The transition period is generally recognized as the
most critical period of the lactation cycle for a dairy cow. The endocrine and physiological
changes that ac- company parturition and the onset of milk production negatively affect
immune function and DMI. As a re- sult of nutrient demand for milk production increasing
faster than DMI, cows experience a period of negative energy balance of varying duration and
intensity. Early lactation is also a critical time for metabolic disorders, of which most occur
during the first 2 wk postpartum (Goff and Horst, 1997). The extent of negative energy
balance is related to the incidence and severity of most metabolic disorders, such as ketosis,
displaced aboma- sum, retained placenta, and susceptibility to infections (Grummer et al.,
2004). In addition, improved energy balance has been proven to result in greater reproduc-
tive efficiency through earlier resumption of ovulatory cycles (Butler, 2005). Early resumption
of cycles is important because conception rates increase with suc- cessive cycles (reviewed by
Butler, 2003). Enhancing nutrient intake seems, therefore, imperative to maxi- mize health
and reproduction of periparturient cows.