Wheat farming

The development of the seed drill has made the planting of cereal crops over a wide area very economic. Early horse-drawn seed drills operated simply by gouging a furrow in the soil and allowing a steady flow of seed to drop into it. Modern seed drills are more sophisticated, using disk-like cutting wheels to open the seed furrow, the depth controlled by other wheels that roll across the soil surface. The wheat seed is dropped individually into the seed furrow, then the furrow is closed by rollers at the back of the drill.

Prior to the advent of mechanised agriculture, the only way wheat could be harvested was by hand, using scythes (see main image). A laborious and time-consuming process, scything requires considerable skill and patience. In the eighteenth volume of his Naturalis Historia, Pliny the Elder describes a device of Roman construction that may be the first semi-automated harvesting machine. Known as a reaper, the machine was a simple box with a toothed front edge, rather like a comb, fitted with two large wheels. An oxen was harnessed behind the reaper and pushed it through the cereal crop, the ears of grain sheared off by the comb-like front edge to collect in the box behind. Reconstructions of the reaper can only harvest grain grown at low plant densities, and could only harvest crops sown by hand.

Before mechanised agriculture threshing, the process of separating grain from the inedible chaff, was performed using handheld tools called flails. These were simple in design having a long handle attached to a shorter piece of wood that served to strike the grain. This method, though used for much of agricultural history, was labour-intensive and time consuming. The industrial revolution brought about the advent of the automatic threshing machine, a horse-drawn contraption powered by a steam-driven traction engine. These machines required only a small number of operators and could quickly manufacture threshed grain and straw from cereal crops, making many farm labourers redundant.

Winnowing, the process of extracting the separated grain from the chaff, dust and other debris, was formerly performed using a winnowing fan. A winnowing fan is a bowl-shaped structure made of wicker, into which the grain and other material were placed. The material was then tossed lightly into a breeze which would blow off the dust and other debris, leaving only the grain behind. Winnowing also removed pests such as weevils that live in stored grain. In the 18th century the Chinese developed a rotary winnowing machine, where a human operator turned a fan that blew the debris from the grain inside a wooden container. These winnowing machines were exported and saw use in Western Europe.

Modern combine harvesters can harvest grain at the high plant densities achieved by modern seed drills. They are known as combine harvesters because they perform all the harvesting, threshing and winnowing functions of more primitive machines within a single unit using just one operator. The wheat crop is aligned and passed back by a rotating pickup reel at the front of the harvester, the base of the stems then severed by a toothed blade that slides back and forth in a similar manner to a hedge trimmer. The wheat is channelled by two horizontal augers to the centre of the vehicle and passed along a conveyor to a threshing drum that separates the wheat grain from the straw. The grain is shaken free and allowed to accumulate in a hopper in the underside of the harvester before being drawn upwards using another auger and out of an outlet pipe. The straw and dust are passed out of the back of the harvester. A previous limitation of combine harvesters was the inability to harvest grain on land with a steep gradient, but this has been recently overcome by incorporating hillside levelling, where the combine harvester is tilted hydraulically to compensate for the topography. German farm machinery retailer Cornways have a timeline of development on their website.

Kansas State University have published a highly detailed resource booklet on wheat cultivation.


Soils for growing wheat

Wheat grows best in a well-drained loamy soil. The term Ďloamí is textural and reflects the particle size distribution of the soil and the relative quantities of the sand, silt and clay size fractions. Sand particles are defined as between 0.02 and 2 mm across, silt particles 0.002 to 0.02 mm, and clay particles less than 0.002 mm. Definitions of a loamy soil vary, but classifications agree that they contain approximately a third of each size fraction. The particle size distribution determines the drainage, ventilation and water retentive properties of the soil, all of which have an effect on the growth of food plants such as wheat. For example, if there is too high an abundance of large particles, such as in sandy soils, the large spaces between the particles known as macropores prevent nutrients being retained and are thus weathered away quickly. Conversely if there is too high an abundance of very small particles, such as in clay soils, the soil is likely to become short of oxygen and waterlogged. The amount of soil drainage will influence the concentration of free protons that determine acidity and exchangeable cations, both of which affect plant nutrition and growth. The texture of a loamy soil is also heavily dependant on the quantity of organic matter that serves to bind soil particles and support the soil structure.

The loamy soils that wheat requires can be further classified according to the degree of weathering that the soil has undergone. The soils in which wheat and other grasses grow best are known as mollisols, being relatively young and exposed to relatively little weathering. Thus many of the original nutrients remain and have yet to be weathered away, and more fragile mineral components such as mica have yet to be broken down physically.

If the particle size distribution and organic matter content of a soil is not favourable, growing wheat and other cereals can be difficult. Problems can be eased by using artificial soil conditioners that modify the structure of the soil, such as gypsum and some artificially synthesized organic polymers. Such soil conditioners are expensive and uneconomic over a wide area, but have been shown to improve wheat productivity when applied in quite small quantities.

