It's sorghum. If there aren't fields of it nearby, would be interesting to know how it got there.

Are the seeds used as part of "hen scratch"? We have some chickens and I deeply mulch their coop with leaves, grass, or whatever, and later I used it when composted around the garden.
There are three such plants growing.

That's probably it. It is used for livestock feed and in bird seed mixes (among other things), so I wouldn't doubt it would be included in hen scratch, too. You can read the package and see if it's listed specifically as an ingredient. This part of your garden will be for the birds!

It will be listed as Milo (Sorghum bicolor). It is a standard ingredient in wild bird feed.