Weed control

The labels of paraquat products must be consulted to determine whether an adjuvant should be added to the spray tank, or whether their use should be avoided.
Surfactants (wetters) are often included in the formulated product and any additional adjuvants are unnecessary and may cause problems. On the other hand, some products may benefit from a tank-mix adjuvant and options will be listed on the label.
Foliar herbicides like paraquat can face challenges on the surface of leaves and in reaching places where they will be most effective. Leaves are covered with a waxy cuticle, which forms a considerable barrier, especially to water-based sprays. Quite simply, wax repels water.
Crystalline wax as shown in the image, is particularly repellent. Spray droplets are prone to bouncing-off leaves or sitting on the top of wax crystals and not wetting the surface - as essential for good uptake. Once inside a leaf, there are further barriers to movement. Surfactants and other adjuvants are used to help overcome these barriers to effective weed control.1
What are adjuvants?
Adjuvants are products with biological, chemical or physical effects, which aim to improve herbicide performance and are increasingly used with other crop protection products too.
What are the essential things farmers need to know about spraying paraquat? Paraquat is a fast-acting, broad spectrum, post-emergence herbicide. The label of paraquat products must always be read before use. However, farmers have basic questions such as: how big should weeds be; should another herbicide be tank-mixed or an adjuvant used; will rain after spraying affect activity; can paraquat help fight resistant weeds? 
This article outlines the main practical aspects of using paraquat for weed control.
Safety before, during and after spraying
Safety is paramount.
Secure Storage
Always store agrochemicals in their original containers and under lock and key, to restrict access to only those people who need to use them and to keep out children, non-users and domestic animals. Never decant pesticides and leave them in unmarked containers.
There are five Golden Rules for safe use:
Exercise caution at all times
Read and understand the product label
Wash after spraying
Maintain the sprayer
Use personal protective clothing: wear a long-sleeved shirt, long trousers and waterproof footwear for spraying, plus eye protection and gloves when handling the concentrated product
Appropriate disposal
Herbicides such as paraquat are now more important than ever as people who were raised in the countryside now choose to live in towns and cities.
In Europe and North America, urban populations have exceeded rural ones for decades. As a result, no longer able to rely on an abundant supply of labor, farmers have adopted new technologies and production has had to intensify to ensure that crops are adequately cared for in order to produce enough food.
Now, in Asia, urban populations are about to overtake rural ones and in less than 20 years Africa will follow suit (Fig 1).1 A recent paper in the journal Pest Management Science has reviewed the increasingly important benefits herbicides bring as the whole farming industry strives to ensure food security for all.2
Herbicides for weed control have been one of the key features of more intensive food production. Weeds compete with crops for the resources essential for good yields: light, water and nutrients. They may also attract pests and diseases, make crops difficult to harvest and contaminate foodstuffs with their seeds. Without effective weed control crops cannot make best use of fertilizer, further limiting yield potential. In fact, it has been estimated that weeds still deprive the equivalent of one billion people of food.3
Volunteer plants – those that grow from seed shed by the previous crop – are weeds that bring the same problems as wild ones, or worse.
Volunteers can form a ‘green bridge’ from one crop to the next carrying insect pests and fungal diseases. Volunteers reduce yields and quality, and hinder crop management. They can be difficult to control, especially if they are growing in a new crop of the same species.
Options for controlling volunteers before planting the next crop can be particularly limited if they carry a herbicide tolerance trait to glyphosate, glufosinate or both. However, recently reported research has shown that paraquat boosted by the addition of a PSII inhibitor herbicide provides a good option to control volunteer corn (maize) before a new crop is planted1. More information about PSII inhibitors and herbicide modes of action can be found here.
Volunteer problems
Concerns that 2012 could be a bad year for volunteer corn were raised in the southeast US last year when lodged crops and late harvests from flooded fields pointed to significant grain loss2. This season there have, indeed, been reports of widespread problems in the US3,4.
Controlling weeds in potatoes is a problem for many farmers. Paraquat is a very useful herbicide for potatoes because it can be used to burndown weeds from before planting right up to the point at which the potato sprouts start to emerge from the soil1. This allows for weed control measures to be tailored to the weeds present. As potatoes may take up to four weeks to emerge, significant weed growth may have occurred after planting. Weeds can quickly overrun potato crops if they are allowed to grow unchecked before the crop leaf canopy closes.
They compete with potato plants for light, water and nutrients, so if they are not controlled weeds reduce yield and quality. In addition, large weeds entangle the crop and interfere with harvesting. Weeds may also encourage pests and diseases. Filling the crop canopy, they restrict air flow, increasing humidity under which fungal diseases, especially potato blight (Phytophthora infestans) thrive. Blight can devastate potato crops so intensive use of fungicides is needed to achieve control.
Paraquat is inactivated and immobilised immediately on contact with the soil, so cannot affect the seed potatoes, or later, the developing crop of tubers.
