Estimating digestible and metabolizable energy concentrations of ruminant diets
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Most commonly-used ruminant nutrition models are based on metabolizable energy (ME) or net energy (NE), but these values cannot be directly measured by commercial feed testing labs. Therefore, equations using concentrations of commonly measured nutrients and maybe in vitro or in situ digestibility measurements are used. Depending on the complexity of the model, ingredient ME or NE values are either used directly to calculate the energy value of the total diet or animal and total diet factors are used to adjust feed energy values to obtain ME or NE values for the total diet. The feed energy equations in the NRC (2001) have been widely used; however, new data and improved methodologies should be incorporated into those equations to improve their accuracy. Major limitation of the old equations include use of nonfiber carbohydrate (NFC) rather than starch, incorrect discount factor for total diets, and overestimation of the digestible energy provided by dietary lipids and the ME provided by protein. The previous method of converting digestible energy (DE) to metabolizable energy (ME) also did not incorporate enough sources of variation. This paper will discuss changes to equations that should result in more accurate estimates of feed energy values.