The objective of this study was to evaluate the effect
of dietary starch content and monensin (MON) on
metabolism of dairy cows during early lactation. Before
parturition, primiparous (n = 21) and multiparous (n
= 49) Holstein cows were fed a common controlledenergy
close-up diet with a daily topdress of either 0
or 400 mg/d monensin. From d 1 to 21 postpartum,
cows were fed a high-starch (HS; 26.2% starch, 34.3%
neutral detergent fiber, 22.7% acid detergent fiber,
15.5% crude protein) or low-starch (LS; 21.5% starch,
36.9% neutral detergent fiber, 25.2% acid detergent
fiber, 15.4% crude protein) total mixed ration with a
daily topdress of either 0 mg/d monensin (CON) or 450
mg/d monensin (MON), continuing with prepartum
topdress assignment. From d 22 through 63 postpartum,
all cows were fed HS and continued with the assigned
topdress treatment until d 63. Cows fed HS had higher
plasma glucose and insulin and lower nonesterified
fatty acids (NEFA) than cows fed LS during d 1 to 21
postpartum. Cows fed LS had elevated early-lactation
β-hydroxybutyrate (BHBA) compared with cows fed
HS. Cows fed HS had greater insulin resistance and
increased plasma haptoglobin in the early lactation
period. There was no effect of MON on postpartum
plasma NEFA. Cows fed MON had higher plasma glucose
compared with CON cows, which was driven by a
MON × parity interaction in which primiparous cows
fed MON had greater plasma glucose concentrations
than cows fed CON. Cows fed MON had lower plasma
BHBA compared with CON, which was contributed to
by a MON × parity interaction in which primiparous
cows fed MON had lower BHBA concentrations than
CON. Starch treatment had no effect on overall liver
triglyceride content. Primiparous cows fed MON had
increased liver triglyceride content compared with CON