Crops that become weeds

There must have been since agriculture began a sort of uneasy relationship between crops and the weeds they generate. Most of the main crops drop seed that becomes buried in soil and emerges in a subsequent crop. Agriculturists have tried and still try without ultimate success to remove this desire of crops to drop their seed. Even global staples such as wheat and maize, that have been intensively selected and bred, still do it. Crops such as oilseed rape do it spectacularly, sometimes losing 10% or more of their seed at harvest, which turns up later as a weed of fields and a plant of waysides.

This dropping of seed is commonly, at worst, a local weed problem, and might over the ages have helped farmers find crop types more suited to their locality by exposing seed and emerged plants to strong selection pressures. But the dropping of seed begins to matter, and even figures in international debates and trade disputes, where the purity of a crop must be assured, as in the special case of GM crops.

The photograph shows barley volunteers - flower heads with many long spiky awns - growing and reproducing in a crop of winter wheat - compact heads, whitish (Living Field collection).

Volunteers, ferals and wild relatives

Dropped seed that emerges and reproduces inside the tilled part of fields has come to be named volunteer (plural: volunteer or volunteers), while similar plants that become established on waysides and waste ground are named feral. The words volunteer and feral have become used in Europe to distinguish the environments in which plants are growing. Volunteers are influenced by what goes on in the field - by soil cultivation, weedkillers, and competition with other plants. Ferals usually live in poor stony ground, often polluted, but have escaped from the annual cycle of control in fields and may have the chance to evolve away from the source crop. Volunteers and ferals may originate from exactly the same variety of crop - it's where they grow that makes the difference. In addition, many crops have wild relatives with which they can cross-pollinate and so exchange genes. This mixing of genes among crops, volunteers, ferals and wild relatives is part of the process by which wild plants are domesticated and crops lose their purity and perhaps even their domestication.

In commercial agriculture, volunteers are usually treated as a nuisance in much the same way as other weeds. The broadleaf (dicotyledonous) volunteers tend in Europe to have longer-lived seed than the grass (monocotyledonous) volunteers from the cereals. The seed of the broadleaf crops can last five to10 years in the soil, even longer. Some volunteers may act as a bridge, carrying over disease between one crop and the next. Volunteers and ferals are not all nuisance, however. Since the beginning of agriculture, they have been an insurance against a shortage of seed. Even today, some farmers harvest volunteer along with the crop, sometimes knowingly, sometimes not when it's hard to tell the difference. Volunteers and ferals generally add to the diversity of plant life in farmland and surrounding urban areas. Some are likely to bring more specific ecological benefits in the form of food and shelter for small animals and microbes. They have also taught us much about the way genetic material can be held, recombined and transmitted.

The potential ecological and economic impacts of GM volunteers and ferals will be examined in a separate article in this series.

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The photograph above shows oat volunteers (lax flowering head) growing and flowering in a crop of barley (more compact head with long awns).

Source crops for today's volunteers and ferals

None of the earliest crops in Britain such as spelt and emmer wheat, or black oat, has left volunteers or ferals. Even the first forays into growing oilseed rape in the 1200s or the more serious efforts in and after the 1500s left none, as far as is known. Certainly, no rapeseed was found in the soil seedbank during many studies in the 1900s. So most of today's ferals seem to come from recent crops, probably an indication that they do not normally survive for long. In total, however, volunteers are among the commonest of today's arable weeds.

  • Cereals - barley (Hordeum vulgare), wheat (Triticum aestivum), oat (Avena sativa) - leave volunteers that remain in a field for a few years, but they seldom stray outwith cultivated ground. They are sometimes seen in great swathes where a contractor has emptied or cleaned some machinery in a field, but they soon disappear. They have no sexually compatible wild relatives in Britain, but wheat, for example, can breed with species of Aegilops in southern Europe. 
  • Oilseed rape (Brassica napus) has become a widely recognised source, leaving volunteers and ferals that can persist in a locality for years, growing as weeds or wayside plants, sometimes being transported around the countryside on wheels, rather than residing in one spot. Volunteers of this plant are among the top ten most common in-field weeds. Oilseed rape does not exist as a wild plant but it can cross-pollinate with its close relative Brassica rapa. It has become a well studied case, because of its importance to GM oilseed rape in Europe.
  • Potato (Solanum tuberosum) leaves both vegetative tubers, sometime called groundkeepers, and seed when a crop is left long enough to reproduce. The vegetative plants can last a few years but the seed is highly persistent, probably more so than that of oilseed rape. The transmission of disease over time is the perhaps the main concern about volunteer potato.
  • Beet (Beta vulgaris) offers perhaps the most biological interest in this topic since its various crop-types (sugar, fodder, vegetable), volunteers, ferals and wild relatives are all the same species in Europe and exchange genes freely. Volunteer beet is a serious nuisance weed in many parts of Europe, but hardly so in northern Britain, where not much beet is grown as a crop. An extended case study will be described for the Beta complex below.
  • Beans (Vicia faba) and peas (Pisum species) are occasionally found as volunteers and ferals. They do not commonly persist for long periods and generally cause little problem. Both are legumes and fix nitrogen, but the contribution of volunteer beans and peas to the nitrogen economy of fields will be small. Both are visited by pollinators.

