From Wikipedia:
2,4-D was developed during World War II by a British team at Rothamsted Experimental Station, under the leadership of Judah Hirsch Quastel, aiming to increase crop yields for a nation at war.[citation needed] When it was commercially released in 1946, it became the first successful selective herbicide and allowed for greatly enhanced weed control in wheat, maize (corn), rice, and similar cereal grass crop, because it only kills dicots, leaving behind monocots.
[edit] Mechanism of herbicide action
2,4-D is a synthetic auxin, which is a class of plant growth regulators. It is absorbed through the leaves and is translocated to the meristems of the plant. Uncontrolled, unsustainable growth ensues causing stem curl-over, leaf withering, and eventual plant death. 2,4-D is typically applied as an amine salt, but more potent ester versions exist as well.
Normally recommended 3 weeks before planting grass.