Year-round access to good quality forage is a physiological priority for ruminants and an economic priority for farmers. Ensiling is a method for preserving moist crops based on organic acid production by lactic acid bacteria under anaerobic conditions. However, silage can be a vector for undesirable microorganisms, impairing crop preservation, animal performance or the health of both animals and humans. Major problems in silage processing are due to failure to apply good manufacturing practice (GMP). To improve silage preservation and guarantee the quality of this animal feed, silage additives such as chemicals, enzymes and bacterial agents can be employed. The purpose of the present paper is to review existing information on the desirable and undesirable microorganisms involved in silage processing, and possible methods, using GMP or silage additives, of limiting silage degradation and its economic effects and health impact on animals and humans. (C) 2013 Elsevier B.V. All rights reserved.