This reduction seems to be correlated with bubble formation in the rumen fluid. The legumes or small grains gravitate more quickly to the lower regions of the rumen for their digestion. A small amount of gas production in rumen contents is more likely to produce froth than a high level, provided the bubbles do not release quickly, float to the top, and coalesce into a large bubble at the top of the rumen, thus enabling the animal to eliminate this gas by belching. Each particle of feed and each clinging gas bubble pushes an equal volume of fluid upward. Thus, the lower the gaseous digestion takes place in the rumen, the more the water level is raised.
Studies show that raising the level of fluid in the rumen can mechanically prevent belching of gas.
If the clinging bubble production becomes extensive, a frothy mass develops and almost completely fills the rumen, obliterating the normal fluid level. This froth normally is not belched because it is not free gas and does not trigger the belching mechanisms. Even if the animal did belch, the amount of gas expelled would be negligible because of the feed, water, and gas mixture. We do not understand completely why gas bubbles produced by small grains, legumes, and ground feed cling to food particles. Other factors contributing to the problem include chemicals such as saponins, saliva, pectin, and the level of water-soluble protein.