Eight groups were considered as treatment groups and received sulfadiazine+trimethoprime (Sultrim), tylosin tartrate, Salinomycin, Lincomycin+Spectinomycin (Lincospectin), Enrofloxacin, Chloramphenicol, Chlortetracycline and Furazolidone. Another group as the control did not received any antibiotic up to the end of experiment. For each group 68 “ Ross “day old chicks were placed in 4 pens (17 birds in each). Vaccination programme was similar for all groups. After Lasota vaccine (at 15 and 31 days of age), the antibiotics were given to the treatment groups at 16-20 and 32-36
days of age in the drinking water, but salinomycin was used in the feed from 16 days of age up 103 days before slaughtering. Before and after usage of drugs (ie, 15, 21, 31, 41 days of age) and also one day before slaughtering (day 51) chickens and their feed were weighed in each pen and the mean of body weight (B.W.), weight gain (WG.), Feed intake (F.l.) and feed conversion ratio (FCR) were calculated in different groups and stages. At the end of the rearting period it was concluded that mean of B.W. and W.G. and total FCR of all treatment groups were better than control. Lincomycin + Spectinomycing group had the highest B.W. and the lowest mortality, and Salinomycin group had the best total FCR, but there was not any significant difference between groups (P 0.05). The Lincomycin + Spectinomycin and the control groups had the lowest and the highest mortality rate respectively. The difference between them was significant (P 0.05).
)