Maize cropping

Maize, or corn as it is always called in North America, along with wheat and rice is one of the world’s top crops. As the current craze for ‘maize mazes’ suggests, it is truly an ‘amazing’ crop.

Maize is often seen as a highly advanced crop grown for breakfast cereals, sweet corn and popcorn - fast-foods of western society. However, maize also provides more basic 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.

In a world increasingly aware of its dependence on oil and the climate change issues arising from its use, there is growing interest in the production of bioethanolDescription 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 http://www.eere.energy.gov/afdc/fuels/ethanol.html 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. from the starch in maize as an alternative fuel.

Maize, perhaps more than any other crop, reaches both high and low extremes of sophistication, mechanisation and technology in crop production. But, all farmers need to maximise the yield and quality of their produce, while saving the costs, time and labour 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-selectiveDescription A chemical product used for eliminating all types of weeds (annual and perennial grasses and broadleaved weeds). Authoritative On-line References and Resources http://www.weeds.iastate.edu/ An invaluable source of contemporary information about herbicides and weeds from Iowa State University. herbicide which, by using in integrated weed managementDescription 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 http://www.ipmcenters.org "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 solutions to crucial weed control issues.

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 as:

• Intensive use of agrochemicals and resistance arising in weeds, pests and diseases
• Excessive water use in irrigation
• Soil erosionDescription 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 http://soilerosion.net/ 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-offDescription 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 http://www.sowap.org/index.htm SOWAP (Soil and Water Protection) is a collaboration between industry, NGOs, universities and farmers to test a range of site-specific soil management methods, based on the concept of conservation tillage. It has looked at economic and environment aspects including effects on soil erosion and run-off. and leaching of agrochemicals
• Loss of habitats and effects on biodiversityDescription 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 http://earthtrends.wri.org/ 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.
• Spread of pollen from genetically modified (GM) crops

Using paraquat in weed control programmes can also address a number of environmental issues concerned with maize growing. Paraquat can be used in conservation tillageDescription 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 http://www2.ctic.purdue.edu/Core4/CT/Definitions.html Core4Conservation is part of the Purdue University-based Conservation Tillage Information Centre. systems to maintain a managed, non-competitive organic cover which provides habitats to encourage biodiversity and helps prevent soil erosion.

Paraquat does not pollute soil or surface waters because it is immobilised and deactivated immediately on contact with soil. For farmers and consumers who do not want to grow or eat GM food, paraquat can be relied upon to effectively control weeds regardless of whether a crop is GM glyphosate tolerant or not.

Paraquat is an essential tool for maize farmers

Paraquat is an extremely versatile tool in the maize farmer’s battle against weeds. It can be applied from before a crop is planted to before it is harvested.

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. 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 ploughing to control weeds. Not disturbing the soil helps prevent erosion and maintains a healthy soil.

In recent years, 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, in integrated weed management systems, is helping to avoid problems of weed shiftsDescription 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 http://www.weeds.iastate.edu/mgmt/qtr00-1/popdyn.htm A classic article on weed population dynamics on the Iowa State University Weed Science website. and resistance.

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

What is Maize?

Maize (Zea mays) is an annual grass which has been domesticated as a crop over several thousand years, originating in Central America. As a grain crop, maize has several unusual or even unique characteristics.

Firstly, and most obviously, it looks different to the small grain cereals like wheat, barley and rice. It grows much taller, typically 2-3 m, and does not have an ear on the top of the stem. Maize 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.

Secondly, commercial varieties of maize in more technologically advanced farming systems are actually ‘hybrids’. These are bred by crossing two dissimilar parent varieties which means that the offspring are particularly vigorous and high yielding. However, the genetics of hybridization means that the harvested seed can not be used to grow a successful crop and a fresh supply of first generation hybrid seed must be bought.

Thirdly, maize is physiologically different to many other crops in that it has a system of photosynthesis which is called ‘C4’ (as opposed to ‘C3’). This means that it uses less water for a given level of yield.

Maize is an important starch crop forming the 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, dietary protein needs to come from elsewhere as maize protein lacks two amino acids essential for humans and animals, lysine and tryptophan. 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 which improves the availability of niacin.

Where is maize grown?

Maize is one of the world’s top three most widely grown crops. At 147 million ha harvested in 2005, it is third in area, but top in terms of grain production.

About 40% of all maize is grown in the Americas (Fig 2). 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.

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.

Some harvest data for main producing countries are shown in Table 1. World maize production has increased by 34% since 1995 from only an 8% 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 over 9 t/ha and those in many developing countries at 1-2 t/ha is huge.

Table 1. Maize production in leading countries in 1995 and 2005 (FAO data).    

How is maize grown?

Maize is a ‘row crop’.

In more 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 enable weed control by mechanical cultivators or hand hoeing or slashing while the crop is growing. 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 14% of all corn grown (2005). This makes maize the second most widely grown GM crop to soybeans, of which 60% of the global crop area is GM.

