I, like allot of people, have hay that is stored in a barn and hay that is stored out in the weather. For those that cut there own, they know the nutritional value goes down, each and every time it is rained on, after it is laid on the ground, as with all crops. The cattle producers experienced enough, that buy in hay, want to know the hay producer personally, or have test data at hand, to ensure the quality.
Does anyone that has their hay tested, pull their specimens from the outer layer of hay, that has been rained on? I don't, nor do i think anyone else does. If they did they would find that it has very little or no nutritional value whatsoever. It's kind of like eating celery. It takes more energy to chew and digest than will ever be gained from eating it. So why force your animals to try and survive on it. I for one do not. This is a cost trade off for not having a barn to store it in.
The barn kept hay is another story altogether, as if it is rained on it is usually not stored inside for fear of spontaneous combustion. I keep and feed this hay to my weaned calves be cause it is generally more nutritious.