Disclosed is a bolus comprising: a bolus body comprising a matrix material andoneor more biologically beneficial substances contained in and/or on the matrix；whereinthe exterior surface of the bolus body is shaped so as to have a least onerecessedportion； and at least part of the recessed portion of the bolus body is coatedwith amaterial which retards or inhibits release of the beneficial substance(s) fromthematrix.
