The NE g value of a feed is calculated by determining the energy deposited as fat and protein in body tissue from feed consumed above that needed for maintenance. To make this determination, representative cattle are first slaughtered to determine the energy contained in their body tissues at the start of the experiment. Then remaining cattle are slaughtered at the end of the experiment, and the Mcal gained from the feed fed is equal to the Mcal in the body tissues at the end of the trial minus that calculated to be in the body tissues at the start. For example, in the same 600 pound steers if an average of 18 pounds of alfalfa hay were consumed daily and 10 pounds were required daily for maintenance, then 8 lb per day were used for gain. Mcal per pound.
All feeds have higher net energy values for maintenance than for growth.
Roughages are lower in both NE m and NE g than concentrates for two reasons. First, because of their fiber content less of the energy is digestible and more is lost as feces. For example, corn contains 1.34 Mcal of ME per pound and has a heat loss of approximately 0.55 Mcal per pound . Prairie hay contains 0.75 Mcal of ME per pound and has a heat loss of 0.42 Mcal per pound when both the corn and prairie hay are fed in a typical growing ration, dry basis. The proportion of the heat produced from a ration that is usable during cold weather primarily depends on the total net energy intake, wind velocity and air temperature.