1 The effect of feeding subtherapeutic levels of salinomycin and avilamycin on growth performance of broilers and its relation to microbial bile salt hydrolase activity in the small intestine A. Knarreborg, R. Engberg, S.K. Jensen, and B.B. Jensen, Department of Animal Nutrition and Physiology, Danish Institute of Agricultural Sciences, DK-8830 Tjele Introduction The health and growth promoting effects of feeding subtherapeutic levels of antibiotics to chickens are well documented (Eyssen and De Somer, 1963; Engberg et al., 2000) and it is generally accepted that the beneficial effects of antibiotics are mediated through their action on the intestinal microflora. However, the mode of antibiotic actions is poorly understood. One of the proposed mechanisms is an inhibition of the bile salt-hydrolase active microorganisms. The taurine-conjugated bile salts chenodeoxycholate (TCDC) and cholate (TC) are the major constituents of avian bile. Conjugated bile salts enter the small intestine through bile secretion and play an important role in the digestion and absorption of dietary lipids (Haslewood, 1967). Several of the gram-positive bacteria species (lactobacilli, enterococci, clostridia) indigenous to the alimentary tract are capable of cleavage of the amino acids moiety from the steroid nucleus of conjugated bile salts (Cole and Fuller, 1984). Unconjugated bile salts are less efficient in emulsifying lipids, and thus, microbial deconjugation of bile salts is believed to impair fat retention and consequently depress the growth of chickens. Aim A study was conducted in order to select the antibiotic growth promoter to use in a future study on the influence of microbial deconjugation of bile salts on the fat retention in broilers. Concurrently, an in vitro methodology was established to measure the bile salt hydrolase activity (BSH-activity) of the microflora in the small intestine of chickens. Materials and methods Animals and diets A total of 72 day-old male chickens (Ross 208) were housed in 18 cages (4 birds per cage). Each cage was allocated randomly to one of 9 dietary treatments (two replicates). The experimental diets consisted of a wheat-soya based meal feed (control diet) supplemented with either salinomycin or avilamycin or combinations of both (Table 1). Table 1. The dietary treatments Treatment (mg/ kg feed) Control (0) Avilamycin (10) Avilamycin (20) Salinomycin (40) Salinomycin (60) Avilamycin (10) + Salinomycin (40) Avilamycin (20) + Salinomycin (40) Avilamycin (10) + Salinomycin (60) Avilamycin (20) + Salinomycin (60)