Wheat

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Planche botanique d'un épi de blé

Contents

General Information

The two main cultivar groups are bread wheat (Triticum aestivum) and durum wheat (Triticum durum). The consumption of wheat is constantly increasing in many countries. The comparably high price of imports is a considerable burden on economies and provides an incentive for domestic wheat production.

For white flour, only the inner part of the grain is used, which is rich in starch and gluten. Milling of the whole grain gives darker but healthier flour; as it includes the germ (containing oil and vitamins) and the bran (the "shell" or sheath, containing minerals)

In general, wheat can be cultivated with few inputs and little mechanization. However, irregular and variable rainfall, a short growing season, periods of extreme heat, poor soils and aggressive pests and diseases can make it a challenge to grow this crop successfully.

Agro-ecological requirements

Wheat is essentially a crop of the temperate to subtropical climates. The dry season and moderate hot summers are conducive to a good wheat crop. Due to the wide adaptation of different cultivars of wheat to a variety of growing conditions, today it is grown from the moderate to cold climates of the far northern and southern latitudes all the way to the equator, and from sea level to altitudes of more than 4500mt. Compared to other cereal crops, wheat's soil and water requirements are quite specific. Bread wheat and durum wheat have somewhat different ecological requirements.

Diversification strategies

Under temperate conditions, the repeated cultivation of wheat is quite common in conventional agriculture. Continuous cultivation of wheat, however, commonly results in elevated levels of weed competition and soil-borne diseases (foot and root rots) and leads to nutrient depletion and decreasing yields. A proper crop rotation underpins successful wheat cultivation.

It is recommended that a cereal crop should not be grown on a plot of land more than twice in succession. Wheat should not be grown more often than every third year. It should be grown in rotation with crops that do not act as hosts to the same pests and diseases and that suppress weeds well. The ideal partners of wheat in crop rotations are pulses or other legume crops. Legumes do not transmit diseases to wheat, cover the soil densely and supply nitrogen to the subsequent crops.

The best crops to precede wheat are legumes or tuber crops. Wheat responds well to the nitrogen supplied by the legume. Tuber crops leave a fine tilt, thus easing soil preparation before wheat. The cultivation of wheat as the second crop after a legume is also recommended. Growing wheat after another cereal crop increases the risk of pests and diseases.

Wheat can also be intercropped. Valuable intercropping partners are chickpea, barley, mustard, pea, long-duration pigeon pea, gram lentil or safflower. Wheat may also be intersown with a short-season crop (chickpea, lentil or grass pea) towards the end of the wheat's growing period, provided that there is enough moisture in the soil. Alternatively, a cover crop can be sown in after the second weeding of the crop, before the wheat plants head. Subsequent passages with a tine-weeder, tine hoe, or rake mixes the seeds with the topsoil and improves their germination.

Disease management

Pest and disease management can be both direct and indirect. On average, diseases and pests destroy 20% of the potential grain harvest, including losses during storage. In some cases, storage losses are the most significant. Losses due to diseases can be effectively controlled by: growing cultivars with corresponding tolerances or resistances, applying an appropriate crop rotation and ensuring good growing conditions.

Pest management focuses predominantly on preventive measures, and great efforts are being made to breed varieties that are resistant to nematodes, the Sunn pest and Hessian fly. These pests can significantly damage wheat yields. In dry climates, direct measures against diseases that rely on humid conditions for infection are generally not necessary, although occasional treatments may be appropriate in humid climates which are more conducive to the spread of diseases. The most widespread and dangerous rust forms are stem rust (Puccinia graminis f.sp.) and brown wheat rust. These only affect wheat. In cooler climates, stripe or yellow rust is also widespread. Rust diseases infect the leaves and sometimes the spikes and can reduce yield by up to half. Other important diseases are spot blotch, head scab, foot/root rot (Fusarium spp.) and Sclerotium foot rot (Corticium rolfsii). Other diseases such as tan spot (Pyrenophora tritici-repentis), powdery mildew, speckled leaf blotch (Mycosphaerella graminicola), glume blotch (Phaeosphaeria nodorum), Alternaria leaf blight (Alternaria spp.), loose smut (Ustilago nuda f.sp. tritici), Rhizoctonia root rot (Rhizoctonia spp.), bacterial leaf streak or black chaff (Xanthomonas translucens pv. undulosa) and barley yellow dwarf luteovirus can be regionally significant.

