Can you plant them in the fall? Or is it better in spring
I believe they can be planted in the fall, they are killed by a frost and have no set number of days until they stop growing.
Here, we plant them in the Spring to get the most forage out of them. But, unless you have water, they may burn up in the summer there.
Eagle Brand seeds That is what I've planted and the deer keep them mowed down. I'm sure that there are other places to get less expensive indeterminate soybeans (otherwise known as forage soybeans) vs. grain type soybeans which are determinate (grow for "X" number of days).
I haven't searched enough to find such a place.