Summarized from chapter 2 of Resistance in Weeds from Herbicide Metabolism, available at https://onlinelibrary.wiley.com/doi/chapter-epub/10.1002/9781119686699.ch2 

Author Dilpreet Riar et al give a clear explanation of how synthetic auxin herbicides, like 2,4-D, work and their beneficial uses. Auxin herbicides copy the action of naturally occurring plant hormones. In small amounts, auxins help plants grow, but in large amounts they discourage growth. This is why applying 2,4-D can eliminate unwanted weeds. One benefit of 2,4-D is that it mainly controls certain plants called dicots while leaving crops like corn, a monocot, unharmed. Because of this, 2,4-D has been an important tool in weed control since it was first used in 1945. Today, it still plays a key role in farming by filling the gap left by other herbicides in weed control.