“Tis the season” – or rather, a month or so after the season – of holiday feasts, mounds of whipped potatoes, mouth-watering pastries.
Of course, everyone in my house carefully monitored their caloric intake and restricted their holiday meals to high-fiber salads and yogurt. Of course. And starch? We tried not to think too much about it.
But let’s think about starch now. Starch is often a major part of livestock diets (and also, I’m afraid, of human diets), particularly diets for high-producing animals. Every time we feed corn or barley or oats, we are feeding starch. And starch is not like high-fiber forages; it needs to be managed carefully.
All our lives we’ve heard about starch but, actually, what is it? Let’s start with glucose – the simple six-carbon sugar that is the basis of most metabolic processes. If we link two glucose molecules – namely from the No. 1 carbon in the first molecule to the No. 4 carbon in the second molecule – we create the compound “maltose.”
The 1-4 linkage is angled in a distinct way called “alpha.” Therefore, maltose is composed of two glucose units held together by an “alpha (1-4) linkage.” Now start adding glucose units, one after another like a string of boxcars in a railroad train.
The result is one type of starch molecule called “amylose” – a long and relatively straight polymer of glucose units. Amylose molecules can be big, with hundreds or even thousands of glucose units.
A second type of starch molecule is “amylopectin.” This molecule contains straight strings of glucose just like amylose, but it also contains extensive branching because some glucose molecules are linked by their No. 1 and No. 6 carbons (alpha [1-6] linkage).
This branching makes the amylopectin molecule more compact than amylose, like a tightly wound lattice, and also more soluble. The important point is: Starch granules are mixtures of both types of molecules, with the percentage of amylose generally less than 30 percent of the total.
Here are two more important points. The first is: Only plants synthesize starch. Plants make it for a very simple reason – energy storage. Starch molecules contain quite a bit of easily available energy in a relatively condensed format. But what about animals?
Well, animals don’t make plant starch, but they do synthesize glycogen, which is a glucose-based storage molecule similar to amylopectin. Glycogen molecules are much smaller than amylopectin molecules. They are more highly branched and more condensed. Most glycogen is stored in the liver, although some is also located in skeletal muscle.
The second point is about linkages. If we take a long-chain amylose molecule and, instead of linking adjacent glucose units with alpha (1-4) linkages, we use a beta (1-4) linkage (beta indicates a different angle in the chemical bond), the resulting molecule is not starch, it’s cellulose – a very different molecule indeed.
Back to starch. Starch is highly digestible. In monogastric animals – species like hogs and chickens and humans – starch digestion is a relatively straightforward affair. Starch molecules travel down the gastrointestinal tract to the small intestine, where the pancreatic enzyme amylase breaks them up into small carbohydrate molecules like glucose, maltose, isomaltose and limit dextrins.
These are quickly absorbed into the blood. But in ruminants like cattle and sheep, starch molecules are exposed to digestion in three different places as they travel through the gastrointestinal tract: the rumen, the small intestine and the large intestine.
In the rumen, microbes ferment most starch molecules into volatile fatty acids (which are nutrients used by the animal) and sometimes lactic acid. However, some starch molecules pass intact through the rumen. The amount depends on the type of starch and its resistance to microbial fermentation. Most starch that reaches the small intestine is digested, as in humans.
And the few remaining starch molecules that reach the large intestine – well, we’re not exactly sure what happens there. Many of these molecules are resistant to microbial action. There is some fermentation and digestion, to be sure, but we are not very clear about the mechanisms.
Now things get a bit complex because starch is not starch is not starch. Grains – which are really large seeds – are composed of many layers. The hard outer seed coating is called the “pericarp.” Beneath that is a series of thin layers including the “aleurone,” which contains some protein.
Beneath that is the bulk of the grain: the starchy endosperm, which is a mass of starch granules.
Differences in the proportions of these layers cause differences in the types of starches: flinty, waxy, non-waxy, opaque, etc. You may have heard of these terms. They all have different cooking characteristics and also different rates of rumen fermentation and resistance to digestion.
Starches also differ in their ratio of amylose to amylopectin, which can cause profound changes in their rumen fermentation patterns. For example, a high percentage of amylopectin tends to increase the speed of starch fermentation and acid formation.
When plant breeders select for grain characteristics, they are actually selecting for things like the relative percentages of the different starch layers and the ratio of amylose to amylopectin.
Starch digestibility depends on other things as well. Processing grains (rolling, crushing, steam-flaking, ruminating, etc.) can speed up starch fermentation by increasing the surface area of the starch granules and also by cracking the hard pericarp, which allows more rumen bacteria to attach to the granules.
These industrial processes, especially those with heat and pressure, can also cause starch “gelatinization,” which is the irreversible destruction of the crystalline structure of the starch granules. Gelatinization can increase access by digestive enzymes and rumen microbes.
When it comes to feeding cattle, sheep and goats, starch is a little like Dr. Jekyll and Mr. Hyde. On one hand, the starch in grains is a readily available, highly digestible source of energy.
Starch provides glucose units directly to the blood, and with good management it can increase animal performance and help support a lush and healthy population of rumen microbes. That’s the main reason we include grain in our rations.
On the other hand, if too much starch enters the rumen too quickly, or if the starch is exceptionally accessible for fermentation, rumen microbes will ferment it into large amounts of lactic acid, which is a stronger acid than the normal products of rumen fermentation.
The result is rumen lactic acidosis (either subacute or acute, also called “grain overload”). In the subacute (chronic) version, the rumen pH routinely drops to less than 5.5, which kills many fiber-digesting bacteria. This reduces fiber digestion, feed intake and animal performance.
The acute version, however, is a metabolic catastrophe. When a ruminant engorges on starch, rumen bacteria quickly produce so much lactic acid that the rumen pH drops below 5 (it is normally above 6). The animal suffers from dehydration, cardiovascular damage, renal failure and death.
Acidosis is not usually a problem with forages. Most forages don’t contain large amounts of starch. Sure, forages may accumulate a small amount in the leaves after photosynthesis, and the lower parts of plants may contain some starch reserves, but the bulk of forage dry matter is primarily fiber, not starch.
But grains ... well, grains like corn and barley are essentially packages of starch. This makes sense, actually, since those big storage molecules come in mighty handy for the seedlings when they first emerge into the harsh world.
But our sheep and cattle are not tiny grass seedlings, and the rumen did not evolve to handle large amounts of starch. So rations that contain much starch can be a challenge.
For example, wheat is known as a very “hot” feed. Translation: Wheat starch can ferment so quickly in the rumen that it carries a high risk of acidosis. That includes wheat grains, of course, and also wheat byproducts like bakery waste and wheat flour.
Let’s be more complete. Based on our knowledge of the amylose-to-amylopectin ratio and also many years of practical experience in feeding animals, we can rank the common grains for their risk of causing acidosis, from high to low: wheat, barley, corn, oats and, finally, sorghum.
So should we avoid grains in our rations? That is a larger question that cannot be answered here. (I suspect that a good answer would involve calculations about money and profit.)
But including grains in a ration – which is the same as putting starch in the rumen – carries some risk. If we understand the characteristics of starch, we can better manage that risk.
Woody Lane, Ph.D., is a livestock nutritionist and forage specialist in Roseburg, Oregon. He operates an independent consulting business and teaches workshops across the U.S. and Canada. His book, From The Feed Trough: Essays and Insights on Livestock Nutrition in a Complex World, is available through Woody Lane.
Woody Lane, Ph.D.
- Lane Livestock Services
- Roseburg, Oregon
- Email Woody Lane, Ph.D.