Gas is produced as a natural by-product of digestive fermentation in the rumen. This gas contains approximately 30% methane, 60% carbon dioxide, and trace amounts of other gases. Gas production in a hungry cow’s rumen on good pasture can be up to 2 litres per minute and must be expelled quickly if the animal is to consume sufficient feed. In order to remove this gas, the animal belches about once a minute. If the gas release is disturbed, foam forms in the digesta and causes the belching mechanism to fail. Usually the foam is of low persistence and occurs only in small amounts. In contrast, the formation of large amounts of rigid foam of a high persistence is lethal. Gas that cannot be expelled by belching causes bloating, and if severe enough, internal pressure on the vital organs surrounding the rumen (i.e. heart and lungs) results in death.

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The main problem of this complex metabolic disorder is its rapid progress and the difficulty of predicting its occurrence under field conditions. The complexity results from the interaction of plant, animal, and microbial factors which lead to foam formation. Pasture bloat predominates in animals grazing fresh forage legumes, especially in spring. Maximum foaminess of the rumen content occurs 1 to 2 hours after the cow begins eating bloat-causing forage and is reduced to the pre-feeding level 4 h later.

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Pasture bloat is caused by grazing plants such as lucerne, wheat pasture, various clovers, succulent grasses, hay and crops such as brassicas. Plant proteins, saponins, pectins, hemicelluloses, bacterial polysaccharides, and peptide slimes produced by the rumen microbes - either solely or acting in combinations – can be responsible for stable foam formation. Bloat is also associated with high levels of potassium and low levels of sodium in the rumen, a characteristic of grazing oat and wheat crops in the winter.

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Small plant particles and foam-stabilising polysaccharides appear to play a crucial role in the process of rumen foam formation. Fine plant particles originate from decomposed chloroplasts. These in rumen fluid facilitate gas bubble coalescence. The bloat potential of crops depends on their digestibility by rumen bacteria. Other compounds are also involved in the development of pasture bloat to varying degrees. Saponins, which occur in significant amounts in both lucerne and clover, are known to form stable foams at very low concentrations.

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In addition to the plant characteristics, the animal's susceptibility to bloat plays a major role. In general, greedy cattle have a higher likelihood of developing bloat.

 

BLOAT PREVENTION

The most common methods of preventing pasture bloat include pasture management and grazing control, feeding of roughage supplements, and the administration of antibacterial and anti-foaming agents. Pasture management may involve using grass or silage-legume mixtures, considering crop maturity and choosing forage with low bloat potential. Feeding ruminants dry hay before or during grazing, as well as slowly adapting the animals to high risk pastures are some of the safest bloat prevention methods.

 

However, even with good pasture management bloat can still occur. When it does it is crucial to ‘break the foam’ as the animal starts to bloat. The best way of doing this is by providing the animal with a bloat block containing anti-foam forming materials, such as Olsson’s Bloat Liq.