It has been postulated that intake of feed mixtures will increase with increasing levels of concentrate until a peak is reached, whereafter it will decline again due to a lack of roughage to ensure a healthy rumen environment. Thus, it should be possible to determine the most desirable level of inclusion of a particular roughage in a feedlot diet for example, if peak intake results are also substantiated by observations that the rumen is in a satisfactory functional condition. The postulate referred to above was tested in a series of 6 experiments with cattle and sheep involving the following roughages: Eragrostis curvula hay milled through screens with different apertures, lucerne hay, wheat straw – also milled through different screen apertures, sunflower hulls and sugarcane bagasse in the pelleted and unpelleted form. The studies confirmed the postulate and clearly emphasized that peak intake is a function of type of roughage, its particle size, the mass/volume ratio of the feed mixture and also type of animal (sheep or cattle). It was also evident that the most desirable level of inclusion of a roughage in a feedlot diet can be established in this way. The digestibility of different feed mixtures is apparently not necessarily additive due to feeding level effect~ and probably also, associative effects.