Everyone knows a livestock producer who has lost animals to bloat. Because of the risk, some folks flatly refuse to graze their stock on dense stands of clover or alfalfa. Others kind of shrug, saying they’ve rarely lost animals and don’t worry about it much. So … what gives?
Legume bloat. Sometimes called frothy bloat. What is it, exactly? Well, let’s consider what happens in a rumen.
We can think of the rumen as a large pouch containing a lot of liquid with a small cap of gas near the top. In the liquid lives a vast population of rumen microbes that make their living by metabolizing the carbohydrates, proteins and fiber dumped into the rumen when the animal eats something. Because the rumen contains little to no oxygen, the microbial process for digesting these substrates is called fermentation. Two of the main products of rumen fermentation are the gases carbon dioxide and methane. (Think of a septic tank.) Hour after hour, microbes ferment feed particles deep in the rumen liquid and release these gases into the surrounding liquid as tiny bubbles that then rise to the surface. A thin film of molecules at the surface creates the boundary between the rumen liquid and the gas above it.
Now the key part: Under normal conditions, these gas bubbles float upward through the rumen liquid, and when they reach the surface, they merge briefly with the boundary layer, pop open and vent their gas into the gas cap. Periodically, the animal eructates (scientific jargon for belching) and releases some of this gas into the atmosphere. You know, the greenhouse gas effect and all that. So far, so good.
But in a bloat condition, something else occurs. The boundary layer assumes a different character – its surface tension increases. This has profound effects on the rumen environment because now when those tiny fermentation bubbles float to the surface, they can’t pop through the boundary layer to release their gas. Instead, they remain intact and accumulate as a stable foam just beneath the surface.
But as this foam begins to form, the rumen bugs (remember them?) are still merrily creating more gas bubbles beneath the surface. Unfortunately, there is no direct feedback mechanism to tell the rumen bugs to stop their fermentation. As more bubbles float to the surface, they add to the other bubbles and accumulate, like a beer foam growing larger and larger. This foam soon builds up an immense pressure inside the rumen. The rumen expands. It eventually expands so much that it causes the side of the cow or sheep to bulge outward. (Which side? The left side. Just stand behind the animal and look forward. The side that’s opposite the right side is the left side.)
As the bloat gets worse, the animal becomes uncomfortable and stops eating. But this changes nothing in the rumen. The rumen bugs continue their fermentation unchecked, at least in the short term, because the rumen already contains lots of feed. Eventually, in a few hours, the bulging rumen becomes large enough to interfere with the movement of the diaphragm, causing respiratory distress. And then, death, either from asphyxiation or a stress-induced problem, such as a heart attack.
Why does bloat occur?
Surprisingly, after years of research, we’re definitely not sure of the exact mechanisms. But we do know that certain conditions will encourage the formation of that stable foam.
First, something must be present that increases surface tension by binding surface molecules together. This something is probably a protein or a group of related proteins that occur most frequently in certain legume forages. Second, rumen gas production must be vigorous, which means that the microbial fermentation must be healthy and active. Third, foam stability is enhanced when the rumen pH is lower than 6, which implies the presence of lots of soluble and quickly fermentable carbohydrates. Fourth, enough cations (calcium, magnesium or potassium) must be present to help stabilize that surface film. And finally, natural antifoaming agents such as salivary mucus or certain lipids (fats) seem inactivated or at low levels.
When thinking about bloat, everyone concentrates on the forages, so for a moment let’s focus on forages, especially the legumes. Some legumes will cause bloat, but not all. Alfalfa can cause bloat, of course, and so can many true clovers (the trifolium species, such as white, red, subterranean, kura, alsike, crimson, etc.). The sweet clovers (melilotus species) and hairy vetch (Vicia villosa) can also cause bloat.
Legumes that don’t (or rarely) cause bloat include the trefoils (lotus species, such as birdsfoot trefoil and big trefoil), crownvetch (Coronilla varia), cicer milkvetch (Astragalus cicer), sainfoin (Onobrychis viciifolia) and the lespedezas (lespedeza species). Most tropical legumes also don’t cause bloat.
But legumes are not the only culprits that cause bloat. Young succulent grasses can sometimes cause bloat, especially small grains, such as wheat, barley and oats. These are all annuals, and annual grasses generally contain higher levels of nonstructural (nonfibrous) carbohydrates than perennial grasses.
One common theme of bloat-causing species is that they contain proteins that, under certain conditions, will bind rumen surface molecules more tightly together and thus prevent gas bubbles from breaking through. Perhaps these proteins form some kind of slime on the surface of the rumen fluid, but we don’t know for sure.
Interestingly, the legumes that don’t cause bloat also contain these problematic proteins, but they also contain something else: elevated levels of condensed tannins – complex molecules that bind to these proteins and reduce their solubility in the rumen fluid. This effectively prevents these proteins from altering surface tension.
Another common theme in bloat is the increased risk with forages that contain low levels of fiber and high levels of soluble carbohydrates and proteins. This occurs when the forages are young and growing – when they are so palatable that animals will consume large amounts in a short period of time.
So, here’s the bloat scenario: A healthy, hungry cow or sheep grazes a lush legume pasture and also consumes a good level of minerals. The plants are young, succulent, palatable and low in fiber. The high intake of quickly fermentable carbohydrates rapidly reduces the rumen pH below 6 and supports active microbial fermentation with vigorous gas production. And, these forages contain certain soluble proteins that can drastically increase the surface tension of the rumen fluid.
That’s the background story for bloat. There’s more, of course, especially about high-risk situations. We’ll cover these some other time.