Potato Facts2,3 Over the past 20 years potato production has doubled in developing countries.
Lower costs and favorable environmental effects when using paraquat: these were the key conclusions from a four-year comparison of weed control methods in bananas, recently completed by the University of the Philippines.
The trial was conducted on a commercial plantation on Mindanao, the large southern island of the country. Bananas make a significant contribution to the economy of the Philippines being a major fruit export. New plantations are being established on more sloping land where soil erosion poses a real threat to sustainable production. Filipino authorities estimate that 623 million tonnes of soil are lost annually from 28 million hectares of land.
Erosion can be reduced by maintaining a vegetative cover on the soil. So, it is important to manage weeds so that they give as much cover as possible to help avoid erosion, but not to allow them to compete with the crop. Two weed control programs were compared. In the field trial, traditional methods of hand weeding and slashing with a machete called a bolo were compared to a monthly spray program in a sequence of two applications of paraquat followed by one of glyphosate. In the latter two years of the trial, these regimes were modified to match changing patterns of weed growth, and weeds were controlled only by paraquat when a threshold of 30% ground cover was breached.
Paraquat in Samoa is not only helping farmers to provide a staple food but is also enabling taro to become a very important export crop.  Ever wonder what fuels the impressive size and power of the awesome Pacific Island rugby players? Look no further than taro. Taro is a tropical starchy root crop which is a staple food in many subsistence communities, particularly in the Pacific islands. And while the rugby team members most likely have a broad selection of meals from which to choose, for many people living in these areas taro is an essential part of their diet.
As a tropical crop, protecting taro from weeds is critical. Weeds thrive in such hot and humid climates, robbing yield. Hand hoeing crops is one option, but, not only is this time consuming and labour intensive, but it means other opportunities, particularly for education in poorer communities, may be missed. Using the right choice of herbicide provides an effective alternative. Weed control is critical in dryland taro, particularly in the first 3 months. Weeds thrive in such climates and a huge range of annual and perennial species have to be controlled by hand-hoeing or by spraying herbicides. 
Other facts about taro Read more about the use of paraquat in taro here
Read an in-depth article about taro cropping here
Weeds do not hit the headlines like droughts, insect plagues or even swine flu, but cause substantial human misery, quietly and constantly, notes one of the UN Food and Agriculture Organisation’s (FAO) most renowned weed experts. Ricardo Labrada-Romero quotes figures produced by leading environmental organisation Landcare Research (New Zealand) which indicate that uncontrolled weeds cause crop losses equivalent to 380 million tonnes of wheat every year. When FAO reckon that more than one billion people in the world are hungry, this has a huge impact.
In terms of farmland needed to grow such a quantity of wheat, using a worldwide average yield of 2.8 tonnes per hectare (FAO, 2007 estimate), this equates to a staggering 135 million hectares (335 million acres), an area twice the size of France, wasted for food production.
Weeds cause most crop losses*
Equivalent Wheat Land Lost
                               (million ha)
Weeds                         135
Diseases                     121
Introduction
Herbicides control weeds by interfering with how they grow. This is achieved by a number of different ‘modes of action’ (MOA) which all either ultimately stop seeds from germinating or establishing as seedlings; prevent plants from making essential carbohydrates, proteins or lipids (oils and fats); or desiccate leaves and stems. Knowing a herbicide MOA is important to understanding how to use a herbicide most effectively. It is a major factor in both herbicide selectivity and weed resistance.
The symptoms observed on weeds sprayed with herbicides express the MOA. In herbicide R&D, when new chemicals are screened experts carefully observe the detail and timing of the appearance of symptoms to gain clues as to the MOA. Fully understanding a MOA may take years of research by plant physiologists, biochemists, molecular biologists and many other scientific disciplines. The precise MOA of paraquat is very well understood - for more information, click here.
Knowing how a herbicide works in detail - its 'mode of action' (MOA) - is important to understanding how to use it most effectively. Herbicide MOA is a major factor in weed control spectrum, crop selectivity and weed resistance.
Herbicides control weeds by interfering with how they grow. Different MOAs all ultimately either stop seeds from germinating or establishing as seedlings; prevent plants from making essential carbohydrates, proteins or lipids (oils and fats); or desiccate leaves and stems.
Paraquat’s MOA involves diverting the flow of energy captured from sunlight in photosynthesis to produce highly reactive free radicals which destroy cell membranes to quickly desiccate leaves. This happens within hours in bright sunlight because of the high levels of energy running out of control. Almost all green plants are affected by paraquat making it a broad-spectrum, non-selective herbicide. You can read more and watch a video showing how paraquat works by clicking here.
 
Mode of action fact file 280+ herbicide active ingredients
20+ herbicide MOAs
1980s: last new MOA introduced
1 MOA in one field spells trouble