Imagine it’s early morning on a warm summer day, a delightful pastoral scene. You see a herd of cattle grazing hungrily through a lush green field of alfalfa or white clover. By noon, however, a few cows begin to look uncomfortable. They separate themselves from the herd. Some lie down with protruding left sides, gasping for air. It’s the classic nightmare of legume bloat.
We observe, first and foremost, that bloat occurs in animals grazing alfalfa or clover. So, logically, we blame those forages for the problem. But if we do this, we are reducing the bloat problem into a binary true-false question: Either use a forage or don’t use it. That’s the easy way – to simplify things – but we would be making a serious mistake because these plants are excellent forages; they are important nutritionally, economically and agronomically. Many of us can’t simply avoid those forages and find suitable alternatives.
Draw up the formula
The reality is: Legume bloat is not a simple true-false question; bloat is a complex problem of many factors. In this month’s article, let’s look at bloat in a new way – that these factors can be related in some sort of equation, like the mathematical equations you solved in algebra class. You surely remember these equations. They come in the form of “X + Y = something.” Building on this logic, we can think of bloat as the “something” after the equation’s equal sign, the mathematical result of numerous factors. And the two factors in our bloat equation are (X) the concentrations of various compounds in the plant and/or the rumen, and (Y) the rates of change of certain events in the rumen.
For the X-value in our equation, here are some concentrations:
- Of specialized plant proteins which increase the surface tension of rumen fluid and facilitate the formation of a stable foam
- Of tannins in some plants which can interfere with those proteins
- Of water in the forage which defines its lushness
- Of certain minerals in the diet, such as calcium, magnesium and potassium
- Of hydrogen ions in the rumen, which can be expressed as the rumen pH value
- Of rumen fermentation gases like carbon dioxide and methane
- Of the rumen levels of soluble carbohydrates, which are substrates for fast microbial fermentation
For the Y-value in our equation, here are some rates that can change over time:
- The rate of feed intake (Hungry animals eat faster than satiated animals.)
- The rate of rumen gas production (Microbial fermentation produces a certain amount of gas during a given period.)
- The rate of cell rupture (Plant cells break apart to expose their fermentable contents to rumen microbes.)
Concentrations and rates of change form an equation with a relatively predictable set of factors. Each factor carries some risk, and when these risks converge, the result is legume bloat. Mathematical. Inevitable.
A good thing about our equation is: It gives us some tools. If we understand the numerous factors in the equation, we can manipulate them. If we can manipulate them, we can alter the results of the equation. In other words, we can better control the risks.
Playing out the risks
Let’s describe a couple of classic high-risk bloat situations and use the factors in our equation to assess and reduce those risks.
That lush field of alfalfa or white clover – is it a nutritional opportunity or a deathtrap? Let’s alter some factors. Rather than allowing hungry cattle onto the field, let’s first fill those animals with hay or let them graze in a nearby grass pasture. Satiated animals that enter the target pasture will graze the legumes more leisurely, which slows down the release of those bloat-causing proteins. Also, another trick: We can open the gate into that target pasture in the afternoon rather than the morning. By then, the dew will be gone, reducing the lushness of the forage. And yet another trick: We can cut the forage and let it wilt a bit. Wilting reduces the water content of plants and makes the forage safer.
Here’s another scenario. A common recommendation to avoid bloat is to plant a field with additional forages, such as grass or a non-bloating legume, with the goal of reducing the proportion of alfalfa or clover in the sward to 40% or less. Well, maybe. There are a few practical problems with this. A multispecies field may not suit our marketing requirements for that hay. Or the alternative species may have agronomic drawbacks. For example, non-bloating legumes like birdsfoot trefoil grow slower than alfalfa and require considerable babysitting when they are young. Or different forages may have different palatabilities, which can make grazing management more complex.
Also, even if the legumes are less than 40% of the biomass, that 40% is calculated across the entire field. In practice, an individual animal may stumble onto a patch of pure alfalfa or clover. For a few hours, that animal may be consuming a diet of nearly 100% legume. Could this really happen? Well, do you know any cattle who would say to themselves, “Hey, self, I’d better include some grass with my diet to avoid that pesky bloat problem”? Me neither. Therefore, planting alternative forages doesn’t automatically reduce bloat risk, but it may impact finances. I would evaluate this option with a sharp pencil.
Small steps into pastures
Another traditional recommendation to avoid bloat is to introduce cattle gradually into high-risk pastures, a little more each day. Really? In general, the highest risk for bloat occurs when animals first enter new pastures, usually within six to 10 hours. The strategy of gradual introduction effectively duplicates this high-risk situation again and again, which actually increases the risk of bloat. As an alternative, we can reduce risks by introducing animals into a legume pasture only once and then keeping them there until we need to move them. Canadian research with cattle on pure stands of alfalfa have demonstrated this quite clearly. Remember: The highest risk occurs when animals are hungry, when their rumen bugs are not yet adapted to those fermentation substrates. Keeping animals continuously in the same field resolves both these problems.
Post-frost challenges
Then there is frost. We all dread walking into legume fields the morning after the first autumn frost. The issue here probably involves the rate of cell rupture in the plants. Frost damages plant cells, which dramatically increases the rate of cell rupture. When animals graze plants after a frost, those ruptured cells release their bloat-causing proteins into the rumen fluid much faster. So what can we do? We can put chemicals into the rumen which reduce surface tension, such as commercial bloat-guarding products containing poloxalene or, in New Zealand, pluronic detergents. These chemicals are surfactants, similar to industrial defoaming agents. They reduce the surface tension of the rumen fluid and prevent the formation of a stable foam. No stable foam, no bloat.
One practical issue with these surfactants: They must be present in the rumen fluid before the problem begins. After a frost, or after the cattle enter a new legume pasture, it’s too late to begin feeding these chemicals as supplements. We need to make them available in the rumen prior to the high-risk situations. (The classic bloat treatment of drenching the animal with mineral oil is our frantic method of adding surfactants after the bloat symptoms occur.)
Grazing strategy risks
Finally, can grazing strategies affect the equation? Let’s think this out. In a set-stocking situation, animals enjoy a king’s luxury. They can graze at leisure and select from all the feeds in the paddock, a bite here, a bite there, maybe come back to a spot after a couple of days and bite again. But think about this for a moment: In those first bites, do animals prefer to eat leaves or stems? Leaves, of course – the youngest, tastiest leaves in the field – which are very high in fermentable carbohydrates, which support a vigorous fermentation rate and reduce rumen pH more than stems, thus increasing risk.
In contrast, cattle grazing in an intensive management system become quite focused about their grazing. As soon as they enter a new grazing cell, they attack the forages, eating leaves and stems. The higher proportion of stems means a higher level of fiber in the rumen during those first critical hours, which reduces the risk of bloat. It may not be a big change, but for some animals, it may be just enough to affect the bloat equation.
Legume bloat. An equation. The level of risk. No problem. And after we solve the bloat equation, perhaps I can teach my cattle to understand algebra.
