I think I would do anything I can to avoid the soybean hay but if that is your only choice, my thought is to quickly get a sample and send it out for testing.
Don't send it to where they do testing for cow hay - they test for different things with cows.
I use Equi-analytical but I know there are other labs - you may have one in your state that your Ag guy might be able to help with.
Ag guy --- that just came to me - lol lol lol Find out who your Ag guy is and talk to him.
Second, do you have a truck to where you might be able to make a long drive and bring a load of grass hay home? I am assuming you only have one horse and it's on your property?
We have a dually with a full back seat. We can easily bring 30 small squares home and that should last your horse quite awhile.
If someone wants to sell you a roundbale: While I am against roundbales, especially when there's only one horse, I would want to know the hay was stored under cover and I'd go over it with a fine tooth comb looking for excess amount of weeds and smelling for mold.
The best thing would be to store it under cover (set it down on a strong pallet and tarp it), then go thru the agony of cutting it up - probably with a chainsaw.
That would keep your horse from eating himself to death and it would keep a lot more of the roundbale from going bad out in the weather.
Also, we have plenty of hay in Tennessee - any chance you could get down this way? If you could find someone with a truck and even a race car trailer, you could take back enough hay to either get you completely thru the winter or sell part of it.
My husband has a gigundus tool box on his race car trailer but between it and the dually, we can still come home with 125 - 130 bales if it's tied down really good.
If looking for hay down here might be an option for you, PM me with your location and I will see if I can't locate someone. I am in southern Middle Tennessee but I am the Admin on a Tennessee Horse forum with members from all over the state. I feel sure somebody could help you.
Do you get the sense I am really really hesitant on the soybean hay
I didn't even know there was such a critter until I Googled it and gave you that link. I was raised on a dairy farm in Ohio but we couldn't grow soybeans up there, so I know very little about their use