Paraquat is glyphosate's bodyguard
The extensive adoption of GM soybean varieties tolerant to glyphosate has led to farmers using this non-selectiveDescription A chemical product used for eliminating all types of weeds (annual and perennial grasses and broadleaved weeds). Authoritative On-line References and Resources http://www.weeds.iastate.edu/ An invaluable source of contemporary information about herbicides and weeds from Iowa State University. herbicide for weed control too much and too often. Although glyphosate is encouraging the continued adoption of no-till, with all the benefits to soil conservation that brings, excessive use is also encouraging glyphosate resistant weeds1. Up to three million hectares in Brazil are now estimated to be infested with five weed species which are no longer controlled by glyphosate.
However, an integrated weed control system has been developed to ensure the benefits of glyphosate can be preserved. This involves continuing to spray glyphosate for burndown, but following just before or just after planting the crop with an application of a paraquat-based herbicide. This contains a second active ingredient, diuron. Diuron is a soil residual herbicide, but at the low rates used in this product it has little or no residual effect, but enhances the activity of paraquat.
Like paraquat, diuron works by interfering with photosynthesis, but in a different way. So-called PSII inhibitors like diuron slow photosynthesis. (Read more about herbicide mode of action in the Knowledge Bank). This means that when mixed with paraquat they effectively simulate a cloudy day. Paraquat’s fast action is well known to be especially quick in bright sunlight. If a sprayed plant is placed in darkness there is no effect. In full sun, paraquat’s movement quickly stops as leaves desiccate. Spraying paraquat on a dull day means that there is less solar energy for paraquat to utilise in destroying plant tissue, so the herbicide is able to move further through sprayed plants because desiccation is slower.
The paradox is that, although paraquat may appear to be initially less effective on a cloudy day, several days later it will be seen to have given a particularly high level of weed control which will persist for longer.
The chart to the right shows the progress of control of the tough broadleaved weedDescription The leaves are "broad" as opposed to the "narrow" leaves of grasses. Also called 'dicots' having two seed leaves, while grasses are 'monocots' having one seed leaf. Authoritative On-line References and Resources http://www.iwss.info The International Weed Science Society. Borreria alata in a Brazilian coffee plantation.
The performance of paraquat at 200 grams/hectare is much stronger and longer lasting when 100 grams/hectare of diuron is added. This effect was statistically significant at all assessment timings.
Mixing herbicide modes of action is helping Brazilian farmers to avoid glyphosate resistant weeds and to grow more soybeans on existing arable land.
- Vila-Aiub, M M et al (2008). Glyphosate-resistant weeds of South American cropping systems: an overview. Pest Management Science, 64, 366–371
- The brand name for this paraquat-diuron mixture is Gramocil.
- The brand name for the leading paraquat product is Gramoxone