Assessment of Energy Value of Feeds

The amount of energy that a feed will supply to an animal can be estimated by measuring the amount of energy that is produced when the feed is completely burnt. This gives the Gross Energy (GE) content of the feed, measured in MJ (or kilocalories). However, it is only of limited use since all feeds (apart from those with a very high fat or oil content) have about the same gross energy content (18 MJ/kg DM). Some other means of evaluating energy values is clearly needed, since straw obviously has a lower energy content than maize grain, but the two have very similar gross energy contents.

Digestible Energy Content (DE)

A more useful measure of the energy value of feeds is the estimate of digestible energy content (DE). This takes account of the amount of energy that is lost in the faeces, having not been digested and absorbed by the animal.

For pigs, this is a good estimate of the amount of energy provided by a feed. For ruminant animals, a large amount of energy may also be lost as methane, as a result of rumen fermentation.

The energy in the feed, minus that lost in the faeces, urine and as methane, is known as the metabolizable energy (ME), and is the term usually used to assess the energy value of ruminant feeds. It is also used for poultry, as their excreta contain both faeces and uric acid (the birds’ equivalent of urine).