Hay bales stored outdoors that do not form a good protective thatch layer can mold up and the dry matter losses can penetrate deep within the bale.
(OSU Beef Team Newsletter)
Forming a Protective Thatch on Your Hay Bales
A thatch forms from oxidation of the exposed outer layer of grasses to sunlight and moisture on the outside of a bale. This layer can be a protective barrier from the elements, protecting the inner contents of the bale.
Factors that INCREASE the probability of thatch formation include:
Fine stemmed grasses
Factors that DECREASE the probability of thatch formation include:
Course stemmed grasses
Woody stems and weeds
*If you have re-seeded a right-of-way with a lot of annuals, it would be best to not store that hay too long outdoors if baled. You could feed it right away or store those bales inside. Hay bales from annual grasses are more prone to weathering and will lose quality more rapidly if stored outdoors due to the course and hollow stems in those grasses.