Hello,

I think this is common cottonwood tree seedlings, Populus deltoides.

https://pestid.msu.edu/weeds-and-plant-identification/common-cottonwood-populus-deltoides/

Repeatedly mowing them will eventually kill them. 

 If these seedlings are sprouting off a root from a mature cottonwood tree nearby, this will happen each season. If these seedlings came from seed, next spring when you see the cottonwood seeds (white bits of fluff blowing about in the air) floating about you can try a pre-emergent herbicide labeled to control crabgrass and some broadleaf weeds on the grass. Apply it according to the label on the package. This may prevent tree seed from sprouting— I could find only one reference to using pre-emergent this way, and it was for ash trees not cottonwood. So you would have to ‘try it and see if it works for you’. Caution- it may also prevent grass seed from sprouting, so don’t apply a pre-emergent if you plan to apply grass seed in the 6-8 week period the pre-emergent is effective. Most pre-emergents are applied to lawns to prevent crabgrass, and so are also known as crabgrass preventer. 

Another concern is, if you are near any open water(lake or river), do not use any herbicide that is not labeled for use near water.

If you can get the grass to fill in and grow thick, you will have fewer weedy plants sprouting. For articles on growing thick lawns see —-

https://www.canr.msu.edu/home_lawns/

Here is a link with information on cottonwood trees so you can determine if a mature cottonwood is nearby(within 100 feet the roots could reach your lawn). Their leaves dangle and twist in the breeze, giving the tree a sparkling look in the sun:

http://dendro.cnre.vt.edu/dendrology/syllabus/factsheet.cfm?ID=64

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