Monensin is an ionophore antibiotic that changes the population of rumen bacteria. The aim of this study was to determine the effects of a monensin controlled-release capsule administered 3 weeks before calving on blood metabolites (serum beta-hydroxybutiric acid (BHBA), cholesterol (CHOL), urea, creatinine (CREA), gamma glutamyl transferase (GGT), aspartate aminotransferase (AST), total protein (TP), phosphorus (P), magnesium (Mg+), calcium (Ca++), glucose (GLU) and plasma non-esterified fatty acids (NEFA) concentration) and milk yield before and after calving. A total of 50 Holstein-Friesian cows were selected from the same flock. Blood samples were taken 3 weeks before the expected calving date and during weeks 1, 2, 4, 6, and 8 after calving. The cows were divided into two groups: a study group (Group M, n = 40) and a control group (Group C, n = 10). Group M received orally a cylindrical device (Kexxtone, ELANCO Animal Health, Guelph, ON, Canada), and group C received no treatment. BHBA and NEFA were lower (P < 0.05) in group M compared with group C in weeks 6 and 8 after calving. GLU was higher (P < 0.05) in group M compared with group C in week 4 after calving. Urea was higher (P < 0.05) in group M compared with group C in week 1 after calving. There was no statistically significant difference (P <0.05) between the two groups in milk yields. Although milk yield was similar in both groups, the changes in the concentrations of BHBA, NEFA, GLU and urea show that a monensin CRC could be used as a glycogenic precursor.