Can botulinum toxin be grown or kept from denaturing in an UNWRAPPED 50 pound hay bale?
Clostridium botulinum toxin is present ubiquitously in soil. As such it is more than plausible that hay bails, which come into contact with soil can and I should expect almost probably will be infected with these bacteria. However it is not the mere presence of the bacteria itself which causes poisoning, it is the toxins they produce when the appropriate conditions are met. These toxins are produced as a consequence of spore formation. 

Traditionally in human cases, the presence of toxins has been linked to home canned foods, where there is an anaerobic environment with a ph &lt; 4.5 (http://cid.oxfordjournals.org/content/41/8/1167.full). It is conceivable that such an anaerobic environment exists within a hay bail (even if not wrapped). The following site from the ontario minestry agriculture and food (http://www.omafra.gov.on.ca/english/livestock/horses/facts/info_haylage.htm) suggest that the pH certain methods of hay bailing have pH &lt; 5. This would seemingly confirm hay as potential source for botulinum toxins.

With regards to testing the hay for C. botulinum I would expect there to be a very high chance of finding this, due to soil contamination, however this does not imply this was a definite source for poisoning. A more conclusive test would be for the toxin. So in summary it is logical to suggest that hay could be the toxin source, but this does not mean it was the source.