Although the introduction of glyphosate tolerant maize has simplified weed control there are issues of adverse changes in the weed flora and resistance to glyphosate developing. Paraquat has an important role to play in avoiding these problems by its use in integrated weed management systems.

Soil erosion and conservation tillage systems

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 mouldboard 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 cropsDescription 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 http://attra.ncat.org/attra-pub/covercrop.html 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.

In the US, the total no-till area is forecast to grow to over 80 million acres by 2008, of which around 50% will be cropped with soybeans, and maize and cotton will take 30% and 20%, respectively. In Brazil, from less than 400,000 ha in 1990/1, there were 6 million no-till hectares in 2001/2.

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

Maize crops need protecting from weeds, pests and diseases to produce the best yields and quality of grain. The susceptibility of maize to the European Corn Borer and the resulting damage to yields, led to the development of GM corn expressing the Bacillus thuringiensis (Bt) toxin helping to eliminate the pest as it feeds on the crop. Maize is attacked by many pests and diseases. These include:

Insects
Insect pests of maize can cause severe damage by eating the roots, burrowing in the stalks, or feeding off leaves and grain. Important soil pests include the northern corn rootworm (Diabrotica barberi), western corn rootworm (Diabrotica virgifera virgifera), white grubs (Phyllophaga spp.) and wireworms (Limonius spp.). Aerial pests include the corn leaf aphid (Rhopalosiphum maidis) and 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. Certain pests which were not present in certain countries were ‘imported’ probably through plant material in fields near airports first. Without control by local predators, they spread to sometimes become a major pest in a country (eg corn rootworm in France).

Diseases
In general, maize is not as susceptible to foliar fungal diseases as are the small grain cereals and the climates in which it is grown, being drier, do not encourage fungal infections. However, in the earlier part of the season in more temperate regions, seedling diseases such as ‘damping-off’ caused by fungi such as Pythium spp. and Rhizoctonia spp. can be a problem. Similarly, if the latter part of the season is damp, ear diseases like corn smut (Ustilago maydis) can cause yield losses. There are also leaf blights, eg caused by Helminthosporium maydis and viral diseases such as maize dwarf mosaic virus.

Weeds
Weeds are a major problem in maize, whatever the climate. Some predominant grass weeds found in most growing regions in the US include barnyardgrass (Echinochloa crus-galli), crabgrasses (Digitaria spp.) and foxtails (Setaria spp.). Major broad-leaved weeds include pigweeds (Amaranthus spp.), ragweeds (Ambrosia spp), morningglories (Ipomoea spp), nightshades (Solanum spp.), cockleburs (Xanthium spp.), and velvetleaf (Abutilon theophrasti). In Europe, one can add Johnsongrass (Sorghum halepense) and Chenopodium spp. to the list.

Weed control in maize can be highly sophisticated. Because it is grown in wide rows, mechanical cultivation to control weeds growing between the rows is possible and still practised, but most weed control in developed countries is by using herbicides.

Herbicides 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, sometimes mixed with residual herbicides. Some residual herbicides require incorporating in to the soil before the crop is planted. 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 herbicidesDescription 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 http://www.weeds.iastate.edu/ 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.

Burndown

In no-till or conservation tillage systems weeds or a cover crop such as rye, wheat or oats have to be controlled by a burndown herbicide, either paraquat or glyphosate. 2,4-D is often mixed with both of these to improve control of some difficult weeds. A paraquat-diuron mixture is used in Brazil. Paraquat-based burndown sprays are more reliable than glyphosate in controlling annual weedsDefinition Weeds that complete their life cycle within one growing season, or year. From seed to flowering to seed before the year ends. Authoritative On-line References and Resources http://iws.ucdavis.edu/ The International Weed Science Society represents individual associations around the world. Details of these regional associations are listed. and cover crops at earlier growth stages when the weather is cold and rain falls soon after application. Particularly 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 perennialsDescription 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 http://iws.ucdavis.edu/ The International Weed Science Society represents individual associations around the world. Details of these regional associations are listed.. However, intensive use of glyphosate has resulted in changes in the spectrum of weeds towards species which are harder to control and the evolution of resistant biotypes in some species which are no longer controlled by glyphosate. This has led to the development of integrated weed management systems.

Integrated weed management

Reduced tillage cultivation systems affect weed infestations through combined effects of soil management and herbicide regimes leading to ‘weed shifts’ where the more favoured species become more dominant in the weed flora: annual grasses, small-seeded broad-leaved weeds and perennials increase. Without inverting the soil by ploughing, 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. Herbicides affect weed spectrum by strengths and weaknesses in control as a result of their particular mode of action, and, ultimately, resistant mutations may arise and those biotypes become predominant for a given species.

