Vegetable cropping

Vegetables are essential to a healthy diet. In many countries, eating at least five portions a day is recommeneded for health.

Although the term ‘vegetable’ implies coming from the vegetative parts of plants, such as leaves, stems and roots, a few fruit, notably tomatoes, are also commonly called vegetables. Therefore, the classification is a culinary one. Arguments over whether tomatoes are fruit or vegetables have even been aired in court. In 1893, the US Supreme Court ruled that tomatoes are vegetables for legal purposes, in particular, relating to trade tariffs in force at the time.

Vegetables also straddle agriculture and horticulture. Some are grown on a field scale like, say, wheat or maize, many are grown in sophisticated glasshouses, while all are grown by smallholders and in domestic gardens. Peas and other legume vegetables have bacteria associated with their roots which convert nitrogen from the air into forms which can be used by plants as nutrients and these remain in the soil to fertilize following crops. A vegetable break crop, such as peas or potatoes, prevents the build-up of pests and diseases in cereal rotations and provides an opportunity to control weeds by alternative approaches.

Paraquat has an important role to play in vegetable cropping because its unique characteristics are particularly suited to the challenges posed by controlling weeds in these diverse crops. The benefits from using paraquat are not only directly concerned with its chemical and biological properties, but also with associated agronomic techniques like minimum tillageDescription Least possible soil disturbance for preparing a seedbed by reducing cultivation and avoiding the use of a plow. It can involve direct drilling, broadcasting into existing stubbles or adopting a strategy of reduced tillage. The main benefits are to reduce tillage energy consumption, to conserve moisture and soil structure, to reduce disturbance of soil organisms and to retain plant cover to minimize erosion. Authoritative On-line References and Resources http://www.ecaf.org/ The European Conservation Agriculture Federation promotes best soil management practices based on minimnal soil disturbance by reduced tillage. and inter-row weed control.

Paraquat is an essential tool for vegetable growers

Paraquat is an extremely versatile tool in the vegetable farmer’s battle against weeds. Because it is deactivated on contact with soil it can be sprayed to burn down weeds immediately before sowing seeds or transplanting 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 where vegetables are grown for early markets. These systems do not rely on ploughing to control weeds. Not plowing helps to 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-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, with a different mode of action, 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 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 resistanceDescription The inherited ability of a plant/weed to survive a dose of a herbicide normally lethal to that species. Authoritative On-line References and Resources http://www.weedscience.org/in.asp The International Survey of Herbicide Resistant Weeds monitors the evolution of resistant species and assesses their impact. All confirmed instances of new cases are listed..

Although paraquat is a non-selective, broad spectrum herbicide, if small amounts land on leaves there is little or no crop 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 other sections of the Paraquat Information Center or by referring to the Paraquat Fact Sheet.

Major vegetables grown around the world

In 2005, according to FAO, there were 66 million ha of vegetables grown in the world. This figure includes of potatoes (19 million ha) but excludes nearly 5 million ha of watermelons and other melons which are sometimes included with vegetables.

Vegetables are often intensively managed and with high input costs, but are usually very profitable. In the UK in 2004 the average gross margin for wheat crops was around £600/ha, while potatoes averaged £4000/ha. In less developed countries, vegetables provide an important source of nutrition and cash from trade in local markets. Vegetables also have a relatively greater importance in the agricultural economy of some countries. Although grown on less than 2% of the cropped area of half the countries in the EU (excluding potatoes), vegetables account for 8%, 10% and 11% of the cropped area in Italy, Belgium and the Netherlands, respectively.

Fresh, frozen or canned vegetables are much closer to consumers than broad acre arable crops, and international food processors and retailers now determine many of the methods used in their production. Furthermore, they demand carefully planned provision of supplies to rigorous standards of quality and uniformity, albeit in sometimes volatile markets.

Table 1. Global league table of vegetables: millions of hectares harvested in 2005 (FAO).

Vegetables for health

‘5-a-day’ programmes promote the inclusion of vegetables in healthy diets. Vegetables are low in calories and fat, and high in water content, therefore having an important role in managing body weight and controlling appetite. This contributes to a reduced risk of developing diabetes through obesity. Diets high in vegetables are known to reduce the risk of many cancers, strokes and heart disease. They are also associated with lower blood pressure, fewer cataracts, and less risk of diverticulosis, lung disease, and osteoporosis.

Vegetables provide vitamins and other phytochemicals such as antioxidants essential to health. Vitamins are needed in tiny quantities but they can only be made by plants and are vital to animals for functioning, maintenance and protection of their bodies. These plant chemicals contribute to health by removing toxins, stimulating the immune system, ensuring a positive balance of hormones, as well as having anti-bacterial and anti-viral effects. Vegetables also provide minerals and fibre.

Table 2. Nutritional and health value of vegetables.

