The higher value is for ruminants like sheep and cattle, and the lower value is for horses – and they may differ by more than 10 units. TDN is a measure of a feed’s energy value. Can one hay sample really have two different energy values? What’s going on here?
The short answer is: yes. Hay can have two energy values, or even more than two, depending on who is consuming that hay. To explain this, we must look at the architectural differences between ruminants and non-ruminants.
Taste of survival
Let’s think, for a moment, about one of the most compelling principles of life: food (really the search and acquisition of nutrients). Quite simply, food is one of the driving forces of evolution. The need for food controls some of the main design features of animals.
The second critical principle is the type of food. Of all the edible materials in the world, which one is the most abundant? What potential feed material is so common that it could support millions of animals with enough energy for survival, reproduction and other evolutionary useful activities?
It’s not sugar. It’s not protein. It’s cellulose (fiber). The bulk of most green plants is composed primarily of cellulose and other fibrous compounds. But there is a catch: Even with all this potential feed around, no mammal can use it directly because no mammal makes the cellulase enzyme necessary to digest this fiber. Bummer.
The fermentation factor
But over the millennia, nature devised an elegant solution. Although mammals don’t secrete the enzymes for fiber digestion, many microbes do. These microbes can digest fiber by a process called “fermentation.”
Therefore, mammals who spend their time eating forage have evolved gastrointestinal (GI) tracts containing regions that house these microbes, areas where microbes can cheerfully ferment the fiber.
In turn, those mammals can digest the nutritious products of microbial fermentation and even gain nutrients from the microbial bodies themselves. It is a brilliant two-stage system that allows forage-eating mammals to derive their nutrients from fiber.
Other species that have prowled the earth – like sabertooth tigers and cave bears and man – evolved a different type of GI tract. They didn’t need to digest much fiber because they simply ate the animals that did.
And that brings us to the problem of TDN for horses.
Rules of the rumen
Put yourself in the position of a gastrointestinal architect: Your job is to adapt a GI tract to digest fiber. Your major tool is a sac containing microbes that are good at fermenting things, and you must insert that fermentation sac somewhere along the GI tract.
However, you also must obey one immutable rule for all mammals: The true stomach (abomasum) must be positioned in front of the small intestine with nothing inserted between them. You cannot alter this arrangement.
Therefore, you really only have two main choices – you can either put the fermentation sac in front of the stomach or you can put it after the small intestine. (Here, “in front” means upstream and “after” means downstream.
The technical terms for these choices are “anterior to the abomasum” and “posterior to the small intestine.”)
The first choice gives us the basic architecture of ruminants like sheep and cattle. All their feed first goes into their fermentation sac (called a rumen) before it enters the true stomach.
Ruminants also have the ability to chew cud, which means they can regurgitate rumen contents for another chew, which causes further physical breakdown of the fiber. Rumen fermentation of this fiber produces metabolic products that are absorbed across the rumen wall.
After leaving the rumen, the residual feed mass passes into the true stomach and small intestine for further digestion. Also, the bodies of dead rumen microbes flow out of the rumen into the stomach and small intestine, where they provide even more nutrients for digestion.
Ruminants, clearly, are designed to extract a maximum amount of energy from fibrous feeds. From an engineering perspective, a ruminant’s digestive tract features multiple opportunities and sequential digestion (rumen and then small intestine) and redundant mechanical systems (chewing cud).
In contrast, the second choice gives us the basic architecture of the horse. Its fermentation sac is the large intestine (called the colon), which in horses is very large. The basic design of the horse is that feed first goes into the true stomach and small intestine before it passes into the colon.
This means that digestible nutrients, like starch and sugars and most proteins, are digested and absorbed from the small intestine, just as it is in humans.
Fiber, on the other hand, cannot be fermented until it travels all the way through the GI tract and into the colon. Microbes in the colon ferment this fiber, and then the metabolic products of this microbial fermentation are absorbed across the colon wall.
The residual mass then leaves the colon and becomes manure. Unlike ruminants, however, horse architecture provides no second chances for additional digestion of fiber or microbial bodies.
Scoring the TDN
Also, in case you’ve never noticed, horses don’t chew cud. Which means that horses can’t smash and grind fiber as well as cattle and sheep. Because horses cannot crush fiber as well as ruminants, they don’t ferment fiber as extensively as ruminants.
In other words, fiber is usually less digestible in horses than in ruminants. This characteristic becomes increasingly important as the feed contains more fiber or contains fiber of poorer quality.
There we have it; the reason for the two energy values in a feed is the basic architecture of the gut. Sheep and cattle are more efficient than horses at extracting energy from fiber. TDN is essentially a score of the ability to extract energy from feeds. The differences in TDN between ruminants and horses reflect their differing efficiencies of fiber digestion.
This TDN divergence becomes greater as the percentage of potentially digestible fiber in feeds increases. For example, a good-quality pasture (i.e., young grass at 20 percent protein with low fiber levels) can have TDN values of 65 percent for ruminants and 61 percent for horses – a difference of only four units.
But a poorer-quality hay (i.e., first-cutting grass hay at 10 percent protein with high fiber levels) can have TDN values of 58 percent for ruminants and only 45 percent for horses – a difference of 13 units.
Good reference tables and laboratory reports list separate TDN values for the various species. However, some publications still contain only one TDN value per feedstuff.
So if you’re trying to balance diets for cattle and horses, and you only have one TDN value, how do you apply this value to both species? Maybe you should find a better reference table. 
Woody Lane 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 his website.
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Woody Lane
- Lane Livestock Services
- Roseburg, Oregon
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