The microbial fermentation of forage material in the rumen leads to the production of a large amount of gas which is then absorbed into the rumen wall, passed into the omasum or, most commonly, expelled by eructation (belching). The eructation of gas is prevented by the development of a stable viscous foam in the rumen. This foam may hold the gases in large pockets (free gas bloat) or small pockets (foamy bloat), and it may be caused by feeding a high-concentrate ration in the feedlot, by feeding legume hay, or, most commonly, by grazing pastures which contain a large portion of leguminous material.

Failure to expel the gases held in the foam results in the rumen becoming distended so that it either presses against the diaphragm, immobilizing it, or constricts the main dorsal vessels through which blood passes to and from the heart. In either case, animal death is due to asphyxiation. Bloat is the cause of substantial losses to cattlemen throughout the world. In New Zealand, some 17,000 to 20,000 dairy cattle die from bloat each year. In the United States, it is estimated that farmers lose livestock valued at about $80 million yearly, while in Canada, annual losses are in the region of $22 million. Consequently, means of preventing bloat have been widely studied. The most common approach has been to try to prevent or reduce the development of foam in the rumen by reducing, by plant breeding methods, the amount of foam-causing substances in the plant material.

The saponins, which are capable of increasing surface tensions to levels that can withstand the gas pressures which develop in the rumen, were the first substances to be considered as the main causal agents of bloat. While they do cause increased and irregular respiration, which is frequently found in bloating animals, it has proved difficult to associate saponin levels with incidents of bloat.

MacArther and Multimore, in 1966, working in British Columbia, presented evidence to show that another important foaming agent was a plant protein known as 18S. They showed that incidents of bloat were associated with high 18S levels in a range of legume species. Alfalfa contains 4.5% to 5.2% 18S protein. Other legumes known to cause bloat (red clover, white clover, sweetclover, and alsike clover) contain similar amounts. Legumes which do not cause bloat (birdsfoot trefoil and sainfoin) and the grasses contain less than 1% 18S protein. This protein substance, which is so named because its sedimentation coefficient is 18 Svedberg units, is also known as the fraction I protein. It has a molecular weight of 500,000 and, since it is one of the cytoplasmic proteins, is readily digestible. Also, any reduction in fraction I protein would seriously impede photosynthetic rates, since it consists of ribulose 1,5-diphosphate carboxylase, the main carboxylating enzyme in the C3 pathway. The heritability of this trait is high, and selection would rapidly decrease 18S protein levels; however, both quality and quantity of forage production would decline if 18S protein levels were reduced by plant breeding methods.

Studies of 18S protein indicate that stable foam found in the rumen results from protein molecules, which are spherical when in solution, reaching the surface of the rumen fluid without being broken down by microbial action. Under these circumstances, the molecules uncoil, become insoluble, and are then capable of stabilizing the foam. Studies in New Zealand show that both fraction I and fraction II proteins (a mixture of proteins with molecular weights between 10,000 and 200,000) act in the same way to stabilize foam. Recent work has shown that certain tannins are capable of precipitating the proteins found in bloat-causing foam. The nonbloating legumes contain large amounts of these tannin substances. While the plants are alive, the tannins are held in vacuoles to prevent precipitation of the plant's own protein. The world collection of the genus Medicago, which is available at the University of Alberta, has been surveyed for tannin content. None of the Medicago species, nor any of the cultivars in common use, has been found to have high tannin levels. Consequently, it would seem that for the present, management, rather than plant-breeding solutions, must be used to combat bloat.

Bloat prevention may be achieved by a combination of pasture management and the use of antifoaming agents. The proportion of bloat-causing Iegumes in a pasture should be 51% or less. Before turning an animal into a pasture containing a high proportion of a bloat-causing legume (over 30%), it is wise to feed a grass, hay, or forage containing tannins (e.g., sudangrass).

The antifoaming agent most commonly used is poloxalene (a polyoxypropylene- polyoxyethylene block polymer), which is sold under the name Bloatguard. It is available in molasses "licks" or may be mixed into the concentrate part of the feed. Antibiotics will also prevent bloat, as will a number of natural oils, such as soybean oil, corn oil, peanut oil, or olive oil. However, these substances are rapidly degraded in the rumen and consequently call for large and frequent doses, which are costly to administer.

In Australia, antibloat capsules are available. These are 15-cm gelatin cylinders, 4 cm in diameter, which split down the middle and are hinged on one side. The capsule, which splits open in the rumen, becomes too large to be regurgitated, and releases a foam dispersing detergent at the rate of 6 g/day for 24 days.