Besides the method proposed by AchingBack I would suggest the following test:
1/ Try to find out which fuse is feeding a horn signal by removing one by one;
2/ With the horn fuse removed try to start an engine.
Usually horn circuit is separate from a starter circuit (at least in my tractor).
If an engine could be started with the battery well charged, it could mean you have a horn issue, which could come on after long beeping. If an engine couldn't be started, it could be battery failure.
That would help you to determine either you have a battery issue, or a tractor electrical issue.
Keep us informed on the results.