I agree with the others, we try to keep a smorgasbord available to keep them interested and to have something year round.
Our staples are clover and chicory in the spring, kale/turnips (rape) and oats in the fall.
If I was only going to plant one thing in the fall, it'd be oats.
One more tip, use an exclusion cage to see usage, we stake down a 24" diameter x 3 foot tall cylinder, this will really tell you whats going on. Otherwise you might think your crops didn't do well when they are actually getting eaten.
Here's some oats from last season: