Roundup® (a trade name used by Monsanto) and other herbicides based on glyphosate (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.
Glyphosate-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 HowStuffWorks article 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, glyphosate gums up enzymes in plants to kill them. Glyphosate kills plants like antibiotics kill bacteria.
If you've been following farming news or the genetically modified food debate, you know that glyphosate-tolerant seeds are now available -- you can buy genetically modified corn, soybeans, etc. that are immune to glyphosate. These plants produce an enzyme that performs the same function as EPSP synthase but is not inhibited by glyphosate.
The question of safety is a hard one to answer because there is a lot of polarized and conflicting information. Here are a few things we can probably say with some certainty:
- Given the amount of glyphosate sprayed on the planet every day, it is probably safe to say that glyphosate 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 glyphosate (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.