There are herbicides for weeds only and those for grass and weeds; how do the chemicals make a distinction between the two?
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Every living thing has a specific “pathway” of how chemicals are converted into useable energy. These processes typically use very unique types of protiens. So if you can target the protiens that exist in one type of weed (typically broadleaves like dandelions) and not the grasses, then you can kill the weeds not the grass.
Edit: there are other types that work differently but the most common used ones are the ones I described.
It’s not that the chemical itself is making a distinction, it’s that it’s only bioactive in certain plants. It’s the same thing that lets us take antibiotics which kill bacteria but don’t harm us: the molecule is only harmful under certain conditions. We can take advantage of slight differences in the way different organisms perform important chemical processes that are vital to their survival to target one specific organism over another.
So, the herbicide interrupts a critical reaction in the weed which kills it, but has no affect on the plants we want to save.
And sometimes it’s simpler: a weed may have more surface area to absorb the herbicide (broad leaf), whereas a blade of grass has less. In one case the dose is toxic and the other case not, or less so.
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