Final answer:
The sudden death of a bull with black, bloody discharge from all orifices and incomplete rigor mortis is most likely due to Anthrax, which is caused by the bacterium Bacillus anthracis.
Step-by-step explanation:
The most likely diagnosis for the bull with a black, bloody discharge from all orifices and an incomplete rigor mortis despite being dead for a day would be Anthrax. Anthrax is a zoonotic disease caused by Bacillus anthracis, a gram-positive, endospore-forming bacterium. The symptoms described, such as bloody discharges and incomplete rigor mortis, are consistent with the hemorrhagic presentations seen in anthrax infections in livestock.
Other diseases like botulism and tetanus, caused by Clostridium botulinum and Clostridium tetani respectively, do not typically present with such symptoms. Moldy sweet clover toxicity leads to bleeding disorders but does not present with black, bloody discharges from all orifices, and grass staggers, or hypomagnesemic tetany, causes neurological signs such as muscle tremors and seizures, not the symptoms observed in the bull.
Final diagnosis of anthrax can be confirmed through collection and analysis of blood samples or by examining sudden death without typical signs of diseases in livestock.