SUMMARYField‐baled hay was incompletely preserved using propionic or formic acids. Results were most satisfactory, although moulding byAspergillus glaucusgroup was still frequent, when die hay contained less than 30% water and propionic acid was used. However, much more propionic acid was needed than in laboratory experiments probably because poor distribution of acid in the bale, resulted in under‐treated pockets of hay where moulds tolerant to propionic acid could grow and then spread to areas that would otherwise have been adequately treated.In Dewar flasks, moulds, particularlyPaecilomyces variotiand theA. glaucusgroup, could spread from untreated hay into hay containing 2% w/w of propionic acid. Addition of 4 or 10% of propionic acid protected adjacent untreated hay from moulding. The concentration of propionic acid necessary to prevent flasks from heating and moulding increased with thickness of the untreated layers and with water content. Redistribution of acid in the bale after treatment was slow and much variation in acid levels occurred even between adjacent 1 cm segments of grass following storage. Spontaneous heating of moulding hay could cause migration of propionic acid in hay, assisting the spread of fungi. A proprietary additive did not prevent moulding at any hay water content tes