pre m and sandburs

[walleyefisher]

got a problem i have a property with sandburs and today
i treated it with pre-m but after reading a post on here
i think i might have treated it to early

question 1, when is the best time to treat sandburs
in minnesota?

question 2, since i treated it with pre-m today can i
spray for dandelion's in a couple of weeks
and put down some pre-m at the end of
may or is that to much herbicide in a one
months time?

question 3, what is better granular pre-m or liquid
pre-m for sandburs?