Botulinum toxin is the neurotoxin protein created when botulism spores grow. The requirements for growth and/or for keeping the toxin from denaturing would seem to be very difficult to create in bale of hay.
There are well documented incidents of botulism in horses who are eating hay, all the references I found were centered around hay in large plastic wrapped bales.
I have been involved in an event where the presumed DX is botulism secondary to ingestion from hay from last year. In this case the hay is second cutting timothy hay (making it 6 plus months old), in rectangular 50 pound (22 kg) bales that are not wrapped, have been barn kept, and when purchased from the vendor where stacked in rows 4 feet wide, by 6 feet (2 meters) high. The bale then spent more then a week on an shelf (chrome platted heavy wire) with good air circulation.
Three pet house rabbits appear to have been effected. with 2 dead and one recovering from a case of descending paralysis. Several tests are underway, but my understanding is that it may be difficult to conclusively show either the botulism in the rabbits, or significant presence of 'botulinum toxin' or 'clostridium botulinum' in the hay (samples from the mangers of effected animals are being tested).
Is it possible to logically conclude that Botulinum toxin could, or could not, be created and/or survive in a bale of hay meeting the above criteria?
If there is an awareness of a plant based neurotoxin that would cause similar symptoms that may be growing in the hay field please offer that in an answer at Gardnening.