<jats:p> Three experiments were conducted to determine the effects of dietary supplementation with a novel antimethanogenic compound (AM) on methane production and growth in Brahman (Bos indicus) steers. The compound was a chemical complex of bromochloromethane (BCM) and α-cyclodextrin, which is chemically stable when added to feed, thus overcoming the highly volatile nature of BCM. In these experiments the AM compound was administered to steers as a mixture with different feed supplements. In Expt 1 the effect on in vivo methane production of feeding steers the AM compound was determined using a confinement-type respiration chamber. Methane production of AM-treated steers (0 ± 2·4 mL/min) was lower (P < 0·001) than that of control steers (205 ± 5·2 mL/min) over 28 days. In Expt 2 we determined the effect of AM treatment over 12 weeks on growth of steers fed on a low quality roughage diet. The most marked effect of AM treatment was reduced (P < 0·01) voluntary roughage dry matter intake (DMI), and reduced (P < 0·01) acetate : propionate molar ratio (A : P) in rumen fluid. Average daily liveweight gain (ADG) (0·22 ± 0·01 kg/day) and feed : gain ratio (F : G) (20·7 ± 1·46 kg DMI/kg liveweight) were not significantly affected by AM treatment. In Expt 3 we determined the effect of AM treatment over 10 weeks on growth of steers fed on a medium quality roughage diet, in steers that were either treated or not treated with a hormonal growth promotant (HGP; oestadiol 17β). AM treatment reduced (P < 0·05) DMI below that of steers not treated with AM, whereas DMI was not significantly affected by HGP treatment. Both AM (P < 0· 1) and HGP (P < 0·05) treatments separately reduced A : P ratio in rumen fluid. AM treatment had no significant effect on ADG, whereas ADG of HGP-treated steers was higher (P < 0·05) than that of steers not treated with HGP (0·76 ± 0·27 v. 0·60 ± 0·027 kg/day). F : G was reduced (P < 0·01) by HGP treatment. F : G of both HGP-treated steers and those steers not treated with HGP was reduced (P < 0·05) by AM treatment. We conclude that feeding steers with this novel AM compound enables the potent antimethanogenic properties of BCM to be realised under commercial conditions, and that prolonged use over 10–12 weeks is associated with an improved feed conversion efficiency in steers fed on better quality roughage diets.</jats:p>