In two separate experiments plastic silos were left opened to air for 3 days and then sealed. The composition of the resultant silages was compared with silages made in silos sealed immediately after filling. In the first experiment Lolium multiflorum of high water soluble carbohydrate (w.s.c.) content (27.3%) and low crude protein (c.p.) content (10.7%) was used and although variations in composition between the inside and outside layers of the “unsealed” silage occurred, both treatments produced well‐preserved silage. In a second experiment using a later cut of similar grass but of low w.s.c. content (10.8%) and high c.p. content (23%), delayed sealing resulted in silages of high pH value with relatively high butyric acid and volatile N contents and low lactic acid levels. Dry matter losses during ensiling were also high in these aerated silages.