Depends on how wet it is.

If we are irrigating our alfalfa and the plants have excellent moisture in the leaves and stems, they can handle 24F for four hours and bounce back.

If they're water stressed, you can see the growing points frost off at 28F four four hours. If the water is on (ie, actively watering), you will see frost damage anywhere the pivot was watering when the temp gets down to 30F. I've put more than one "frost wedge" into a field when I thought it was going to stay above 35F and the temp went down to 29F (or lower).

The same clowns who are telling me what the average temperature will be 100 years from now can't tell me accurately within five or six degrees what the temperature will be tonight, less than 12 hours away.

BTW -- if it is knee high now, that should be some pretty hot hay, unless it is already going to bloom. Here, our hay is knee high at the bud stage, which is California dairy quality.

Given that it is getting pretty late in the season, if it were here on this place, I'd be cutting that. Then again, we're at 6,000' above sea level, and you can get highly variable weather at this time of year. One snowstorm this late, with a big cold pulse behind it, and I might never get the hay baled and off the field. I dunno what your weather patterns are like.

That's me -- I'm not pretending to know how to do hay in your country.