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. 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 chart to the right shows the progress of control of the tough broadleaved weed 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.
- 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 in countries where it is registered is GRAMOCIL®.
- The brand name for the leading paraquat product is GRAMOXONE®.
- GRAMOCIL® and GRAMOXONE® are Registered Trademarks of a Syngenta Group Company.