In the US, intensive use of glyphosate, particularly since the introduction of GM glyphosate resistant varieties, has led to the development of resistant weeds in cotton and soybean crops which may be rotated to maize. Horseweed (Conyza canadensis) has become a problem across tens of thousands of hectares of soybeans in many eastern and southern states since 2000. More recently, glyphosate resistant common ragweed (Ambrosia artemesifolia) has been recorded too. Glyphosate resistant horseweed has also become a problem across tens of thousands of hectares of cotton in Tennessee, Missouri, Arkansas and N. Carolina since 2001. In 2005, glyphosate resistant palmer amaranth (Amaranthus palmeri) was recorded in cotton in Georgia. In Brazil, ryegrass (Lolium multiflorum) resistant to glyphosate has been noted a small area of soybeans, currently less than 100 ha.

As an alternative broad spectrum non-selective herbicide to glyphosate, paraquat has a valuable role to play in integrated weed management (IWM) systems. In the US, it is becoming accepted that over-reliance on glyphosate and the concurrent reduction in diversity, will exacerbate weed resistance problems. No more than two applications of glyphosate should be applied to any one field over two seasons. Paraquat can provide the 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

Time 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 Viet Nam, Dr. Nguyen Truong Thanh of the National Plant Protection Institute, and Ha Dinh Tuan and Le Huy Hoang of the Vietnam Agricultural Science and Technology Institute have found that using paraquat and not ploughing can improve maize production and alleviate environmental problems by saving time and costs, and reducing soil erosion, amongst other benefits.

In the north west of Vietnam, large areas of maize are grown in the rainy season on steeply sloping land (10-25º).

All operations, including plowing, sowing and weed control are generally done manually. High labour requirements, low yields, soil degradation and the danger of forest fires from burning slashed and dried weeds are serious problems.

Applying paraquat 2 days before planting, followed by spraying it between the rows of maize 30 days after emergence, has been compared to traditional farmer practice in large plot field trials.

Paraquat effectively controlled almost all weeds (some of which can grow 1.8 m tall) after 1-2 days.
Growing maize without ploughing and using paraquat saved, on average, 50 days/ha of manpower otherwise needed to hoe, cut, dry, and burn weeds.

Soil erosion in the peak rainy month of July was reduced from 4.2 t/ha to 2.2 t/ha (Fig 1).

Yields have been similar under both growing systems, but the savings in time and costs from using paraquat have been very substantial: on average 0.75 million Vietnam Dong/ha = € 35/ha.

CASE STUDY: Paraquat in Guatemala

Using paraquat for weed control on smallholdings in La Nueva Concepción and Guadalupe Parcelamientos in Guatemala resulted in better weed control than laborious hand hoeing and better yields than glyphosate. Paraquat also reduced losses of soil from the sloping fields because, unlike when hand-hoeing, the soil was not disturbed, and unlike when using glyphosate, only weed shoots were controlled leaving the roots intact to help anchor the soil.

In Guatemala, many smallholders, especially in the highlands, hoe weeds from their maize crops by hand. Those using herbicides generally prefer paraquat over glyphosate because of crop safety. To assess the merits of paraquat or glyphosate compared to hand hoeing, large trials were conducted on gently sloping maize fields (6-8% inclination) over two seasons.

Using herbicides was much more effective than hand weeding. Paraquat was slightly better than glyphosate in controlling broadleaved weedsDescription The leaves are "broad" as opposed to the "narrow" leaves of grasses. Also called 'dicots' having two seed leaves, while grasses are 'monocots' having one seed leaf. Authoritative On-line References and Resources http://iws.ucdavis.edu/ The International Weed Science Society represents individual associations around the world. Details of these regional associations are listed. such as Ipomoea and Commelina.

Paraquat treated plots yielded up to 17% more maize than those where glyphosate was used. Although, on average in these two seasons, yields from hand hoeing were similar to those where paraquat was used, in the longer term the greater losses of soil from hand-hoeing, including the high losses of micronutrients recorded, would be expected to decrease yields.

References & Resources

Maize Associations

US National Corn Growers Association: http://www.ncga.com/

Maize Agronomy and Production:

United Nations Food and Agriculture Organisation FAOSTAT: http://faostat.fao.org/faostat
Purdue University Corn Growers Guidebook: http://www.agry.purdue.edu/ext/corn
Iowa State University ‘The Maize Page’: http://maize.agron.iastate.edu
National (US) Information System for the Regional IPM Centers: http://www.ipmcenters.org/cropprofiles/ListCropProfiles.cfm?typeorg=crop&USDARegion=National%20Site

Glyphosate Resistance:

http://www.weedscience.org/Summary/UspeciesMOA.asp?lstMOAID=12&FmHRACGroup=G

GM

International Service for the Acquisition of Agri-Biotech Applications (ISAAA): http://www.isaaa.org/
Economic Research Service (ERS): http://www.ers.usda.gov/Data/BiotechCrops

Conservation Tillage

No-till farming portal: http://www.notill.org/links/new_links.html

Environmental Issues

WWF