A total of 1010 dry cows and pregnant heifers was randomly selected from 25 dairy farms near Guelph, Ontario, Canada to receive either a controlled-release
capsule of
monensin or a placebo at 3 wk prior to expected calving. Serum samples were obtained at the time of treatment administration, and both serum and milk samples were collected at wk 1, 2, 3, 6, and 9 postcalving. The threshold used to define subclinical
ketosis was selected a priori at a concentration of > or = 1200 mumol/L of
beta-hydroxybutyrate. Using this threshold, the prevalence and incidence of subclinical
ketosis were significantly reduced (50%) by
monensin treatment. The duration of subclinical
ketosis for cows that had been treated with
monensin was also shorter than that for cows treated with the placebo.
Monensin treatment significantly reduced the incidence of subclinical
ketosis when the threshold was defined using higher concentrations of serum
beta-hydroxybutyrate (1400 and 2000 mumol/L). In addition,
monensin significantly reduced the prevalence of positive milk
ketone tests.