Maize cropping

Maize (corn) along with wheat and rice is one of the world’s top crops. Maize provides not only the fast-foods of western society - breakfast cereals, sweet corn and popcorn – but also the staple foods for much of the world’s population in developing countries where it is used to make porridge, bread and tortillas. All around the world, maize grain is a basic livestock feed, and the crop can be cut while still green to make silage as a winter feed. Also, over recent years maize has been increasingly used as a feedstock for the production of bioethanol


Bioethanol is ethanol of biological origin. Crops containing sugar or starch grown for energy use include sugar beet, sugar cane, maize and wheat. "2nd generation" bioethanol will be made from cellulose from, e.g. waste straw and stover, willow and popular trees, sawdust, reed canary grass, switchgrass, Miscanthus.

Authoritative On-line References and Resources The US Department of Energy's Office of Energy Efficiency and Renewable Energy has am Alternative Fuels and Advanced Vehicles Data Center containing key information on all biofuels.
Maize, perhaps more than any other crop, reaches both high and low extremes of sophistication, mechanisation and technology in crop production. All farmers, however, need to maximise the yield and quality of their produce, while saving the costs, time and labor needed to grow it. Protecting maize from weeds, pests and diseases is essential to avoiding heavy losses in yields and quality of grain. Weed control is usually most important. Paraquat is a 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.
herbicide which, when used in integrated weed management


A decision support system for crop protection which focuses on long-term prevention or suppression of pest problems with minimum impact on human health, the environment, and non-target organisms. IPM takes into consideration all available pest control techniques and tactics (cultural, mechanical, biological, chemical). IPM emphasizes the growth of healthy crops for better productivity with the least possible disruption to agroecosystems.

Authoritative On-line References and Resources "The USDA's National Site for the Regional IPM Centers' Information System provides information about US commodities, pests and pest management practices, people and issues."
systems, can provide the solution to weed problems.
As maize is grown so widely, and often intensively, its production can create significant environmental impact. WWF, the world's largest and most experienced independent conservation organisation, cites the major environmental issues in growing maize to include:
  • Intensive use of agrochemicals and resistance arising in weeds, pests and diseases
  • Soil erosion


    Displacement of solids (soil, mud, rock and other particles) usually by the agents of currents such as, wind, water, or ice by downward or down-slope movement.

    Authoritative On-line References and Resources This site brings together reliable information on soil erosion from a wide range of disciplines and sources. It aims to be the definitive internet source for those wishing to find out more about soil loss and soil conservation.
    and degradation
  • Water contamination by run-off


    The occurrence of surplus liquid (like rain) which originates up-slope and is collected beyond the ability of the soil to absorb it. The surplus liquid then flows away over the surface to reach the nearest surface water (pond, lake, river).

    Authoritative On-line References and Resources

    US Geological Survey's Water Science School
    and leaching


    The natural process by which water soluble substances are carried downward through the soil into groundwater.
    of agrochemicals
  • Loss of habitats and effects on biodiversity


    The variety of life in all its forms, levels and combinations. Includes ecosystem diversity, species diversity, and genetic diversity (IUCN, UNEP and WWF, 1991).

    Authoritative On-line References and Resources EarthTrends is a comprehensive online database, maintained by the World Resources Institute, that focuses on environmental, social, and economic trends. Statistics on biodiversity indicators are available.
Using paraquat in weed control programs can also address a number of environmental issues concerned with maize growing. Paraquat can be used in conservation tillage


Any tillage and planting system that covers 30 percent or more of the soil surface with crop residue after planting to reduce soil erosion by water.

Authoritative On-line References and Resources

Purdue University-based Conservation Technology Information Centre.
systems to maintain a managed, non-competitive vegetative cover which provides habitats to encourage biodiversity and helps to prevent soil erosion. Paraquat does not pollute soil or surface waters because it is immobilised and deactivated immediately on contact with soil.

