Abstract: A structural element (2), suitable for use in the manufacture of a release device for intraruminal administration of an active agent to a ruminant animal, is of a compacted material, which material comprises a mixture of powdered iron, graphite and optionally copper, the graphite being present in the mixture in an amount of from 2 to 7 wt %, the copper in an amount of from 0 to 5 wt % and the iron in an amount of from 88 to 98 wt %, by weight of the total weight of iron, copper and graphite. A plurality of structural elements (2) may be assembled to provide a structural unit (4) of a release device.

Abstract: A bolus for delivery of pulsed doses of biologically active material to a ruminant comprises a series of distinct and separate bolus elements. Each bolus element is constructed of a degradable outer sheath and a core of biologically active material. The elements are adapted to separate following administration to the ruminant, and to disperse in the rumeno-reticular sac. In due course the elements degrade or corrode to release the biologically active material. Example boli include bolus elements of different respective corrosion characteristics and, as such, are capable of releasing pulsed doses of the active material to the ruminant.