Paraquat is rainfast in record time

Weed control operations with paraquat in a tea plantation: is it going to rain?Paraquat controls weeds effectively even if it rains within 15 to 30 minutes of spraying. This excellent rainfastness is unique amongst herbicides and gives farmers important practical benefits. More work can be achieved in between showers and weed control will be better than when using alternative herbicides such as glyphosate or glufosinate and surprised by unpredicted rainfall. Paraquat’s rainfastness was immediately obvious from its early development in the 1960s in the plantations of South-East Asia.1 In tropical rainy seasons, as elsewhere, sudden rain can wash recently applied foliar-acting herbicides from weed leaves reducing or eliminating their efficacy.

Experimental evidence

Field experiments in Malaysia have shown the great advantages of paraquat’s incredible rainfastness.2 Long-term weather data indicate that there is a 50% chance of any day’s weed control operations being adversely affected by rain. This can be expensive in terms of extra herbicide and labor needed for re-spraying. In addition, continued weed growth and crop competition in the period before any re-spraying, should it prove necessary, is obviously undesirable. Using simulated rain, paraquat was found to be rainfast within 15 minutes of application, ie no difference could be measured between the degrees of weed control with or without wash-off. Clearly, sufficient paraquat must have been absorbed within that short time for maximum efficacy. However, glyphosate and glufosinate gave poor weed control even when the rain-free period was six hours. 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. Results are shown in Fig.1 Spray.Seed (250 g paraquat/ha) Glyphosate (730 g salt/ha)        
            Fig. 1. Rainfastness of paraquat and glyphosate products on annual ryegrass (Syngenta, 2002).

Practical experience

Rain events can be quite complex: rain-free period after spraying, rainfall intensity and duration are the main features that will affect herbicide run-off from target weeds. A short, sharp shower could have similar effects to a longer period of steady rain. Very light rain can even help by redistributing spray deposits over leaves and stems. Weed characteristics are important too. Leaves are covered in waxes, some forms of which repel water more than others. Those that allow droplets to spread, facilitate faster herbicide penetration due to the greater contact area. However, weed populations are usually a mixture of species with various leaf-types and rainfall events all vary. Typically, over paraquat’s many decades of use, a good guide has been found to be that a rain-free period of less than half an hour is sufficient to achieve maximum levels of weed control.4 Not only is paraquat absorbed into weeds very quickly, but it is very fast acting. This allows any decisions on re-spraying to be made probably within a day of any problems. With slow acting glyphosate, the likely level of control would not be obvious for several days or even a few weeks depending on temperature.

Other benefits

Paraquat’s rapid penetration into weeds has other benefits related to rainfastness. In coastal estuarine rice growing areas of Indonesia paraquat is the only herbicide that can be used effectively in land preparation. There are only a few hours between incoming tides pushing back river water on to the land, washing any applied herbicides from weed leaves. Paraquat is the only herbicide that can be absorbed fast enough to ensure sufficient active ingredient gets into plant cells to give any useful levels of weed control. When used in a no-till


Also known as conservation tillage or zero tillage is a way of growing crops from year to year without disturbing the soil through tillage ie cultivating the soil usually with tractor-drawn implements.

Authoritative On-line References and Resources  A portal for on-line information about no-till farming.
system, preparing the land for planting rice using paraquat takes only seven to ten days on a typical one hectare farm. Traditional systems take one to two months. With paraquat one family can grow crops on two to four hectares, harvesting two crops each year. Rice production can increase by 60% at each harvest or by 120% over one year. Ultimately, thanks to what is effectively rainfastness, tidal rice farmers using paraquat can triple their annual income.


  1. Jeater, R S L (1964). Evaluation of paraquat and diquat for weed control in rubber. Weed Research, 4, (2), 133 – 141
  2. Lim, J L & Ngim, J (1997). Evaluating non-selective


    A chemical product used for eliminating all types of weeds (annual and perennial grasses and broadleaved weeds).

    Authoritative On-line References and Resources An invaluable source of contemporary information about herbicides and weeds from Iowa State University.
    herbicides for rainfast properties in Malaysia. The Planter, Feb 1997
  3. Syngenta Australia technical bulletin (2002)
  4. Bromilow, R H (2004). Paraquat and sustainable agriculture


    Management and conservation of the natural resource base and the use of technological and organizational change in a manner that ensures continued agricultural production from the land for present and future generations. Such practices conserve land, water, and plant and animal genetic resources. They are environmentally non-degrading, technically appropriate, economically viable, and socially acceptable. Sustainability rests on the principle that we must meet the needs of the present without compromising the ability of future generations to meet their own needs. Therefore, stewardship of both natural and human resources is of prime importance.

    Authoritative On-line References and Resources Information from the USDA's Alternative Agricultural Systems Information Center.
    . Pest Management Science, 60, 340 – 34


The brand name of the leading paraquat product is Gramoxone. Spray.Seed contains a mixture of paraquat and diquat