It is now 90 hours ( 4 days and a bit) since Lad ate some cubes from that bag which had a weasel carcass in it. There have been no symptoms of botulism or other ill effects.
My vet tried to find a source of antitoxin in case botulism developed. There is none available in Canada. The only source found was in New York state, at a price of $3000 per vial :/ No way. Even if it were affordable a courier would have taken too long to reach this somewhat remote area.
Lesson learned: even if your human nose tells you the food smells OK, believe your horse when he snorts at it and hesitates to eat.
We seem to have dodged the bullet. (Thank you, Lord.)
Bonnie and Lad