Windmill grass (Choris verticillata) has been moving into Iowa in the last few years. This summer I am seeing more of it than ever. This is a spreading, warm-season weed with a light green color. It gets its name from its distinctive seedhead that looks like a windmill. The seedhead will detach from the plant when the seed is mature and it will roll like a tumbleweed and spread its seed to other turf areas. It is also known as tumble windmill grass in some regions.
Roundup will kill it non-selectively, but it is a great seed producer and it will come back. The new herbicide Tenacity (mesotrione) is labeled for it. I have not tried this yet myself, but I hear that it works well if you are persistent. If anyone has experience with this, let me know.
I took the pictures below this morning near Nevada, Iowa. This is typical of where it occurs. I generally see it along curb sides and in compacted area, although it can show up in more open turf areas.