NB: Vitamin A = retinol, B1 = thiamine, B3 = niacin, B6 = pyroxidine, B9 = folic acid, C = ascorbic acid, E = tocopherol.

* Red peppers are richer sources of vitamins than green peppers

The preparation of vegetables in cooking is important in determining their nutritional and health-giving properties. For example, boiling spinach for only 4 minutes, as opposed to steaming, halves the content of vitamin B9 (folic acid) important for growth. On the other hand, processing tomatoes into sauces, etc, makes lycopene, a potent antioxidant (which makes tomatoes red), more available to the body

Challenges in growing vegetables sustainably

Vegetable growers face many distinctive challenges. These include:

• Erosion of light soils
• Leaching of pesticides from highly irrigated light soils
• Soil compaction from intensive cultivations
• The need for early harvests and multi-cropping
• The need to supply produce to strict requirements for quality and freedom from pesticide residues

In Europe, integrated crop management (ICM) systems have been introduced to minimise the impact of vegetable production on soil, water, air and 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.. A report commissioned by the European Commission outlines the improvements ICM systems are designed to make to reduce the environmental impact of crop production and encourage sustainable farmingDescription 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 www.nal.usda.gov Information from the USDA's Alternative Agricultural Systems Information Center. systems¹. These cover reductions in nitrate and pesticide leaching, pesticide residues, 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., spray drift, and carbon dioxide emissions; improving soil nutrient balance and soil and air quality; and protecting flora and fauna. There are many examples where vegetables are targeted.

In the US, ICM has been widely adopted in vegetables. For more information, the website www.ipmcenters.org includes agronomic profiles of many vegetable crops across the country by state.
Paraquat has a key role in the quest for sustainable vegetable production by controlling weeds that would otherwise seriously reduce productivity, in conjunction with other agronomic practices that protect the environment and help to reduce the overall use of pesticides.

Soil Erosion

Erosion of soil by wind and water can be a common problem on the generally lighter soils used for vegetable production. Vegetables tend to be grown on light soils because of the need for ready and frequent access to fields to drill to production schedules; fertilize and protect from weeds, pests and diseases, and to harvest promptly. Many vegetables are grown in wide rows with little crop cover for several weeks during establishment, risking soil erosion. Root vegetables can also contribute to erosion by retaining soil and removing it from the field in harvesting.

Weed control can exacerbate erosion problems if the wrong herbicides are used. Glyphosate and residual herbicides are the worst offenders. Glyphosate is systemic, controlling weeds right down to the roots, but paraquat is a contact herbicide which only burns down weed shoots. Roots are left intact and provide an anchoring effect in the soil. Residual herbicides prevent new flushes of germination of weed seeds through the season. This leaves the soil bare and, therefore, more susceptible to erosion. Paraquat has no residual activity, being inactivated immediately on contact with soil. Managing weed infestations with paraquat keeps weeds from competing with the crop, yet allows them to re-establish after a few weeks, anchoring the soil and also providing habitats for beneficial insects and other predators of potential crop pests.

Leaching of Pesticides

Light soils are also more prone to leaching of nitrogen and agrochemicals. These problems are exacerbated by the use of frequent irrigation keeping the soil moist. Residual herbicides, in particular, are often prone to leaching because they have to be mobile in the soil to exert their effects on buried seeds. Paraquat, however, is immediately bound to soil particles on contact. This binding is extremely tight preventing any movement of paraquat in the soil, regardless of rain or irrigation, so there is no leaching at all. This has been confirmed under laboratory conditions and in field studies involving vastly exaggerated rates of paraquat over 40 years.

• In their review report for paraquat the EC concluded that “all studies indicate that paraquat is immobile” and that “paraquat will not be used under conditions where contamination of the saturated zone occurs” (EC, 2003)
• In their RED document, the US EPA declared that “paraquat dichloride was shown to be very immobile in soil” and that “paraquat is not expected or considered to be a groundwater concern from normal paraquat dichloride use patterns” (US EPA, 1997)

A light cover of non-competitive weeds as they recover from a paraquat spray also makes a contribution to removing excess soil nitrogen which might otherwise leach.

Soil Compaction

Highly mechanised production with intense tillage and heavy traffic potentially causes problems with soil compaction. However, using paraquat for weed control allows the adoption of minimal tillage systems which do not rely on controlling weeds by burial from plowing. Abandoning the mouldboard plow in reduced or zero tillage (‘no-tillDescription 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 http://www.no-till.com/index.htm A portal for on-line information about no-till farming.’) systems saves cash, time and fuel, improves soil structure, reduces erosion and provides havens for wildlife. Minimal tillage systems used for establishing vegetables in California (Mitchell et al, 2004) illustrate the sort of machinery and technology requirements to make widespread adoption feasible in these crops. These include:

• Equipment to complete several operations in one pass.
• Cultivators and drills matched to bed systems which need to be kept for several crop cycles.
• Machinery capable of effectively incorporating 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 residues of previous crops.
• Global Information Systems which allow precision alignment of cultivations and other operations.
   

