Milk NPN and noncasein nitrogen (NCN) were determined
using methodologies proposed by IDF (1964)
and IDF (1993), respectively. The concentration of CN
nitrogen, true nitrogen (TN), and serum nitrogen (SN)
were calculated by difference, where CN nitrogen =
TN − NCN; TN = total nitrogen − NPN; and SN =
TN − CN nitrogen. Efficiency of milk production was
calculated by dividing milk production by DMI. Milk
energy secretion (MES) was calculated according to the
NRC (2001) equation: MES (Mcal/kg) = (0.029 × %
fat) + (0.0547 × % CP) + (0.0395 × % lactose), and
the milk energy (Mcal/d) was obtained by multiplying
the milk energy concentration of daily milk by daily
milk production.