Integrated Crop Management (CPA Book)

Crop Rotation


Introduction

The notion of growing different crops in sequence has long been recognised as an effective means of improving soil fertility and reducing the impact of weeds, insect pests and diseases. The earliest and best known use of the technique was the traditional Norfolk four-course rotation practised at the end of the nineteenth century. This consisted of a sequence of roots (usually turnips or swedes), barley, clover and wheat.

In terms of nutrition, the basic principle of rotation is the alternation of crops like cereals that exhaust the supply of soil nutrients ( exhaustive crops) with those that replenish them, such as legumes (restorative crops). Protection against pests is achieved by interrupting the life-cycles of many crop-specific diseases, insect pests and weeds so that, by the time the same crop returns to a field in the rotation sequence, the pest numbers have been severely depleted. The cultivation's between crops, particularly where ploughing and burial of surface trash occurred, also helped to prevent 'carry-over' of some less specific pest problems, while surface cultivation before and after sowing root crops grown in widely spaced rows was useful for controlling troublesome weeds. For this reason, such crops are often referred to as cleaning crops.

...benefits of an arable rotation

There were, and still are, other benefits from rotational farming. These include the spreading of risk if one crop should fail because of weather, or if market prices for it become depressed, for whatever reason. Equally important is the spread of labour requirements over the year, so that farm staff are kept gainfully occupied. Finally, a rotation allows a balance, in any one year, between crops grown and sold off the farm, known as cash crops, and those used on the farm for feeding livestock so stabilising cash flow and providing the opportunity for the return of nutrients and organic matter to the soil.

The impact of modern technology

With the aid of modern farming practices and technology, many of these benefits can be enjoyed without the need to adhere to a rigid rotation. Farmers have been able to switch from traditional mixed farming to all-arable enterprises consisting entirely of cash crops. This simplification has been made possible by the availability of better machinery for cultivating and harvesting, as well as technological developments in fertilisers, crop protection products and plant breeding. These have enabled farmers to carry out work on the land more quickly and efficiently which has reduced the need for labour.

Pesticides have provided cost-effective answers to some massively disruptive pest problems so increasing yields and income. Plant breeding, too, has made available a wide range of varieties suited to different markets and different growing conditions leading to improved quality and yield. As an added bonus, many varieties have inbred resistance to some major pest problems (usually diseases). Today we are on the threshold of the introduction of genetically transformed crop varieties which will exhibit properties of insect pest and disease resistance, and even herbicide tolerance, that would have been difficult to achieve by conventional breeding. It remains to be seen whether the promised benefits will become reality, but the prospect is exciting. Finally the various guaranteed price systems for agricultural produce that have operated since the second World War  have helped to even out cash flow over 'good' and 'bad' years.

...continuous cropping

Cereals are the backbone, in financial terms, of UK farming, and the ultimate simplification was to grow them continuously. In the sixties and seventies, on the thinner chalky soils of south-eastern England, many farmers turned to continuous cropping of cereals and became known as the 'barley barons'. Clearly, this total abandonment of any form of rotation meant that the natural benefits were lost and had to be replaced artificially. A famous series of experiments at Rothamsted Experimental Station have proved that yields of continuously cropped wheat and barley can be maintained almost indefinitely at a reasonable level if fertiliser input is high enough.

...and problems

But the pendulum had swung far enough. Apart from the huge peaks and troughs in labour demand on the farm, other problems began to emerge. Annual and perennial grass weeds, for example meadow grass, wild oats, blackgrass and couch, thrived under continuous cereal cropping and became a major problem that was only partially answerable with herbicides.

A gradual shift to winter croppings made things worse and bought cereal diseases into prominence. Cereal fungicides initially provided the solution but there was no chemical answer to take-all, and reliance on take-all decline was economically unwise. This meant that the number of wheat crops that could be grown in succession was effectively limited to two.

The control of diseases such as eyespot, mildew and Septoria, became totally dependant on one or two chemicals and eventually resistance appeared. Gradually there was a realisation that the traditional benefits of rotation could not be so easily discarded in spite of the advantages of technology.

The challenge was to decide what to grow in rotation with the cereals. potatoes, sugar beet or vining peas and beans were options in some parts of the country and on some soil types. Although the gross margins on many of these crops can be attractive, the capital investment and high variable costs incurred in growing them was often a deterrent. In the livestock areas grass was the obvious choice. More recently, set-aside has been imposed on the rotation. The difficulty was that farmers had trimmed their labour to the bare minimum and were equipped with machinery designed for combinable crops, i.e. those which needed to be sown with a seed drill, harvested with a combine harvester and, perhaps, dried in store.

Salvation came with recognition of the value of oilseed rape in temperate climates and a highly significant improved European price support for it. Winter rape fits well as a follow-up to winter barley and can be handled by the normal cereal machinery. The area grown increased rapidly in the late seventies.

Growers of horticultural crops have the same problems. vegetable growing is highly specialised and intensive. It is a high risk/high return business with success dependant on delivering quality produce at the right time. Often several crops are produced from the same land in one season, resulting in increased risk from some pests and diseases.

Modern rotational farming

Today, even in the most intensive arable systems, some form of alternation of crops is practised. A four-year rotation is now recognised as too short for the effective reduction of some pest problems, and five or six-year sequences are more commonly used. These will nearly always include a succession of two or three cereals between which are grown break crops, one of which is nearly always oilseed rape. Other changes from the original four-course cropping are the replacement of traditional root-crops with potatoes, sugar  beet, carrots and, in some parts of the country, mangolds. Beans and peas have replaced red clover or, alternatively, a 2- or 3-year grass ley break is grown. Other oilseed crops such as linseed and even lupins, have been tried in the UK but both present a number of problems which are not associated with oilseed rape.