To be effective, disease control measures must take into account how the diseases are transmitted. Foot and root diseases such as Fusarium spp., Rhizoctonia spp. and Septoria are soil borne. They develop in crop rotations that fail to interrupt their disease cycles, and are highly site specific. Powdery mildew (Erysiphe graminis) and rust diseases can be transmitted by wind over long distances. Thus, the two groups of diseases require different control measures. Fungal diseases can also be spread through infected seeds. Seedborne diseases can be difficult to control. To prevent their transmission, all seeds used should, wherever possible, be tested for seed-borne diseases and be certified disease free. The most efficient measure against seed-borne diseases remains the use of certified seeds.

Leaf or brown rust (Puccinia recondita f.sp. tritici): This is the most harmful wheat disease of all. It spreads easily and occurs regularly in Africa. Brown rust can affect wheat at all stages, at temperatures between 2 and 32°C, and does not necessarily rely on moisture. Losses occur as a result of a reduction of the green leaf surface. Infected plants normally produce fewer tillers and fewer and smaller grains. The symptoms are red-brown pustules on the upper leaf and leaf sheath. On resistant cultivars the pustules stay small. Black spores develop at higher temperatures. One direct treatment is to uproot and burn affected plants. The main measure, however, is to use resistant varieties of wheat in the first place. The cultivation of a mix of varieties may reduce the infection rate. Tobacco decoction spray is reputed to control rust diseases of wheat, but special attention is necessary to avoid ill effects on humans.

Stripe or yellow rust (Puccinia striiformis): In the tropics, stripe rust is the main disease of wheat grown in the cool highland climates, although temperatures above 20°C stop its growth. Losses occur due to a loss of active leaf surface, reduced root growth and increased water losses.

Spot blotch or brown foot rot (Cochliobolus sativus): Spot blotch is a soil-borne disease. It infects a great number of cereals, grasses and legumes and is distributed in soils throughout the world. The disease affects all parts of the plant, at all growth stages, and can lead to serious damage, especially in arid regions and in drought-stressed plants. After infection in the soil, airborne dissemination causes severe foliar diseases and yield losses (at high humidity). Early infections result in the seedlings dying or in stunted plants, which show tiller abortion, while infections after heading cause premature ripening and small and shrunken seeds. The lower leaves elongate and brown-black lesions that contrast sharply with the healthy leaf tissue appear after heading. The most visible symptom is a dark brown coloured, sub-crown internode.

Other preventive measures are to avoid infested plots, grow resistant cultivars, mix resistant and susceptible cultivars and treat the seeds with microorganisms or plant extracts (mustard which has a high glucosinolate level). The only effective direct measure is to burn the wheat residues after harvest to reduce the pathogen population in the soil (but this means that the organic matter of the topsoil will be burnt too).

Powdery mildew (Erysiphe graminis): The disease builds white to grey-brown fungal cushions with black dots on the leaves that lead to their death and to yield losses. Powdery mildew is only of major importance in highly susceptible cultivars. Its development is enhanced by a high nitrogen supply and dense stands with a close contact between the plants. Preventive measures include using resistant varieties and variety mixtures and avoiding very dense stands and over-fertilization. Pest management

Field pests include various aphids (which may also transmit viruses), termites, grass, bugs, thrips, beetles, grubs, worms, maggots, miners, midges, sawflies, nematodes (of the roots and the grain) and birds. In Africa in particular, migratory locusts regularly destroy wheat crops. If an application of natural insecticides is being contemplated, its impact on the beneficial organisms within the crop must also be taken into consideration. Some certifiers restrict the use of natural insecticides in organic cereals. Potential agents against aphids, caterpillars or mites include pyrethrum, Bacillus thuringensis, rotenone, soaps and the spraying of oils and Neem.

Aphids: aphids pierce and suck on different parts of plants. The most harmful is a mass-attack of aphids on the ears of the wheat, which results in smaller grains with less protein. Fortunately, heavy losses are rare. Aphids develop best in warm and dry climates, and natural enemies are important in controlling this pest. Cultural measures that encourage a high natural biological diversity and promote natural enemies consistently contribute to the control of aphids.

Nematodes: Nematodes are aquatic animals that inhabit the films of water around soil particles. Their larvae attack the roots and stunt the plants. Some species are widespread, others occur locally, some affect many agricultural crops including vegetables, fruit and staple crops, and others only attack specific agricultural crops. Wheat is affected by rootknot nematodes (Meloidogyne spp.) and cyst nematodes (Globodera spp., Heterodera spp.). Most plant-parasitic nematodes live in the topsoil. Species can persist in the soil for several years (as cyst). Most plant-parasitic nematodes encourage fungal diseases. Nematode control focuses on the interruption of the nematode life cycle by crop rotation, the promotion of microbial activity and the use of resistant varieties. No biological means of control are yet known, and experimental use of plant extracts and soil amendments has not been wholly successful. Other cultural measures such as soil solarization (the steaming or flooding of the soil), are quite effective but generally are difficult for farmers to apply.

