Rations include other components that are more compelling, like protein, fiber and minerals. But fats can contain 225 percent more energy than carbohydrates. Under the right conditions, they may represent a source of untapped calories for growth and lactation. Such a potential; perhaps we should look at dietary fats more closely.
Closer examination
To understand fat nutrition, we need to understand how fats are analyzed. Look at any feedstuff reference table or analysis report, and you won’t see a specific value for fat. Instead, you’ll see something called “ether extract.”
Some documents treat this value as synonymous with nutritional fat – but it’s not. It’s close, but it’s not exactly the same. Ether extract is the material in feed that is soluble in ether. True, most fats are soluble in ether, but some are not, and there are quite a few other things soluble in ether that have no nutritional value. Let me explain.
Laboratories analyze fat with a procedure called the “Soxhlet extraction” (pronounced “socks-let”). This is a technique dating from the 19th century using cleverly-designed glassware, hotplates and tubes of cold water. The concept is straightforward enough. Essentially, a ground-up feed sample is bathed in liquid ether.
Anything soluble in ether dissolves into the fluid. After a series of ether washings and recyclings, the residual sample is weighed, and the difference is called the “ether extract.”
By the way, we should never take this technique lightly, even though it may be old and familiar. Ether is short for diethyl ether, which is the chemical name for engine starting fluid – the same explosive stuff that we spray into balky engines (at least the older, less-computerized engines).
The can of starting fluid in my garage has the solemn warning in bold, red letters, “Extremely flammable! Keep away from heat, sparks or fire.” Recently, I saw an advertisement for a commercial Soxhlet extraction system that boasts an “explosion-proof hotplate.” As they say in many advertisements: Do not try this at home.
So what exactly does the ether extract procedure extract? One thing it extracts is nutritional fat. The basic sub-unit of nutritional fat is called a “fatty acid,” which is a long-chain molecule composed of carbons and hydrogens with a carboxyl group (-COOH) on one end and a methyl group (-CH4) on the other end. Fatty acids differ from each other principally in the number of carbon atoms and carbon-to-carbon double bonds in the chain.
Fat, energy and forage
From a nutritional perspective, fats that contain 225 percent more energy than carbohydrates are called “triglycerides.” These are molecules composed of three fatty acids linked together with a three-carbon glycerol backbone, similar to a three-horse yolk.
Triglycerides are the primary fat molecules in grains and beans and also in tallow, grease and vegetable oil. For these types of feedstuffs, ether extract does a fairly accurate job of representing their nutritional value.
Let’s recognize the one main reason for triglycerides: energy storage. Long chains of carbons and hydrogens create molecules that are more densely packed with energy than carbohydrates. This energy-dense design is no accident. In animals and plants, triglycerides act as physiological bank accounts which store energy until those extra calories are needed.
Not surprisingly, we find high levels of triglycerides in certain specialized tissues, such as animal body fat and the portions of some plant seeds (grains).
But forages are important feedstuffs too and, from a fat perspective, forages are quite different from grains. First, most ether extract values for forages are quite low, and second, forages contain few triglycerides in their growing tissues.
Leaves and stems contain no specialized energy storage depots, but still they contain a small amount of ether extract. (Somehow, the image of grass plants with globs of fat hanging from their leaves is rather disturbing).
Ether-soluble compounds in forages represent a wide array of functional molecules – galactolipids, waxes, resins, essential oils, pigments and even chlorophyll. Only some of these compounds have nutritional value, like the galactolipids, which are molecules that contain only one fatty acid linked to a single sugar.
Galactolipids are digestible, but their energy values are lower than triglycerides because each galactolipid molecule contains a smaller proportion of fatty acids than a triglyceride molecule.
Most of the other ether-soluble substances in forages have little or no nutritional value, which means that the ether extract values for forages are not very useful. In practice, this error is usually minimal because the forages generally contain so little ether extract (less than 2 percent of the dry matter).
Breaking it all down
One interesting note about fats is that their digestibility values may be wrong. Remember that digestibility is a measurement of the amount of material that disappears from the gut between the mouth and the manure.
To measure this value for fats, we routinely run ether extracts on the feed and the manure. Using a little arithmetic, we can easily calculate the amount that did not appear in the manure.
Dividing this amount by the amount in the feed gives us the apparent digestibility of the ether extract. For example, if an animal consumes 60 grams of ether extract in the feed, and 6 grams comes out in the manure, we can assume that 54 grams were digested and therefore conclude that the apparent digestibility of that ether extract is 90 percent (= 54 / 60 expressed as a percentage).
Unfortunately, as fatty acids move through the gastrointestinal tract, some of them can become linked to calcium or magnesium atoms, making them less digestible. These altered molecules continue traveling downstream through the gut and end up in the manure. Calcium and magnesium salts of fatty acids are called “soaps.”
The problem is that soaps are not soluble in ether. Which means that an ether extract analysis of the manure will not extract them, which means that the ether extract analysis for manure will result in a lower number than it should be, which means that the digestibility for those fats will be overestimated. Still following me?
This error may have broader consequences, particularly for older total digestible nutrient values. The traditional formula for calculating the total digestible nutrient value of feeds involved adding the digestible portions of various nutrient components.
(Historically, this was the formula used before the modern total digestible nutrient formulas were developed.) The digestible amount of ether extract is multiplied by 2.25 to account for the extra energy in triglycerides.
However, depending on the type of feed, triglycerides may not comprise a large percentage of the digestible fats, and if soaps were formed in the gut, then the digestible amount of that ether extract was overestimated. This means that some of those old values for total digestible nutrients may have a small error built into them. Hmmm.
Starting fluid, soaps and hotplates that won’t explode. There’s depth in still waters. Maybe we should read those ether extract values with a little more respect.
