What Is the Difference Between Glyphosate and Paraquat? A Detailed Comparison of the Two Herbicides
Glyphosate and paraquat have one major similarity and five major differences.
One major similarity. Both glyphosate and paraquat are non-selective herbicidal herbicides and are highly effective against stubborn weeds.
The first of the five differences: they kill weeds by different mechanisms.
Glyphosate is an organophosphorus systemic herbicide. It inhibits the activity of certain enzymes within the plant, thereby inhibiting amino acid metabolism. Amino acids are the most important building blocks of proteins; when amino acid synthesis is blocked, protein synthesis is disrupted, which leads to the plant's death. In addition, green plants absorb glyphosate through their stems and leaves, and it is then translocated throughout the whole plant to take effect.
Paraquat is a fast-acting contact non-selective herbicide with a strong destructive effect on the green tissues of plants. It takes effect within 2-3 hours after spraying, and in sunlight the effect is immediate. This herbicide has no systemic translocation effect; it can only damage the part of the plant it touches and cannot destroy the roots or the seeds lurking in the soil, so weeds tend to regrow after application.
The second of the five differences: the timing of action differs.
Glyphosate works best on weeds when they are growing most vigorously. When weeds are small, their leaves are small and photosynthesis is weak, while nutrients from the roots are being translocated upward; at this time less of the herbicide is absorbed through the leaves, and the amount translocated to the roots is very small, so it cannot kill the weeds effectively. Therefore, the best time to apply it is during the period of vigorous weed growth, especially before the weeds flower.
When spraying glyphosate, avoid the hotter times of day so that the solution stays on the leaf surface longer. If the temperature is too high, the solution evaporates too quickly, making it hard to achieve high efficacy.
Paraquat should be applied when the weeds are still small, because paraquat can only kill the above-ground parts of the weeds; therefore it should be applied while the weeds are small, otherwise the roots will keep living and the weeds will easily grow back. Paraquat should also be sprayed at noon on a sunny day. At this time temperature is directly proportional to the herbicide's effect: the higher the temperature and the stronger the light, the better the weed-killing result.
The third of the five differences: toxicity differs.
If glyphosate is swallowed by mistake, within 15 minutes it can cause dizziness, nausea, weakness in the limbs and soreness of the throat. These symptoms are relatively obvious when more than 100 ml has been ingested, equivalent to the amount of two liang of liquor (about 100 ml).
If paraquat is swallowed by mistake, 3 ml can be fatal, and there is no specific antidote; the mortality rate is over 90%. The poisoning symptoms are also obvious, causing multi-organ failure including liver and kidney failure, pulmonary fibrosis and respiratory failure, leading to a painful death.
The fourth of the five differences: market conditions differ.
Glyphosate has now become the most widely used herbicide in the world. In China, however, paraquat's registration and production licence were revoked as early as 2014 and production was stopped, and in 2016 the aqueous formulation was banned from sale and use in China.
The fifth of the five differences: the names differ.
As a little aside, here is a small joke for readers. The names of glyphosate and paraquat are obviously different at a glance, and this is the easiest way to tell them apart.