The dietary availability of carotenes to the animal depends in part on the relative proportions of different isomers and these may alter during ensiling. The proportions of the more available ,%carotenes are higher in well preserved silages than in clostridial silages. Under anaerobic conditions only small losses of carotenes occur with acidtreated silages but the provitamins are degraded rapidly in some fodder plants in the presence of both acids and oxygen. The presence of nitrites in silages is considered to increase destruction of carotenes.