EFFICACY OF MONENSIN FOR THE PREVENTION OF SUBCLINICAL KETOSIS IN LACTATING DAIRY-COWS

A total of 1010 dry cows and pregnant heifers was randomly selected fr om 25 dairy farms near Guelph, Ontario, Canada to receive either a con trolled-release capsule of monensin or a placebo at 3 wk prior to expe cted calving. Serum samples were obtained at the time of treatment adm inistration, and both serum and milk samples were collected at wk 1, 2 , 3, 6, and 9 postcalving. The threshold used to define subclinical ke tosis was selected a priori at a concentration of greater than or equa l to 1200 mu mol/L of beta-hydroxybutyrate. Using this threshold, the prevalence and incidence of subclinical ketosis were significantly red uced (50%) by monensin treatment. The duration of subclinical ketosis for cows that had been treated with monensin was also shorter than tha t for cows treated with the placebo. Monensin treatment significantly reduced the incidence of subclinical ketosis when the threshold was de fined using higher concentrations of serum beta-hydroxybutyrate (1400 and 2000 mu mol/L). In addition, monensin significantly reduced the pr evalence of positive milk ketone tests.