In opposition to cases of acute intoxication by mycotoxins, the risks of chronic infections by repeated ingestion of small amounts are frequent. The economic consequences in terms of losses in productivity and of animal health are difficult to estimate, but potentially important. Various types of moulds can contaminate the forages, on the fields, during conservation (as hay or as silage), or during their consumption. Their development is favoured by certain climatic conditions or the conditions of the conservation and may diminish the feeding value of the forage and/or produce mycotoxins potentially dangerous to the animals' health. The methods of preventing the exposure of livestock to the mycotoxins are to be followed at all the stages of the production. Apart from keeping good conservation practices and from applying chemical silage additives, there exist methods based on the use of biological agents that have been tested with success.