Sunn pest (Eurygaster integriceps Puton): Sunn pests and pointed wheat shield bugs are widespread throughout the rain-fed grain producing regions of northern Africa and south-western and south-central Asia, but losses occur mainly in central and western Asia.

Storage pests: Storage pests include the rice weevil (Sitophilus oryzae), the lesser grain borer (Rhyzopertha dominica), the Angoumois grain moth (Sitotroga cerealella) and the khapra beetle (Trogoderma granarium). Rodents, predominantly the black rat (Bandicota bengalensis), also damage stored seeds. Agents to control storage pests in organic wheat are limited.

Harvest and post-harvest handling

Harvest

The time of harvesting depends on the sowing, the climate and the variety being grown. Irrigation delays harvest, whereas high temperatures speed up maturation. Wheat grains are harvested when the plants turn yellow; the grains have become dry and hard inside, and are of a golden color. Mature wheat grains naturally have a moisture content of 10 to 12%. Commercial farmers harvest the grain with mechanical combine harvesters, which cut the tillers, thresh and winnow the grains all at once. However, most farmers harvest wheat with sickles. If wheat from small fields must be harvested before it is fully mature, it should be stacked in sheaves under shelter to dry.

Post-harvest handling

After harvest, the grains must be threshed from the plant and then winnowed to separate the grain from the chaff, immature grains and impurities. Traditionally, threshing is done by beating the ears with sticks, by trampling or by driving a small tractor over the straw. Alternatively, a wheat sheaf may also be beaten against a low wall or a container, which makes it easier to collect the grains and reduces losses. Manual threshing methods generally result in higher grain losses than mechanical threshing. One of wheat's characteristics is that the grain separates easily from the chaff.

Winnowing by hand is common in the tropics, but is very laborious and does not achieve the same results as mechanical winnowing. Low-cost, hand-driven, or motorized blowers are becoming popular for cleaning and additional drying.

Storage

To ensure a good storage life and reduce losses, wheat grains must be fully dried and cleaned of dirt, insects or bad grains. Moisture content below 13% is considered safe for storage. Incorrect temperature and excessive humidity in the grain after harvest can destroy the baking quality and cause high levels of mycotoxins, which are harmful to humans. The dried grains should be stored in such a way that air is able to circulate, thus preventing the development of moulds. High temperatures and moist conditions should be avoided as this may spoil the grains. Cool and dry storage will protect the grain from fungi and moulds. The storage area should also be secured against birds and rats, both of which also pose a storage problem. On farms, storage in metallic drums, earthen jars or polyethylene containers is common. For larger amounts of grain, bamboo and mud silos are also used. If the seeds are not stored in an airtight container, it may be necessary to re-dry them regularly.

Commercial storage facilities can be used as an alternative to drying and storing the grains on the farm where they are produced. If the grain is to be sold as a certified organic product, the storage facilities must also be certified by an organic certification body.

The most common method of controlling insects in stored wheat grains is to lay them out in the sun. Most insects will leave the grain at temperatures of 40 to 44°C. Treatment of the grains in storage is rarely done in the tropics because it is too costly. Possible treatments include fumigation with CO2 or N gas in closed containers, or treatment with siliceous stone powder.

Sources and References

Bibliography

FiBL, Research Institute of Organic Agriculture, Switzerland (2011). African Organic Agriculture Training Manual. Soil Fertility Management. [Accessed 19 March 2012]

FiBL, Research Institute of Organic Agriculture, Switzerland (2011). African Organic Agriculture Training Manual. Conversion to Organic Farming. [Accessed 19 March 2012]

FiBL, Research Institute of Organic Agriculture, Switzerland (2011). African Organic Agriculture Training Manual. Crop Management. [Accessed 21 March 2012]

FiBL, Research Institute of Organic Agriculture, Switzerland (2005). Organic Cotton Crop Guide. Authors: Frank Eyhorn (FiBL), Saro G. Ratter (BioSim), Mahesh Ramakrishnan (ICCOA). [Accessed 19 March 2012]

FiBL (2011): African Organic Agriculture Training Manual. Version 1.0 June 2011. Edited by Gilles Weidmann and Lukas Kilcher. Research Institute of Organic Agriculture FiBL, Frick. [Accessed 12 July 2012]

FiBL, Research Institute of Organic Agriculture, Switzerland (2011). African Organic Agriculture Training Manual. Soil Fertility Management. [Accessed 19 March 2012]

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