Ask Mr. Smarty Plants

Lest you think you are the only one blessed with this noxious weed, please read this previous Mr. Smarty Plants answer.  You didn't say what was in your horse pasture, but we are assuming it is grass of some sort. The problem with any kind of chemical control is that Cenchrus spinifex (coastal sandbur) is also a grass, member of the family Poaceae; something that kills the sandbur will also kill the surrounding grass. 

Actually, the sandbur species mentioned above is more a product of the Southern United States; it is  likely that what you have in Minnesota is Cenchrus longispinus (did you ever hear a more appropriate plant name?). This University of Minnesota Extension website Sandburs by Deborah L. Brown discusses this plant in Minnesota.

Frequently, when we tackle the treatment of weedy invasive plant, about the only advice we can give is to pull it out. We realize this is probably a big task in a horse pasture. One suggestion that we noted was that the sandbur tended to take hold in disturbed, poor soil, and that enriching the grasses you wanted to stay could discourage the survival of the weeds. Learn to recognize it when it has not yet produced the burs, or seeds, so you can pull it out as the opportunity arises and prevent the next generation, at least of that one plant. Sandburs are definitely survivors, with their seeds adapted to cling to clothing, fur, shoes and skin!  Extra nourishment, high nitrogen fertilizers, and perhaps more moisture can help to enrich the good grasses and crowd out the bad ones.

Cenchrus spinifex

Cenchrus spinifex