Many other arable crops also leave volunteers but not usually in great number. Grasses and forage legumes, such as clover, leave volunteers that reside long term in managed or part-managed swards, but are rarely noted as being volunteer. Indeed, most of the crops discussed in the section of 5000 years on The Plants have left volunteers or ferals in at least some part of the country, even if for a brief period. So Thirsk, in citing an account of roadside madder in Turner's New Herball of 1568, raises a question that is highly topical today (though about feral rapeseed) - did the feral madder come from seed spilled from 'consignments that passed along the road from the port to the clothiers inland' or were they escapes from cultivation at an abbey or gardens nearby.

The photograph above shows left, feral bean growing with weeds, including cleavers, in the margin of a wheat field, and right, two flowering stems (Living Field collection).

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The rapeseed complex - a case study

History and background

The plants of the rapeseed complex are central to much of the debate, and the acrimony of recent years, on the transmission of GM impurity in the human food supply chain. Several plant species are involved, of which three are the most important – oilseed rape Brassica napus, turnip Brassica rapa and cabbage Brassica oleracea. (In this short article, their botanical or latin names will be shortened to napus, rapa and oleracea.) Some of the species, but not all, cross pollinate with each other. Some have wild relatives, other do not. Moreover, the crop species occur in different forms as a result of selection and breeding. Both napus and rapa have root and oilseed forms. For example, the yellowish-orange mash known as neeps in Scotland (swede some other places), is the same species as oilseed rape – both are napus.

It's often thought that the yellow rapeseed crops and their roadside ferals are a modern phenomenon, but they are not. Rapeseed has been grown for many centuries in Britain: it's just that it was not grown much in the 19-hundreds before the 1970s, so there are hardly any old photographs of it and people assume it did not exist before it suddenly appeared about 1975. So for historical authenticity - yellow rapeseed fields should not be seen in colour movies set in the UK in the early nineteen-hundreds, but may be excused in medieval costume dramas.

Rapeseed (mostly napus but some rapa) has been beneficial to modern cropping in northern Europe: it provides a ‘break’ between cereals crops, usually supports a diverse weed flora and invertebrate food web and yields a wide range of products, including the recently marketed cold pressed rapeseed oil. As a vegetable oil, it supplanted animal oils in some industrial applications. The Signal Tower Museum in Arbroath on the Stevensons' Bell Rock lighthouse (closed for refurbishment until Spring 2011) shows how sperm whale oil was replaced as the energy source for the light in the late eighteen-hundreds by colza (a French name for rapeseed, implying it was imported rather than grown locally). The colza was itself was later replaced by coal-derived oils.

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Role of oilseed rape Brassica napus

This is presently the most widely grown crop in the complex - most of the bright yellow fields seen in spring and early summer are of this species. The seeds are crushed for edible oil and animal feed and also certain varieties – commonly known as double high (high in glucosinolates and high in erucic acid) - are used to make light industrial oils. Its form is annual or biennial but some ferals can live several years.

  • Oilseed crop - common and widespread in northern Europe where it is generally called oilseed rape.
  • Root crop - neeps (as in haggis, neeps and tatties) is the same species, bred over generations to enhance the tuber. It is called swede in some parts of the world. Its flesh is yellow-orange, distinct from the white of the turnip. Since oilseed rape and neeps are one species they cross pollinate if they flower at the same time.
  • Volunteers - by far the commonest in-field weed of the rapeseed complex is derived from napus oilseed crops; it is now among the ten commonest weeds of the UK.
  • Ferals - similarly, by far the commonest yellow-flowering feral seen along roadsides and in waste ground is derived from the oilseed version of napus.
  • Wild relatives - none of the same species.
  • Cross-pollination - all napus types can cross pollinate with each other and with forms of rapa but not normally with oleracea. Their genes are carried over many kilometers by wind and insects. They form an interacting, shifting network in the landscape.
  • So where did Brassica napus come from? The cabbage and the turnip are the 'parents' of Brassica napus. How they came to produce this prolific offspring is unclear. It is difficult to get them to repeat it, even in the laboratory.

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The cabbage Brassica oleracea

The cabbage is one of the most useful species. What would we do without its vegetative variants such as sprouts and broccoli! Its nature is biennial or perennial, but some cultivated forms flower in one year.

  • Oilseed crop - if allowed to fruit, the seeds have oil in them, but cabbages are not grown as oilseed crops. 
  • Root crop -  not as these either. 
  • Volunteers - cabbages occasionally leave volunteers; their flowers are a paler yellow than those of oilseed rape, but are rarely noted or identified.
  • Ferals - as for volunteers.
  • Wild relatives - the wild form of cabbage (same species) grows by the sea in certain places, mostly on the tops and faces of steep cliffs. There are some in Scotland - where individuals seem to persist for years. Plants reared from seed taken from a Scottish location in 1998 are still growing at SCRI twelve years later.
  • Cross pollination - since it does not normally hybridise with napus or rapa, its day to day contribution to the spread of genes and impurity in crops is virtually zero.

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The turnip Brassica rapa

The turnip certainly makes the rapeseed cromplex more interesting by its two forms (oilseed and root, just like napus) and its similar appearance to napus. Up close, it is brighter green and hairier than the glaucous or bluish-grey napus or the cabbage. But at a distance its oilseed form looks the same as oilseed napus. The oilseed form was grown in the north of Britain, but has been largely replaced by new varieties of napus. Its form is annual or biennial.

  • Oilseed crop - grown in Scotland as recently as the 1990s, but rare now.
  • Root crop - having mostly white flesh and commonly called the turnip, it is widely grown for human and animal fodder.
  • Volunteers - still around in fields in east Scotland, but the age and exact origin of these populations in uncertain. 
  • Ferals - as for volunteers, found occasionally.
  • Wild relatives - a wild relative of the turnip (same species) – lives in some parts of Europe, for example along river banks in England but not (to our knowledge) in Scotland. In some northern European countries such as Denmark, it occurs commonly in and around fields.
  • Cross pollination - a pecularity of the complex is that the cultivated and wild forms of the turnip will cross pollinate and hybridise with napus. It was previously thought the hybrids were total wooses and soon died, but recent findings in Denmark show that some of the hybrids are as fit as the parents. Wild, volunteer and feral rapa therefore provide additional means for the movement and transmission of genes from oilseed napus.  

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Other species

The wild radish Raphanus raphanistrum, the wild and cultivated mustards e.g. Sinapis alba, charlock Sinapis arvensis and several similar species occur in farmland, often in similar places to volunteer and feral oilseed rape, but with the possible exception of the wild radish are not involved in the movement of genes in the rapeseed complex. Some of these species have entered fields as unwanted impurities in rapeseed crops, so it is very difficult to tell the time of introduction or origin of populations of these species.

A cabbage field of uncommon interest

It is rare for the three species oilseed rape (napus), turnip (rapa) and cabbage (oleracea)  to cohabit the same piece of land, but this occurred in certain fields near Dundee about ten years ago. The fields had previously grown rapa oilseeds in the early 1990s which left volunteers, and in some later years, napus oilseeds, that also left volunteers. Both types of volunteer lived comfortably together for some years with other weeds based in the soil seedbank. The volunteers were not much seen in cereal crops, but in the year in question, about 10 years ago, the field was contracted out to cabbage growers.

Now it is almost impossible to control these volunteers in cabbage, and so all three – the planted cabbage and volunteers of the oilseed forms of rapa and napus grew together. In some places, you couldn’t see the cabbage for the volunteers. The cabbage did not cross-pollinate with the others or in any way alter their genetics – it simply provided the opportunity for them to flourish. Both volunteer forms seeded before the cabbage was ready to harvest and replenished their seedbanks in abundance. Their seed and offspring, which probably includes hybrids between them, will remain in the field for many years to come.

In the ecology of arable systems, coincidences or rare happenings like this start traces that lead on for many years. Which leads us to 2010, when the field is contracted to vegetable growers and again, the napus and rapa volunteers are growing and flowering side by side in the same field.

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Scientific study of the complex

Volunteers, ferals and wild relatives of rapeseed have been studied at SCRI since the early 1990s. The aims are to measure and understand the movement of pollen and seed and to model the evolution of the complex. Since rapeseed was one of the first GM crops to be trialled in Europe, assessments were needed of whether GM varieties would lead to more weedy and troublesome forms and whether GM crops could be grown without breaching EC labelling thresholds for impurity in non-GM crops (presently 0.9% GM in non-GM).

The research is based in large study areas in Tayside, at farms throughout east Scotland and in the arable parts of England. Recently, SCRI coordinated a major effort to draw together data from all over Europe and reach conclusions on the rate and extent of geneflow and its importance for GM-coexistence and environmental risk. The following pages on the SCRI web site give summaries of the various projects and reference to scientific papers and reports.

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The Beta complex

The section on the Beta complex is in preparation. Here are some photographs of wild sea beet in the Adriatic.

Photographs of wild sea beet Beta vulgaris subspecies maritima and its habitat at Torcello in the lagoon of Venice: top left, landscape; bottom left, typical habitat; bottom right, leaves and stems, top right reproductive branches (Living Field/Squire).

General references

Grigson G. 1958. The Englishman's flora. Paperback published 1975 by Paladin, St Albans, UK. 542 pages.

Thirsk J. 1997. Alternative agriculture. A history from the Black Death to the present day. Oxford University Press. 365 pages. ISBN 0-19-820662-3

[Page began 3 June 2010. Latest update 8 August 2010]

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