The nutritive value of ensiled grass can be modified by the fermentation process. Compared to industrial fermentation, silage fermentation is still a relatively uncontrolled process. Consequently, considerable variation in the pattern of silage fermentation occurs on farms (Wilson and O'Kiely, 1990). This makes animal performance difficult to predict accurately and may alter the composition of the most appropriate concentrate to be fed with the silage. The objectives of the present experiment were (1) to compare the growth and rumen fermentation in cattle fed silage, where silage fermentation was altered using organic acids and (2) to determine the relative effects on growth when the silages were supplemented with two forms of concentrate.