REVIEW: Monensin for Grazing Dairy

The results of 14 research studies indicated that grazing dairy cows fed monensin respond similarly in milk yield and milk composition to dairy cows fed in confinement. When compared to controls, grazing cows receiving monensin averaged 0.9 kg/cow per d greater milk production. Milk fat percent was 0.1% less and milk protein percent was 0.05% less; however, daily yield of milk fat and milk protein were slightly greater for grazing cows fed monensin compared with controls. Results from a study in which pasture was supplemented with a partial total mixed ration (TMR) suggest the effects of feeding monensin pre-calving carry over post-calving in early lactation. Feeding monensin increased yields of milk and protein without affecting 3.5% fat-corrected milk production, reduced the loss of body condition score, and improved percentage of pregnancy. The effects of monensin may be related to an improved energy balance of lactating dairy cows fed pasture supplemented with a TMR. Monensin was initially approved for dairy cows in the United States only when included in a TMR. Monensin is now approved for component feeding systems in which cows are fed in a tie stall barn or milking parlor, or as a