Rate of increase in DMI during early lactation is the primary determinant of energy intake and balance. Dry matter intake increases by approximately 1.5 to 2.5 kg/wk during the first 3 wk of lactation (Bertics et al., 1992; Kertz et al., 1991).
Generally, older cows have a more rapid rate of increase in DMI during the first 5 wk postpartum than primiparous cows (Kertz et al., 1991). This difference in feed intake between older and younger cows serves as an argument for separate grouping and management of heifers compared with older cows, at least during the early portion of the lactation.