Ruminant Nutrition: Dairy - Digestibility and Microbiology

Estimation of the energy value of a ration is fundamental for the determination of the quantity required to meet the energy requirement of the animal. Net Energy is the best estimate of the energy value of a feed/ration, but is difficult, time consuming and costly to determine. In addition, it is virtually impossible to measure the energy value of a single feedstuff fed to a representative animal because few, if any, feedstuffs contain the appropriate balance of nutrients required by the target animal. In practice, estimated net energy value of individual feedstuffs used in ration formulation have been derived from equations relating net energy to some more easily determined characteristic of a feedstuff, typically some measure of digestibility such as Total Digestible Nutrients (TDN). This method is reasonably accurate as long as the TDN value used is appropriate for the target animal. For example, it is well established that the TDN value of a ration fed to a lactating dairy cow at a daily intake required to support lactation is significantly lower than the TDN value of the same ration fed to the same cow at a maintenance level of intake when the cow is not lactating. Level of intake is only one of several factors which can depress the TDN value of a ration fed at production levels of intake. Protein quantity/quality which is adequate to support maximal digestibility in an animal at maintenance may significantly depress TDN value when protein is limiting for lactation. TDN measured using sheep may be appropriate for the estimation of the energy value of feedstuffs for cattle where appropriate equations are available. Variation in digestibility of feed is the main source of variation in energy value of feed.