Paraquat is an essential tool for maize farmers

Paraquat is an extremely versatile herbicide. It can be applied from before a crop is planted right up to harvest. A video showing paraquat’s unsurpassed speed of action can be viewed here. Paraquat is deactivated on contact with the soil meaning that it can be sprayed to burndown weeds before planting a maize crop without risking damage to that crop or indeed subsequent crops in the rotation. There are no leaching, persistence or root uptake problems to restrict its use, unlike many other herbicides which have ‘residual’ properties. You can read more about paraquat’s unique soil properties here. Paraquat works well even in cold and rainy weather, unlike most herbicides, making it suitable for use early in the season to control weeds in conservation tillage systems. These systems do not rely on plowing to control weeds. Not disturbing the soil helps prevent erosion and maintains a healthy soil. Intensive use of glyphosate has caused new weed problems as species less well controlled have ‘shifted’ to become more dominant and some species have evolved biotypes which are resistant to glyphosate. Using paraquat as an alternative non-selective herbicide, with a different mode of action, is helping to avoid problems of weed shifts


A change in the weed community within a field i.e. relative abundance or type of weeds. This can be the result of a management practice like herbicide use or any other phenomenon that brings about a change in weed species composition. Species or biotypes adapted to current weed management practices increase, whereas weeds susceptible to those practices decrease.

Authoritative On-line References and Resources A classic article on weed population dynamics on the Iowa State University Weed Science website.
and resistance.
The importance of paraquat in fighting weed resistance to glyphosate and maintaining farmers’ options to use conservation tillage systems is discussed here. Although paraquat is a non-selective, broad spectrum herbicide, if small amounts land on crop leaves there is little or no damage because paraquat does not move through plants systemically like glyphosate. So, paraquat can be used for inter-row weed control to remove weeds growing between the crop rows. Paraquat has a very robust environmental profile. It does not leach and is degraded in soil. Further details of paraquat’s safety to the environment, spray operators and consumers can be found by visiting the different sections of the Paraquat Information Center or referring to the Paraquat Fact Sheet. You can read more about the benefits of using paraquat here.

What is Maize?

Maize has been cropped for thousands of yearsMaize (Zea mays) is an annual grass originating in Central America. It is a tall crop, , typically 2-3 m, and, unlike wheat, barley and rice, has separate male and female flowers. Male flowers emerge as the tassel from the top of the stem after all the leaves have formed, while the female flowers are found at the base of the leaves on the middle of the stem. After pollination, the collection of female flowers forms the familiar cobs. Most commercial varieties have one or two large cobs. Commercial varieties of maize in more technologically advanced farming systems are actually ‘hybrids’. These are bred by crossing two dissimilar parent varieties. This means that the offspring are particularly vigorous and high yielding. However, the genetics of hybridization means that the harvested seed cannot be used to grow a successful new crop and farmers must buy a fresh supply of first generation hybrid seed. Maize is physiologically different to many other crops in that it has a system of photosynthesis called ‘C4’ (as opposed to ‘C3’). This means that it uses less water to achieve a given level of yield. Maize is a staple source of carbohydrate in the diet of hundreds of millions of people. It is also a rich source of some B group vitamins. However, as a protein source maize lacks the amino acids lysine and tryptophan, essential for humans and animals. Maize is also low in calcium compared to other grains. People who rely too heavily on maize in their diets can suffer from the disease ‘pellagra’ which is due to the poor bioavailability of the vitamin niacin. Some indigenous people learned to prepare maize flour with lime to improve the availability of niacin.

Where is maize grown?

Maize is grown over a wide range of climatic conditions from tropical to temperate. In warmer conditions two or more crops may be grown in one year, but in cooler temperate climates, although a valuable forage crop, grain will not fully ripen. About 40% of all maize is grown in the Americas (Fig 1). There, leading countries are the US, Brazil and Argentina. Similar areas to those in North and South America are grown in Africa and China, respectively, although far less intensively. Some harvest data for main producing countries and some developing ones are shown in Table 1. World maize production has increased by 97% since 1995 from only an 36% increment in area grown. This is largely attributable to yield improvements in countries such as the US and Argentina using technologically advanced growing methods. The gulf in yields between the US at 10.0 t/ha and those in many developing countries at 1-2 t/ha is huge.

Table 1. Maize production in leading and developing countries in 1995 and 2013 (FAO data).

  Production (million t) Area (million ha) Yield (t/ha)
  1995 2013 1995 2013 1995 2013
World 517.14 1018.11 136.50 185.12     3.79      5.50
USA 187.96   353.70   26.39   35.48     7.12    10.00
China 112.36   218.62   22.85   36.34     4.92       6.01
Brazil    36.27     80.27   13.95   15.28     2.60       5.25
Mexico    18.35     22.66     8.02     7.10     2.29       3.19
Argentina    11.40     32.12     2.52     4.86     4.52       6.60
India      9.53     23.29     5.98     9.50     1.59       2.45
S Africa      4.87     12.49     3.53     3.25     1.38       3.84
Romania      9.92     11.35     3.11     2.52     3.19       4.50
Nigeria      6.93     10.40     5.47     5.20     1.27       2.00

How is maize grown?

