How do weedicides identify only weed plants for action?

Question

Weedicids (chemicals) are used to kill unwanted plants which are grown with crop.

Answer 1

Ill agree with that there gal.

It mainly has to do with the small but important differences between a weed and a grain or whatever.

It normally blocks or over stimulates a hormonal growth pathway that is in the weed but is different in the plant.

the "more different" the plant is to the weed the better this works

Answer 2

Monocots are much better at rapidly inactivating excess auxins(weedicide) than eudicots. Since crop plants are monocots, they can resist the auxins while weeds(mostly eudicots) die from excess hormones.