my old forage supplier had his tested repeatedly one year, and after 3 weeks there were no changes... so i will use it when it is 4 weeks old. never had a problem so far.
I am using mine now. As long as its dry its fine. It'll be more 'potent' than if you left it 2 months or whatever but it is still usable. Its usable from the day its baled if dry (which it should be).
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I think you are supposed to leave newly cut hay at least three months before using it.
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Bugger that. Personal preference of course.
I have the advantage of making my own hay, it comes off the field beautifully dry and cool, doesn't heat up and hasn't killed any of my horses in the last three years.