This invention belongs to the area of conservation of forage. Specifically, that is possible to use in laboratory works to determine the quality of test silages. The purpose of invention is to quarantee forage ensiling properties by using big bale plastic film, wrapping it over the jar mouth and thereby ensuring anaerobic conditions in the jar. In order to obtain the purpose, closing of laboratory test silos by big bale plastic film is worked out. The method is based on wrapping the big bale plastic film 3 over the mouth 2 of glass jars 1, film which is previously lenghtwise line divided into halves, ensuring sufficient anaerobic conditions and gases exchange between the silos and environment. There is possible to use this method imitating like heavy and light as well as easy fermentation conditions, reducing nutrient losses and ensuring opportunity to regulate the fermentation processes.