Multi-Cropping

Vegetables which are early to market are generally more highly priced. Growers often aim for early harvests that mean planting early in potentially less favourable weather conditions. Unlike glyphosate, paraquat gives reliable weed control when sprayed at low temperatures because it does not rely on weeds being actively growing to kill them. Also, paraquat’s performance is not affected by rain soon after spraying. Paraquat needs only 15-30 minutes before rain to give maximum levels of weed control, compared to glyphosate’s need for several hours.

Many vegetable crops, eg lettuce, brassicas, are often transplanted out into the field, not sown from seed. Because paraquat is immediately and strongly bound to soil it can not be taken-up by plant roots so transplants will not be damaged.

Paraquat is safe to use for inter-row weed control by careful application with knapsack sprayers or from tractor mounted sprayers with shielded spray nozzles. Unlike systemic herbicides which are too dangerous to use, even if small amounts of paraquat land on crop plants they cannot move within the plant to cause damage.

As many crops may be grown in one season with continuous production schedules, a fast turnaround time between crops is critical. Paraquat gives complete control of weeds within a day or two even in cold weather, and within hours in hot sunny weather. This is a critically important asset in contrast to the one to three weeks needed for the full effects of glyphosate to show.

Pesticide residues

Paraquat is sprayed to control weeds before vegetable crops are planted or in between the rows, not over the whole crop. Any paraquat accidentally landing on leaves is tightly bound, just as it is in soil, and finds it very difficult to enter, and even more difficult to move to fruit or tubers. Furthermore, UV radiation in sunlight degrades paraquat on leaf surfaces and soil absorption prevents entry via roots.

Other benefits of paraquat in vegetable growing

Paraquat can be used to control weeds in potatoes right the way up to early emergence of the potato shoots, which readily grow back if scorched. This extends the period of early weed control and reduces the need to rely on pre-emergence residual herbicides.

CASE STUDY: Vegetable Production in China

Mr Shen Shu Ping of the South China Agriculture University has reported on the value of paraquat in growing vegetables in the Guandong region of China, in an article published in the Chinese Journal 'Pesticide Information'. Thanks to paraquat’s fast action the income of smallholder families is increased and the requirements for arduous hand-weeding are reduced.

In Guandong region, one million hectares of high quality vegetables are grown, often with up to eight crops harvested in each field every year. Obviously, this can only be achieved with a rapid turnaround between crops. Paraquat is used on over 40% of the vegetable area and sales continue to grow rapidly.

Vegetable farmers in Guandong use paraquat in several different ways. Firstly, it is sprayed before planting, often in tank mixture with a fertilizer or the residual herbicide butachlor. Later, paraquat is sprayed between crop rows to control weeds using shielded spray nozzles. After harvest it is used to desiccate stubbles, trash and any remaining weeds.

The key benefit from using paraquat in Guandong is its speed of action. After spraying, weeds are killed within 1 or 2 days and during this period the ground can be prepared for the next crop without delay. Crops can be then be sown or planted safely because paraquat is immediately and completely inactivated on contact with soil. With systemic, slow acting herbicides like glyphosate, farmers would have to wait for 10 days after spraying before beginning work on the next crop. Over eight crops this would be a loss of 80 days of production every year.

References & Resources

Vegetable Agronomy and Production

United Nations Food and Agriculture Organisation FAOSTAT: http://faostat.fao.org/faostat/

EC DG Agriculture report: Horticulture in the EU (2003) http://europa.eu.int/comm/agriculture/publi/fact/horti/2003_en.pdf

Integrated Crop Management Systems in the EU. A report by Agra CEAS Consulting for EC DG Environment (2002).

National (US) Information System for the Regional IPM Centers: http://www.ipmcenters.org/cropprofiles/ListCropProfiles.cfm?typeorg=crop&USDARegion=National%20Site

British Potato Council: http://www.potato.org.uk

Mitchell, J, Jackson, L, Miyao, G (2004). Minimum tillage vegetable crop production in California. University of California Division of Agriculture and Natural Resources publication No. 8132.

(http://anrcatalog.ucdavis.edu)

Soil Properties

European Commission DG Health and Consumer Protection review report for the active substance paraquat: http://europa.eu.int/comm/food/plant/protection/evaluation/existactive/l...

US EPA R.E.D. Facts: Paraquat Dichloride: http://www.epa.gov/oppsrrd1/REDs/factsheets/0262fact.pdf

Vegetables and Health

US 5-a-Day program: http://www.5aday.gov/index.html

¹  (http://europa.eu.int/comm/environment/agriculture/pdf/icm_finalreport.pdf)