Differences in rumen volume can have considerable effects on the digestibility and voluntary intake of ruminants. For instance Mould et al. 1982 showed that cattle in Bangladesh had a much greater rumen volume relative to live weight than reported values of Friesian cattle. They also consume much more straw per unit of live weight than Friesian cattle.

Differences between cows in the ability to consume roughage have often been observed. It is however possible that this strategy could be adopted simply by increasing outflow rate and not necessarily by a high rumen volume. Depending on the strategy adopted this would give rise to differences in the digestibility of the roughage consumed.

In order to investigate these aspects in more detail, 22 lactating Friesian cows were given doses of Cr-mordanted straw having been ground through a hammer mill with a 2 mm screen. The cows were fed completely mixed diets consisting of 50% ammonia treated straw and 50% concentrate.