AIMS To evaluate the efficacy of a biodegradable silage coating for the ability to protect timothy (Phleum pratensa) type silage against spoilage and its quality under natural conditions. METHODS AND RESULTS Triplicate mini-silos of silage were prepared for three treatments (1: uncoated; 2: coated with biodegradable coating and 3: sealed with plastic), two types of storage (unprotected or protected from rain) and 10 sampling times (0, 7, 14, 21, 28, 35, 42, 56, 63 and 70 days postensiling). Triplicate mini-silos were opened at each sampling time for microbiological (total aerobic bacteria, lactic acid bacteria, moulds and yeasts) and biochemical analyses [pH, dry matter (DM), water-soluble sugars (WSC), lactic (LA), acetic, propionic and butyric acids content]. The study showed that at day 70, counts of moulds and yeasts in silages protected against rain and coated with biodegradable coating were 5.98 log CFU g(-1) when compared with 5.92 and 3.62 log CFU g(-1) in samples from plastic-sealed silage and uncoated silage, respectively. The pH was low and stable pH (4.34) when compared with uncoated (7.17) and plastic sealed (8.34) silages (P < or = 0.05). A DM, WSC and LA content of 421.7, 13.4 and 20.9 g kg(-1) was, respectively, observed. For silage stored outdoors, a level of moulds and yeasts of 3.77 log CFU g(-1) of silage was also observed in silages coated with biodegradable coating after 28 days of storage. A stable pH showing a mean value of 4 was also observed. The pH, DM, WSC and LA content were, respectively, 4.18, 341.1, 13.34 and 31.8 g kg(-1) in these samples. After 70 days of storage, the level of moulds and yeasts on silage sealed with biodegradable coating was 7.73 log CFU g(-1). A DM, WSC and LA content of 291.9, 5.56 and 10.0 g kg(-1) was, respectively, observed. CONCLUSIONS When compared with uncoated silage, the application of biodegradable coating can preserve the quality of silage for up to a month when exposed to rain and up to 70 days when protected from rain. SIGNIFICANCE AND IMPACT OF THE STUDY Results emphasize the possibility of the use of a biodegradable coating as an alternative to plastic film for sealing horizontal bunker silos.