It apparently has been bouncing around it my subconscious that something was off...I think with a moldy feed problem you are more likely to be dealing with a fungal toxin rather than botulism. The bacteria that cause botulism require an oxygen-free (anaerobic) medium to grow, which is why it is such a concern with canned foods.
With chicken feed, however, mycotoxins produced by Aspergillus, Fusarium, and Penicillium spp. of fungus are relatively common...especially Aflatoxin. Aflatoxin mainly attacks the liver and it's the liver damage which then gives you the neurologic signs and general weakness. Treatment is mainly supportive: remove the affected feed, supplement with vitamins incl selenium, and increase their protein intake by at least 1% temporarily.