chickens fed on rosemary (T5) on both days 21 and 42 of the experimental time. Similarly, the highest Lactobacilli counts were recorded on chicken fed on T5 (day 21) and T3 (day 42). On the other hand, almost all treatment herbs showed a higher Enterococcal count, with the highest recorded for T3 (day 21) and T6 (day 42). The present findings suggest that supplementing lemongrass and thyme leaf powder improves BW performance and gut microbial composition. Likewise, rosemary leaf powder enhances the beneficial microbial composition and reduces pathogenic bacteria. However, the underlying detailed biological mechanisms and dose standardization of these herbs for inclusion in the diet of broiler chickens need to be studied further.