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Virginia Cooperative Extension - Knowledge for the CommonWealth

What is the energy content of this ration?

Dairy Pipeline: April 2006

Robert E. James
Extension Dairy Scientist, Dairy Nutrition
(540) 231-4770; email:

Samples evaluated at the forage testing lab tell us the amount of Net Energy (NE) per pound of dry matter. The numbers are important enough that they may be the first item we look at when the nutritionist sends us a new ration. However, estimating the useful or Net Energy (energy which can be used for maintenance, milk production and growth) is not a very precise process. Gross energy content of the feed is measured by burning a sample and recording the energy released. We must account for the inefficiencies of the cow by deducting energy lost in feces and urine or expelled as methane and other gases. Cows also produce heat in the process of digestion. The only way to account for these losses is to place the cow in a large glass box, hook up a gas mask, collect and evaluate all manure and urine and record the heat produced. Be assured that forage testing labs in the U.S. do not determine energy using this method! So how is energy determined? NE is estimated from a large body of research on energy values of various ration components. For instance, feeds containing lots of fat and starch provide more energy than feeds containing larger proportions of fiber, ash, or minerals. NE values represent the amount of energy we expect the cow to derive from a feed at a given level of intake. Just to make things more interesting, higher intakes reduce energy extraction (via rapid passage through the digestive system) and vice versa. Remember the following when evaluating energy values of a feed.

  1. NE value of a feed is an estimate of useful energy content of feed.
  2. NE values are not absolute like other nutrients such as minerals, vitamins, or protein.
  3. Higher intake means less net energy available per lb. of feed intake.
  4. There is no standard method of estimating NE used by all laboratories across the U.S.

Keep the limitations of estimating NE in mind in ration evaluation. Use all the nutritional information available, including protein, fiber, minerals and vitamins to create rations that promote a healthy rumen, high intake and high milk production.

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