The main objective when considering dietary energy levels in the dairy cow is to achieve optimum economical milk production, not necessarily energy balance, at all stages of lactation. Some cows may attain energy balance by reducing milk yield.

In the early part of lactation, while appetite is increasing, a negative energy balance is acceptable, particularly in the high-yielding cow, at the expense of body reserves. During this time and up to peak yield, maximum use must be made of the cow's feeding capacity in order to attain its full milking potential. This is important, since the peak yield considerably influences the rest of the lactation, as concluded by Broster (1972).

Failure to achieve a sufficient energy intake in early lactation may be reflected in excessive weight loss as well as in reduced milk production, whereas in the post-peak period, too rapid body-weight gain may be associated with over-generous feeding.