Planting into weed-free fields after a burndown treatment gives crops a great start. Although vegetation including weeds, volunteers and cover crops can help protect against soil erosion through the winter, come spring they need removing, and the faster the better.
It is important to get a clean start in no-till and other conservation tillage systems to avoid problems for the new crop, which will reduce yield.
Weeds not only compete with the young crop for nutrients, light and water, but they are also reservoirs for pests and diseases, and can cause blockages in planting equipment.
Paraquat is ideally suited as a burndown herbicide: fast acting, rainfast and controlling a very broad spectrum of winter annuals, including some important weeds often resistant to glyphosate across much of the US, Latin America and elsewhere.
No non-selective herbicide acts faster than paraquat. Effects are often seen within a few hours and weeds can be completely desiccated within a day of application (e.g. in bright sunlight). Glyphosate usually takes one to two weeks to have a similar effect, especially under cool spring temperatures.
Fast action gives farmers important benefits including opportunities for higher yields and better prices through earlier planting; rainfastness in 15-30 minutes; and ease of seeing where weeds have been treated after a break in spraying operations.
Paraquat is rainfast
Paraquat controls weeds even if it rains within 15 to 30 minutes of spraying.2,3 Although paraquat is not systemic it quickly penetrates leaves to reach the site of action in chloroplasts.
The charts below demonstrate the superior rainfastness of paraquat. An experiment conducted in a sophisticated rain simulator confirmed the rainfastness of the paraquat-based product SPRAY.SEED® over glyphosate.3 After spraying, annual ryegrass (Lolium rigidum) was subjected to light rain of 2-3 mm/hour for 30 minutes after various rain-free periods. SPRAY.SEED® was clearly rainfast after only 15 minutes, while glyphosate gave poor control even after 6 hours without rain.
Figure 1. Rainfastness of paraquat (left) and glyphosate (right) products on annual ryegrass (Syngenta, 2002).
Getting the best from paraquat
Paraquat controls a very broad spectrum of weeds, best sprayed when they are between 2.5 cm and 15 cm tall (1”– 6”). These include annual broadleaved weeds and grasses, including many that have glyphosate resistant populations. Glyphosate is a unique herbicide, especially with its ability to control perennial weeds, and must be protected. Using alternative options for burndown reduces the risk of further resistance. In glyphosate tolerant crops, glyphosate can then be reserved for a later selective treatment.
The performance of paraquat can be boosted by the use of an adjuvant (usually a non-ionic surfactant) if this is indicated on the product label. The addition of other herbicides as tank-mix partners can improve activity on certain tough species or add a pre-emergence soil residual element to the weed control.
Photosynthesis inhibitor herbicides, e.g. atrazine, diuron, metribuzin, are especially good partners for paraquat (pre-mixed products with diuron are available in some countries, e.g. GRAMOCIL® Brazil) because they have a synergistic action.4,5 When mixed with paraquat they slow photoynthesis, simulating evening light or a cloudy day.
In full sun, paraquat’s movement quickly stops as leaves desiccate. Spraying paraquat in dull light or in the presence of this type of partner herbicide means slower action, so it is able to move further through sprayed plants before desiccation. This results in a slightly slower, but more prolonged weed control, and is demonstrated by the results of an experiment shown in Fig. 2.5
- International Survey of Herbicide Resistant Weeds.
- Lim, J L & Ngim, J (1997). Evaluating non-selective herbicides for rainfast properties in Malaysia. The Planter, February 1997.
- Syngenta Australia technical bulletin (2002).
- Montgomery, G B et al. (2017). Effect of time of day of application of 2,4-D, dicamba, paraquat and saflufenacil on horseweed (Conyza canadensis) control. Weed Technology, 31, (4), 550-556.
- Norsworthy, J K et al. (2011). Evaluation of combinations of paraquat plus photosystem II-inhibiting herbicides for controlling failed stands of maize (Zea mays). Crop Protection, 30, 307-310.
The brand name of the leading paraquat product is GRAMOXONE®. GRAMOXONE®, GRAMACIL® and SPRAY.SEED® are Registered Trademarks of a Syngenta Group Company.