Effect of monensin on blood ketone bodies, incidence and recurrence of disease and fertility in dairy cows

Twelve trials designed to test the effect of monensin on milk production we re carried out at eight different research farms. Data from these studies w ere evaluated for effects of monensin on health and reproduction. Monensin was added to the concentrate starting either 2 wk before (293 cows) or 5 wk after calving (601 cows) for periods ranging 16 to 37 wk. Applied after ca lving, the incidence of clinical mastitis was reduced in the monensin-treat ed animals. Monensin decreased the rate of intramammary infection (approxim ated by a change from below to above 250,000 somatic cells in milk) in firs t lactation heifers by 13%. Indicated by an adjusted incidence rate ratio o f 0.58 (P = 0.03), the case incidence of noninfectious lameness decreased f rom 31% in control cows to 18% in cows receiving monensin before calving. T he time from calving to first service was shorter for cows fed monensin bef ore calving. Monensin reduced calving to conception intervals in cows with endometritis during the pretreatment period. No significant effects of mone nsin were observed for the duration of intra mammory infection (somatic cel ls in milk above 250,000), infectious lameness, endometritis, cystic ovaria n disease, a summary category of 'other diseases,' times from calving to fi rst observed estrus, and from first service to conception.