Maize is a ‘row crop’

Figure 1.  Global maize area distribution 2013 (FAO).In temperate climates maize is sown in spring and harvested in late summer or early autumn (eg April/May sowings and September to November harvests in the US). In more tropical climates it can be grown all year round. Maize seed is traditionally planted in wide rows (about 75 cm apart), which enables weed control in the growing crop by mechanical cultivators or hand hoeing. Wide rows are still popular in maize even where herbicides are used. Maize, together with cotton, followed closely on soybeans in the second wave of genetically modified (GM) broadacre crops to be introduced in the late 1990s. Now, corn which contains GM traits for controlling insects (‘Bt corn’) or which is tolerant to the herbicide glyphosate, or both, accounts for 32% of all corn grown (2011). This makes maize the second most widely grown GM crop to soybeans, of which 74% of the global crop area is GM. Although glyphosate tolerant maize has simplified weed control it has brought serious issues of weed shifts and glyphosate resistant weeds. Paraquat has an important role to play in avoiding these problems because of its alternative mode of action and by its use in integrated weed management systems.

Soil erosion and conservation tillage systems

No-till corn Broad-spectrum herbicides, led by the introduction of paraquat in the 1960’s, have allowed the adoption and growth of soil cultivation systems which do not rely on controlling weeds by burial from plowing. Abandoning the plow in reduced or zero tillage (‘no-till’) systems saves cash, time and fuel, improves soil structure, reduces erosion and provides havens for wildlife. More recently, conservation tillage systems have been developed as a wider approach to crop production than just tillage. These include the use of cover crops


Cover crops are primarily planted not to be harvested for food but to reduce soil erosion, control weeds and improve soil quality. They are usually plowed or tilled under before the next food crop is planted, in which cases the "cover crop" is used as a soil amendment and is synonymous with "green manure crop."

Authoritative On-line References and Resources ATTRA is the US National Centre for Appropriate Technology's Sustainable Agriculture Information Centre.
and mulches of residues of previous crops to ensure at least 30% of the soil surface is always covered to prevent soil erosion.
These techniques help to reduce soil erosion. Cultivating soil to control weeds can lead to erosion which removes nutrients and organic matter and decreases water holding capacity. In the US Corn Belt, although some of the most highly erodable land has been taken out of production, and erosion has decreased since the ‘dust bowl’ days of the 1930s, the average annual loss of soil by erosion has been estimated to be 14 t/ha, a rate way above what can be replaced by natural processes of soil formation.

Protecting maize crops

Herbicides for weed control can be used at several stages:
  • Pre-planting or pre-emergence: to burndown weeds present when fields are prepared for planting using contact herbicides such as paraquat or glyphosate. Paraquat is safe to use right up until the first signs of emergence. Paraquat can be applied before a crop is planted, or before it emerges, either to the whole field or as a band along the seeded rows. As a contact herbicide which controls only weeds present at the time of spraying, it can be tank-mixed with residual herbicides such as chloractetanilides, atrazine and simazine which are effective in preventing the germination or emergence of new flushes of weeds.
  • Post-emergence: by use of selective herbicides


    A chemical product used for eliminating certain types of weeds only (ie either grasses or broadleaved weeds) and not affecting specified crops.

    Authoritative On-line References and Resources An invaluable source of contemporary information about herbicides and weeds from Iowa State University.
    or inter-row weed control with paraquat based products. Paraquat may be sprayed between the rows after emergence with a sprayer with shielded nozzles. When maize plants are at least 25 cm tall careful application may be made without spray shields providing spray is not allowed to contact the upper two thirds of the plants.
  • Pre-harvest: harvest aids such as paraquat are sprayed to control large weeds to make harvesting easier and prevent the return of weed seeds to the soil.
  • Insect pests of maize can cause severe damage by eating the roots, burrowing in the stalks, or feeding off leaves and grain.
Although there are exceptions, often maize crops are not as susceptible to foliar fungal diseases as small grain cereals and the climates in which it is grown, being drier, do not encourage fungal infections.  

