I have seen lambsquarters send out new shoots on the side, when they are mowed off.  I would not be surprised if marestail does the same thing.  Since you have already mowed it off, you will soon be able to tell us whether you have controlled the marestail, or whether they will start new shoots. 

If they start and send out new shoots,  I would expect these to be harder to kill.  While you don't have as many leaves, you have do nothing to the root system.  And the longer that plant is alive, the hardier that root system is getting.  If you are going to spray it, you need to let that plant get enough leaves on it, so that chemicals like 2,4-D & Powermax have some leaf surface to be adsorbed into the plant.