Kansas, I think that is Beefstake (aka Perilla). Some of them will have a lavender tint to the leaves and some are just green like the ones in your picture. There is also a lot of variation in teeth on the leaves of this species.
Here are a couple of links with pictures for you to look at:
The aggravating thing about Perilla is that you won't even see it until about this time of year, and it all doesn't germinate at once. You will feel like you have it knocked out pretty good, but when you come back to the plot a week later the stuff is everywhere again.
At this time of year I want to get plots planted and/or do some final maintenance and get out, but that stuff won't let it happen. Like you said, it pops up quickly. And I might add that it can make next years seed crop quickly too.
No, never seen a deer touch it. The few sources I can find say the same thing.
The reason there is so much variation in the species is because in Asia they have propagated it as a potherb for centuries, so lots of different cultivars exist. But, I found another source that says the USDA has identified 3 different toxins in it that were toxic to cattle and horses. I just wish they had kept it in Asia............