Maize is attacked by many weeds, pests and diseases including:

Grass weeds:
Barnyardgrass (Echinochloa crus-galli)
Crabgrasses (Digitaria spp.)
Foxtails (Setaria spp.)
Johnsongrass (Sorghum halepense)
Broad-leaved weeds:
Pigweeds (Amaranthus spp.)
Ragweeds (Ambrosia spp)
Morningglories (Ipomoea spp),
Nightshades (Solanum spp.)
Cockleburs (Xanthium spp.),
Velvetleaf (Abutilon theophrasti).
Fathen (Chenopodium spp.)
Northern corn rootworm (Diabrotica barberi)
Western corn rootworm (Diabrotica virgifera virgifera)
White grubs (Phyllophaga spp.)
Wireworms (Limonius spp.)
Corn leaf aphid (Rhopalosiphum maidis)
Two spotted spider mites (Tetranychus urticae)
Cutworms, which are the larvae of moths, can attack roots and shoots. Grasshoppers and locusts can also be serious problems.
‘Damping-off’ caused by fungi such as Pythium spp. and Rhizoctonia spp.
Ear diseases like corn smut (Ustilago maydis).
Leaf blights, eg caused by Helminthosporium maydis
Viral diseases such as maize dwarf mosaic virus.

Burndown and Integrated weed management

In no-till or conservation tillage systems weeds or a cover crop such as rye, wheat or oats have to be controlled by a non-selective burndown herbicide, either paraquat or glyphosate. 2,4-D is often mixed with both of these to improve control of difficult weeds. A paraquat-diuron mixture is used in Brazil. Paraquat-based burndown sprays are more reliable at earlier growth stages when the weather is cold and rain falls soon after application. Under such challenging weather conditions, paraquat will control weeds in a few days compared to glyphosate’s 2-3 weeks. Glyphosate is a systemic herbicide and gives a thorough kill of many perennials


Weeds that return year after year. Some die back in the winter but their roots remain alive and shoots reappear in spring; some don't die back and grow in size and stature the next season.

Authoritative On-line References and Resources

The International Weed Science Society represents individual associations around the world.
. However, intensive use of glyphosate has resulted in weed shifts. When the soil is not inverted by plowing, more small seeds are left on the surface ready to germinate and perennials survive better as the underground rhizomes or stolons are not disturbed or destroyed.
In the US, intensive use of glyphosate, particularly since the introduction of GM glyphosate tolerant varieties, has led to the development of resistant weeds in crop rotations which include maize. As an alternative broad spectrum non-selective herbicide to glyphosate, paraquat has a valuable role to play in integrated weed management (IWM) systems. No more than two applications of glyphosate should be applied to any one field over two seasons. Paraquat can provide an alternative means of effective and sustainable weed control.

Double cropping

Maize is usually grown in a rotation with other crops, often soybeans which improve the fertility of the soil for the following maize crop, reducing the need to apply nitrogen fertilizer. In warmer climates, such as in Brazil, soybeans are grown before a second crop of maize in the same season. To grow a successful maize crop in a double-cropping system, a hybrid must be chosen which will mature more quickly given the later planting as a second crop. Similarly, a fast-turn around time between crops must be achieved and conserving soil moisture is important. No-till techniques and using fast-acting paraquat for weed control can provide this, often giving as much as a 10 day lead over any recommended glyphosate program.

Harvest aid

Corn harvestTime is often a key factor in corn production. In more northerly regions, autumn weather may be closing in, jeopardising harvest. Using paraquat to desiccate weeds in a mature corn crop buys crucial time by speeding harvest. If infested with weeds, a desiccated crop can be harvested more quickly. Residues in the grain are not a problem because paraquat is non-systemic.

CASE STUDY: Paraquat in Vietnam

In Vietnam, using paraquat in no-till systems can improve maize production and alleviate environmental problems. Read more

References & Resources

Maize Associations

US National Corn Growers Association

Maize Agronomy and Production:

United Nations Food and Agriculture Organisation FAOSTAT Purdue University Corn Growers Guidebook Iowa State University 'Corn production' Information System for the Regional IPM Centers

Glyphosate Resistance:

International survey of herbicide resistant weeds


International Service for the Acquisition of Agri-Biotech Applications (ISAAA)

Conservation Tillage

No-till farming portal

Environmental Issues

WWF (World Agriculture and the Environment: a commodity-by-commodity guide to impacts and practices by Jason A Clay. Published by Island Press (2004). ISBN No. 1-55963- 370-0)