Disclosed is a bolus comprising a matrix material and one or more biologically beneficial substances contained in and/or on the matrix； wherein the exterior surface of the bolus body is shaped so as to have either one or more relatively wide and/or shallow grooves or depressions and one or more relatively narrow and/or deep grooves or depressions； or at least one groove or depression which differs, in terms of width and/or depth, at different locations along its length； at least part of the groove(s) or depression(s) being coated with a material which 15 retards or inhibits release of the beneficial substance from the matrix, such that the relatively wide and/or shallow groove or depression, or part thereof, allows for relatively rapid uncoating of the bolus and the relatively narrow and/or deep groove or depression, or pad thereof, causes greater delay in the uncoating of the bolus.
