Did you use a commercial potting soil? I make my own indoor gardening systems and I will run into this often when using cow manure. Cows eat feed, often composed of what you and I would call weeds, but cows call grass, lol. This is likely then a seed that was buried in the bag when the tractor scrapped up the poop. Cow poop is one of the most perfect growing mediums you can have. But it does come with hitchhikers, lol.

Letting it grow out until you get mature leaves will help you ID it. Then you can go to the USDA website and know what it is. You said you live in Michigan? Most likely it is a plains "weed" of some type.

Please do keep up the good work- fun to watch stuff grow!!!

I used 'black dirt' sold by a neighbor about a mile or 2 away. Though the cow manure thing you mentioned would make sense in this situation since I presume that's something they use to produce their black dirt. Things I plant in that stuff tends to kick a lot more butt than stuff growing in the surrounding unfertilized dirt. Didn't know about the USDA website. Gonna have to look into that~

Yup, this is manure based then. Gonna pick up a few seeds now and then. Once they get to this pint, it will be obvious they are not what you planted. Painless enough to yank or leave them. Works either way because you harvest by hand. Fun to watch weeds grow- something to be said for mutts!!!!