An alloy for use in the manufacture of boli for adminstration to ruminant animals, the alloy including a major proportion of magnesium, aluminium in an amount of less than 8 % by weight and copper to enhance the rate of the galvanic corrosion of the alloy, the copper being present in an amount of more than 0.5 % by weight. Example boli include a bolus comprising a tubular sheath made from the alloy which surrounds a core component, the sheath and core being galvanically coupled. Boli including the alloy of the present invention provide relatively high magnesium release rates as well as providing an overall higher proportion of available magnesium for assimilation by the animal.