Although levels of concentrate supplementation are generally lower in organic as compared with conventional dairy cows, forage-only (FO) diets are not very common in organic dairy cows because of the resulting limited dry matter intake (DMI) and lower milk production. However, from the perspective of net food production, FO diets or forage diets supplemented only with by-products from the food processing industry, offer considerable potential because they do not compete with humans for food. The aim of the present study was therefore to investigate the effects of adding a mixture of wheat bran and dried sugar beet pulp [0.56: 0.44 on a dry matter (DM) basis] to a FO diet on DMI, milk production, chewing activity and production efficiency. Seventeen multiparous and three primiparous mid-lactation Holstein cows were randomly assigned to one of two treatments, receiving either a FO mixture with hay and grass silage in equal proportions (FO) or the same forage mixture supplemented with a mixture of wheat bran and dried sugar beet pulp at a rate of 25% of dietary DM (25% BP). The experiment was conducted in a change-over design with two experimental periods of 7 and 6 weeks, respectively. Overall, feeding the 25% BP diet increased DMI and energy-corrected milk (ECM) yield by 1.8 kg d(-1) as compared with cows fed FO. Feed conversion efficiency (kg ECM per kg DMI) and energy efficiency (kg ECM per 10 MJ net energy for lactation intake) were higher in FO, but cows fed FO were in a slightly negative energy balance and also tended to have a higher mobilization of body tissues as compared with cows fed 25% BP. In comparison with FO, cows receiving 25% BP showed less chewing activity per kg DMI or per kg neutral detergent fiber ingested. In conclusion, results from this feeding trial showed that adding wheat bran and dried sugar beet pulp to a FO diet increased DMI and milk yield and improved the energy balance when compared with a FO diet, although the magnitude of the milk yield response was lower than expected.