As increasing amounts of concentrate mixtures containing grains and legumes are fed to animals, the competition between food for human nutrition and feed for animal production is growing and the cost for cereals is rising. In contrast to monogastric animals, ruminants do not depend on a similar food basis as humans and are able to produce highly valuable foods out of roughages indigestible for humans. In fact, the ruminants digestive tract has not evolved to handle large amounts of concentrated feedstuffs. These are convincing reasons to reconsider the extent of concentrate supplementation in dairy farming. The aim of this study was to reduce the concentrate supplementation in an organic dairy herd and to analyse the effects on milk yield, milk contents, live weight, body condition score, health and fertility. Between 2011 and 2013, 35 lactation periods of 27 dual-purpose Simmental dairy cows were recorded. The reduction in concentrate supplementation for the 13 cows from the experimental group was 25 %, based on the amount fed to the 14 cows in the control group. There were no significant differences in milk yield, milk contents, body weight, or body condition score. However, the reduction in milk yield in the experimental group was 10 % and the milk fat to protein ratio was numerically increased. These results suggest that the animals did not compensate for the lower concentrate intake by an adequately higher feed intake from forage, resulting in a reduced nutrient supply. Moreover, there was a tendency showing that cows in the first parity had more difficulties coping with the lower level of concentrate feeding. Fertility performance in the experimental group was even numerically better than in the control group.