Wheat farmers and breeders need to pay close attention to the condition of the soils in which they grow their crops. Ploughing and other agricultural machinery can have an adverse effect on the grain size distribution and macroporosity of a soil, causing it to become compacted. In extreme cases overworked soil can collapse, becoming waterlogged and short of oxygen. Growing crops removes nutrients from the soil, and while they can be replenished to an extent with artificial fertilisers, farmers and breeders often practice crop rotation. Crop rotation limits the amount of time a soil is exploited to grow one crop, being allowed to recover after a certain amount of time or being used to grow an alternative crop with different nutritional needs.

More information on the soils of the British Isles can be found using the following links:


Wheat as Animal Feed

Low quality wheat that has been deemed unfit for human consumption may be used as animal feed. Withdrawing a crop from the human food marked may be in response to damage caused by pests, diseases or frost. The high carbohydrate content of wheat makes it a suitable foodstuff for meat animals that are being fattened or sustaining livestock in harsh conditions during the winter. Grain from wheat-rye hybrids is sometimes used as an animal feed, being rich in protein and readily digestible. Sheep can be fed wheat grain without any processing, but cattle from having wheat rolled or coarsely ground. Rolling wheat simply flattens the grain, splitting the bran and exposing the endosperm inside, yielding a flake. This increases the surface area over which the cattleís digestive enzymes can act. This compensates for the incomplete chewing of grain.

Wheat has qualities that render it particularly useful as an ingredient in livestock feed. The high gluten content of hard wheat is exploited in the manufacture of pellet animal feeds, forming an insoluble mesh to contain other ingredients. However, caution should still be exercised when feeding wheat to farm animals, as the gluten may have a similar binding effect on the contents of the rumen, forming a solid mass that inhibits digestion. In extreme cases overfeeding of wheat to ruminants can result in the complete blockage of the rumen, a condition known as bloat which can be fatal.

For more information on the use of wheat in animal feed, the Ontario Ministry for Environment, Food and Rural Affairs website may be of interest.


Wheat diseases

The diversity of pathogens that infect wheat is wide. Many common diseases of wheat belong to the fungi, but wheat can be parasitized by viral and bacterial pathogens as well. Diseases of wheat may not be specific, and may be affected by pathogens that have a more potent effect on other plants, such as the barley yellow dwarf virus.

Take-all, caused by the fungal species Gaeumannomyces graminis, is a major root-rot pathogen of cereals. The species affects the entire grass family including wheat and other cereals, but different groups of grasses are parasitised by a particular variety of G. graminis. The variety that parasitizes wheat is named G. graminis tritici. The take-all fungus enters the wheat plant through the roots using long darkly coloured strands called hyphae that causes the characteristic black colouration at the base of the stems. Take-all fungus then goes on to block the plantís conductive tissues (xylem and phloem), preventing the transport of water and nutrients to and from the roots. The roots then rot and the plant dies. To present, all attempts to breed wheat varieties that are resistant to take-all have failed. However, take-all can be controlled biologically using a competing species of fungal parasite. Phialophora graminicola is a fungal parasite of members of the grass family including wheat and other cereals. Unlike G. graminis, P. graminicola only infects the outer part of the plant root and does not damage the vascular tissues. Therefore, the entry points to the plant are already occupied and more hostile fungal parasites cannot infect it. This kind of biological pest control is known as competitive niche exclusion. Take-all disease has a habit of easing naturally after a few years of infecting a crop, a phenomenon called take-all decline. When this occurs grain yields may return to economically viable levels. This decline in virulence is thought to be caused by a bacterium, Pseudomonas fluorescens, which develops in diseased crops and synthesizes antibiotics that go on to control the fungal infection. It is believed thatP. flourescens may be responsible for controlling other cereal root diseases.

Edinburgh University have a webpage with detailed information on Take-all disease. The site is no longer maintained and has been archived, but there are interesting images and suggested titles for further reading.

Seedling blight is caused by the fungal pathogen Fusarium nivale. Despite the name, wheat seedling blight is not related to the infamous potato blight that caused the Irish potato famines of the nineteenth century. Seedling blight overwinters as spores in soil, on straw and crop debris, or in infected seeds. Infected seeds develop the disease at an early stage, but plants may also become infected by spores distributed by rain splash. The fungus develops in the ears of grain on the growing wheat plant causing a blighted appearance, the infected grain falling to the ground and continuing the spread of the disease. Close relatives of the seedling blight pathogen, F. graminearum and F. calmorum, cause wheat ear blight. Toxins produced by these pathogens named Deoxynivelenol (DON) and Zearalenone (ZON) can sometimes contaminate flours made from infected wheat. Ironically a close relative of the seedling and ear blight pathogens, F. venenatum, is cultivated as a source of mycoprotein used in some meat substitutes such as Quorn.

