Monensin sodium is widely used to manipulate ruminal fermentation (Bergen & Bates, 1984), with the aim of increasing energy supply to the animal. Monensin has been most widely used in diets of beef cattle and young growing dairy cattle, particularly in confinement management systems, where the ionophore can be mixed directly into the ration. Accurate daily dosing of grazing ruminants with small quantities of rumen modifiers, such as ionophores, proved particularly labour-intensive and expensive until the development of the controlled-release capsule (CRC; Laby et al. 1984). Subsequently a commercially available CRC containing monensin sodium was developed (Elanco Animal Health, Cambridge, NZ) facilitating its use in grazing animals. It has also been tested as a method of reducing the incidence of bloat in lactating dairy cows grazing pastures containing legumes.