A bolus and method of preparing a bolus comprising a bolus body 2 comprising a matrix material and one or more biologically beneficial substances contained in and/or on the matrix. The exterior surface of the bolus body is shaped so as to have a least one recessed portion. At least part of the recessed portion of the bolus body is coated (4, fig 4) with a material which retards or inhibits release of the beneficial substance(s) from the matrix such as wax. The recessed portion can be in the form of longitudinal grooves 6 or a single concave channel (6, fig 4). A pair of co-operating boluses (fig 5) is also provided.