Ergot is caused by a fungal parasite of wheat and other cereals, Claviceps purpurea. Ascospores are produced by the fruit of the fungus that are wind-dispersed and infect grasses. Once the infection is established, the ascospores develop into a fungus that produces secondary spores called conidia in a sweet-tasting honeydew that attracts insects. The insects then transmit the conidia to wheat plants. This alternation between generations is common in fungi. The fungi cells then differentiate into structures to produce the black fruit. These then mature and drop off the wheat and begin to produce ascospores. It is these black fruits that cause the black flecks seen in flour contaminated with ergot. Ergot is detrimental to wheat growth, and is also toxic to animals that eat the infected wheat, causing a condition known as ergotism. Both the grain and the straw become infected and thus ergotism is a risk to both humans and livestock. Symptoms of ergotism include nausea, itching, seizures, headaches and hallucinations. It has been suggested as an explanation for medieval ideas about witchcraft. Extreme cases can result in gangrene in the extremities of the body. Ergoline, the toxin in the ergot fungus that causes ergotism, is the basis of some drugs including those to treat migraines and ease Parkinsonís disease.

Powdery mildew of wheat is caused by the fungus Blumeria graminis. The disease gets its name from the dusty white encrustation of fungal hyphae on the leaves of wheat and other cereals. The effect of powdery mildew can be severe, with loses of up to 40% of the crop yield in extreme cases. Mildew grows best in mild temperatures at high humidity, but does not require liquid water to be freely available to infect a plant. The fungus overwinters in straw and crop debris. In spring it produces wind-borne spores called conidia in flushes every seven to ten days. Infection is most widespread in wheat that is heavily fertilised and densely planted. Imperial College London is part of an ongoing project to sequence the genome of B. graminis, and their website contains detailed information and some interesting images. Ohio State University has complied a fact sheet on powdery mildew of wheat.

Wheat rust is caused by the fungus Puccinia triticina, with different strains that produce diseases on different parts of the plant anatomy. Variations of the disease affect the stem, leaves and grain with circular to oval lesions varying in colour from yellow to brown. Like mildew, wheat rust is distributed by airborne spores produced by the coloured patches on an infected plant, called aecia. These uredospores are very light and may be transported hundreds of kilometres before being washed out of the atmosphere by rain. At the right temperature the uredospores germinate within half an hour of coming into contact with liquid water. After germination, they extend a germ tube that extends across the host plantsí leaf and then inserts itself into a stomata pore, a microscopic opening on the surface of leaves that plants use for gas exchange. Initially the fungus continues to produce uredospores such that it can reproduce asexually, later moving on to producing sexual basidia and basidiospores from small black structures called telia. The basidiospores are dispersed, but are limited by humidity and can only travel a few tens of meters. The fertilised basidiospores develop into the aecia that produce the uredospores and continue the life cycle of the rust fungus. The US department of agriculture has released a detailed booklet on wheat rusts.

The Bayer Crop Science website contains detailed further information on rusts and other diseases affecting wheat, however the site search is limited to binomial names.

Eyespot is caused by the fungal pathogen Oculimacula yallundae. The fungus grows on straw and wheat stubble, reproducing sexually with spores that fertilise to form an asexual ascospore. The ascospores infect a new host plant by the action of rain splash. Once infected the plant develops large spots on the base of the stem, from which the disease gets its name. Advanced stages of the disease include the formation white heads, where the ears of grain take on a bleached appearance, and lodging, where the stems bend and become procumbent. Lodging is caused by the lower sections of the stem, known as internodes, buckle and loose their structure. When this occurs harvesting becomes more difficult, and crops suffer losses to grain rotting close to the ground. Ohio State University has compiled a fact sheet of information on Wheat Eyespot.

Glume blotch is caused by the fungus Septoria nodorum that overwinters in infected straw and grain, and can survive on the leaves of winter-growing varieties of wheat. Dark fruiting bodies called pycnidia develop on the stems and on the head of the wheat plant, producing spores that are spread by rain. Glume blotch spreads most effectively in wet and windy conditions. The spores need to be wet for at least six hours before they can germinate and cause an infection, therefore dry weather can arrest the spread of glume blotch. The fungus inhibits the grain filling stage of the wheat plantís development, resulting in shrivelled seeds with little mass. Further information on glume blotch and other diseases of cereals can be found through the following link.

The government of Manitoba, Canada has a selection of pages on cereal diseases , including glume blotch and others discussed here.

Barley yellow dwarf virus is principally a disease of barley and oats, but can affect wheat to a lesser extent. Barley yellow dwarf virus is classified as an ssRNA virus, meaning that its genome lacks DNA and is made up of a single strand of ribonucleic acid, or RNA. The RNA is not arranged into the helical formation seen in more complex organisms. The virus overwinters in grasses and is transmitted to the host cereals by wingless aphids. Once the disease has infected a small area it is transmitted wider by flying aphids. Diseased plants develop a yellowing of the leaves and stunted growth that gives the virus its name. Viruses such as the barley yellow dwarf reproduce by hijacking the hostís cellular mechanisms to generate their own proteins, dispensing with the cellís correct function and eventually killing it.

Wheat mosaic virus produces patches of tissue death across the leaves, giving a multicoloured appearance from which the disease gets its name. These patches are not caused by the virus directly but by a process called necrosis, where tissues die in response to a viral infection. This helps prevent the virus from spreading to other tissues in the plant. Wheat mosaic virus was first found in 1999 in the Cotswolds, and is transmitted by the fungus Polmyxa graminis. Columbia University runs an online database with information on viruses, including those that parasitise wheat.

These websites offer a collection of more detailed information on the diseases of wheat and other cereals: