Roundup (a trade name used by Monsanto) and other herbicides based on glyphosphate (the generic name) are probably the most commonly applied weed killers in use today. These herbicides are used by everyone from farmers to foresters to gardeners to biologists trying to control invasive exotic plants.
Glyphosphate-based herbicides all work on the same biochemical principle -- they inhibit a specific enzyme that plants need in order to grow. The specific enzyme is called EPSP Synthase. Without that enzyme, plants are unable to produce other proteins essential to growth, so they yellow and die over the course of several days or weeks. A majority of plants use this same enzyme, so almost all plants succumb to Roundup.
If you have read the article entitled How Cells Work, you know a good bit about DNA and how it produces enzymes. In the same way that many antibiotics gum up enzyme production to kill bacteria, glyphosphate gums up enzymes in plants to kill them. Glyphosphate kills plants like antibiotics kill bacteria.
If you've been following farming news or the genetically modified food debate, you know that glyphosphate-tolerant seeds are now available -- you can buy genetically modified corn, soybeans, etc. that are immune to glyphosphate. These plants produce an enzyme that performs the same function as EPSP synthase but that glyphosphate does not inhibit.
The question of safety is a hard one to answer because there is a lot of polarized and conflicting information. Here are a couple of things we can probably say with some certainty:
- Given the amount of glyphosphate sprayed on the planet every day, it is probably safe to say that glyphosphate is not violently toxic to people or animals. People do not have the same enzymes in their cells that plants do, just like human cells and bacteria differ enough that antibiotics kill bacteria cells but not human cells
- On the other hand, most people react badly to glyphosphate (and other chemicals mixed with it) when ingested or applied to the skin, so you want to avoid any contact with the chemical.
- Roundup will kill almost any plant, including aquatic plants, so you want to be sure to avoid spray drift onto other plants or into water. Any pesticide should be applied carefully.
Here are several interesting (and often conflicting!) links that cover a wide range of opinion on glyphosphate: