I expect that there are different kinds of late-stage hay.
Around here, a late-stage hay is grass that started growing in February, and was cut in May or June, because of the rain. It has thicker stems and lower protein, but it has had more than enough water, and is still green and growing when it is cut. It will have flower heads, but may or may not have actual seeds. It is perfectly palatable to my horse, and almost all horses. Though the couple of times my mare has gone on a food strike, it has been over late stage/ first cut hay. But I think that had to do with the variety of grass, not the stage. It was never about timothy, but rather random local grass from a small delivery guy.
I’ve stopped feeding mature/first cut hay for a couple of reasons. It tends to be low-protein. Plus I expect that the higher ADF also sequesters some of the nutrients, making them less available. Also, our late stage/ first cut local grass hay can be off the charts in NSC. And shipped-in late stage/ first cut timothy costs almost as much as second cut. This timothy tends to be very low in both NSC and protein, which some folks need.
But it isn’t bad hay by any means, and the horses eat it.
Now, if the hay was truly mature, gone to seed, gone brown standing in the field, if it was harvested at the end of a long dry summer, that might be another problem.
We are perhaps lucky here to have one climate belt on the coast, and another one a couple of hundred miles inland, plus the open prairie within commercial shipping distance over the mountains, so there are a variety of hay regions and bad weather (at least so far) hasn’t struck all of them in the same year! Plus we have a mix of local, small scale farmers selling out of their fields or delivering for a fee, the feeds stores buying commercially produced hay that they will deliver, plus various hay shipping companies running commercially with their own trucks. The price differential between wholesale hay in the interior and small-lot resale on the coast makes this an attractive proposition for the trucker middlemen!
So while you can certainly find nasty hay that horses won’t eat, or on the other hand premium hay that’s insanely expensive, if you know what you are looking for, you can also get gorgeous hay that suits your horse’s needs, without going broke.
And, of course, any timothy that finds its way down to the coast is going to be good hay of its kind (either first or second cut), because it will be commercially grown, and then bought by a middleman (feed store or hay dealer) who is going to be picky, since he has to sell it on. Maybe there is nasty timothy sitting in a leaky barn up in 100 Mile House, but it isn’t going to turn up in our local feed store. The problematic hay I’ve run across has all been local, small producer, hay. I still don’t know why maresy went off that one batch so emphatically. If I rightly recall, it might have been rye grass (or advertised as such), and perhaps it was too high in NSC or fructans and was giving her gas. Or maybe in reality it was just a stand of canary reed grass someone has cut and baled, and it was giving her impaction. Nothing wrong in terms of colour, smell, etc, and the horses I sold it on to ate it fine. One of life’s mysteries. But I do listen; if my hoover-everything mare won’t eat her hay, she gets new hay.