SUMMARY: An experiment was conducted with 60 male Friesian calves between 5 and 11 weeks of age. The experiment was a 5×2×2 factorial design. The five major treatments were the inclusion of 20 or 40% ground wheat straw or 20, 40 or 60% ground lucerne in a pelleted diet. A methane inhibitor was included in the diet given to 30 of the calves. The diets were given alone or with a chaffed wheat straw supplementad libitum. All diets contained 2% sodium bicarbonate.The weight gain of the calves was greater with the diets containing ground lucerne than with those containing ground wheat straw, but in both cases weight gains were reduced with increasing roughage content of the diets. The efficiency of feed conversion showed a similar trend. The supplementation of the diets with chaffed wheat straw did not affect the performance of the calves. The inclusion of the methane inhibitor in the diets improved the weight gains and efficiency of feed conversion. The daily retention of nitrogen in the calves showed a similar trend to the growth rates.The molar percentage of acetate in the rumen increased as the roughage content of the diets increased while the proportions of propionate, butyrate and valerate decreased. The inclusion of the methane inhibitor in the diets decreased the concentration of total volatile fatty acids (VFA) in the rumen and increased the molar percentage of propionate, butyrate and valerate.