New Question wrote:
We know a person that puts a certain kind of fertilizer on his lawn. The
fertilizer kills what he calls "crab grass" and turns it yellow-brown, while
the rest of the lawn is still green. So this guy always has a lawn full of
large, round, yellow-brown spots. He claims he must kill the crab grass
this way, then plant new grass seeds in the yellow-brown spots.

What is the best way to handle this "crab grass" problem?

Crabgrass grows where regular grass won't, or can't. Make sure you have
healthy grass.

Fertilizer/crabgrass preventer is part of this.

One overlooked part of this is proper mowing. Cut your grass no lower
than 3" in crabby areas; this will prevent the crabgrass from getting
sun. Also, make sure your mower blades are sharp, so your real grass
isn't stressed as much by mowing. Mow often enough that you don't end up
with extra-long grass thatch (or use a mulching mower), but try not to
mow more often than necessary either.