Rotations and ICM

Integrated Crop Management requires a recognition of the fact that a diverse crop rotation is the most effective indirect means of maintaining soil fertility and controlling pests. By the judicious application of modern technology it seeks to enhance, rather than replace, the benefits of rotation that were apparent 100 years ago. The aim should be to grow healthy, vigorous crops producing economically viable yields and quality by making optimum use of natural resources. In this way the reliance on technological inputs such as fertilisers and pesticides is reduced, but the need for them is by no means eliminated. Such a system also optimises cash flow and makes the best use of labour and machinery, just as t did in the nineteenth century.

The actual cropping sequence is central to the whole cropping plan. the most suitable rotation will vary from farm to farm - an indication of why ICM provides guidelines and not prescriptions. Within the cropping sequence consideration must be given to other factors such as varieties own, pest control, nutrient availability, environmental impact, harvesting, storage and marketing. A five year rotation is desirable where crops such as sugar beet, potatoes or oilseed rape are involved. ideally grass leys should be included where ever feasible to improve fertility and build-up soil organic matter. Grass also reduces erosion and nitrogen leaching into groundwater. This would include, for example, the potential conflict of demand for harvesting late winter cereals and drilling oilseed rape.

Finally, from a wildlife viewpoint, a diversity of cropping is important. Furthermore it helps if the various crops are spread around the farm in relatively small units, rather than large blocks, thus creating a mosaic of habitats and a continuity of   food supply and shelter within reasonably short distances. This, of course, has to be balanced with the operational advantages of growing larger areas of the same crop in close proximity.

...variety choice

Varieties should be chosen that not only are suited to the climatic and soil characteristics of the farm but which also meet the requirements of the proposed market. In the modern world, more than ever before, farmers need to be able to meet the demands of their customers. This means identifying in advance what is wanted, and then delivering the product on time to the specification ordered. Plant breeders have produced a wide range of cereal varieties but choice still requires skill. Varieties with a prostrate growth habit help by competing with weeds in the critical early stages of growth. as well as quality, factors such as standing power, drought tolerance and disease resistance should all come into the equation. Total reliance on one or two cereal varieties should be avoided in order to minimise the risk if disease resistance should break down. Soon the choice will be widened by the appearance of genetically transformed varieties offering pest resistance, herbicide tolerance or improved agronomic features. Awareness of these developments and being able to fit them into the overall farm cropping plan is an essential part of ICM.

...pest management

Rotation can assist pest control well beyond the choice of resistant varieties. Mention has already been made of the effect of successive cereal crops on the incidence of diseases such as take-all and eyespot. There are several other examples where the cropping sequence can encourage pest problems. Where these involve crop-specific problems, such as nematodes in sugar beet or potatoes, the solution is top ensure an adequate interval between repeat cropping on the field. Even on adjacent fields it is sensible to avoid growing the same variety or even the same crop so as to discourage the migration of 'mobile' pest problems from one to the other. These include flying pests, such as aphids, as well as wind-borne fungal spores.

Some problems, such as Sclerotinia, can affect several crops that might be included in the rotation: in this case oilseed rape, peas, beans, celery and carrots. Awareness of this is clearly important in planning a rotation since growing any of these crops in close succession increases the risk from Sclerotinia.

In spite of the wide range of available herbicides, some weed problems are still very difficult to control. These include volunteer crop plants, such as potatoes growing as weeds in other crops. Some grass weeds, such as couch and meadow grasses, can also be difficult to control. More recently the problem of herbicide resistance has arisen, especially with blackgrass which, in some areas, is showing a high level of resistance to many of the products commonly used for its control in cereals. The rotation should provide suitable opportunities for the control of these weeds.

Rotations

1. Cereals:

  • Exhaustive (cash) crops;

  • Encourage grass weeds;
  • Continuous cropping encourages take-all, eyespot, nematodes.

2. Potatoes and root crops:

  • High value cash crops;
  • Heavy demand for nutrients and labour;
  • Necessary cultivation's control many weeds: 'cleaning crops';
  • Continuous cropping very unwise: eelworms.

3. Pulse crops (e.g. peas and beans):

  • Cash crops;
  • Natural restoration of nitrogen via bacteria fixation in root nodules;
  • Continuous cropping causes build-up of Sclerotinia, stem rots and eelworms;

4. Oilseeds (e.g. oilseed rape, linseed):

  • Cereal break crops;
  • Oilseed rape drilling and harvest fit well into cereal timetable;
  • Thrive on deep soils;
  • Oilseed rape, pulses, sugar beet should not be grown on same field more than once in 5 years.

5. Restorative crops (e.g. leys, kale):

  • Crops normally fed from field:
  • Allow return of nutrients and organic matter to the soil.

Choosing the right field and measuring environmental impact

Some fields on the farm are likely to be more suitable for particular crops than others. Aspect, topography and soil type need to be taken into account as well as ease of access and possible difficulties operating machinery like combine harvesters. The planned sequence must allow sufficient time for the disposal of any crop residues and the necessary cultivations before drilling or planting the next crop. This means that the cropping plan is likely to include autumn and spring-sown crops. The timings of the cultivations themselves is one of a number of important factors in assessing the environmental impact of the cropping sequence. Damage to the soil structure, disturbance or destruction of nesting sites and removal of food sources can all result from ill-timed cultivation operations. Equally important, the removal of green cover in the autumn months increases the risk of nitrate leaching into groundwater.

...set-aside

Set-aside is now, for most people, an obligatory part of the cropping plan. Set-aside involves taking an area of a farm out of normal arable production in return for payments, provided certain conditions about the management of the set-aside are fulfilled. Although it has been used in the USA since the 1930's set-aside was first introduced into the UK in 1988 as a voluntary measure. The aim was to reduce agricultural production but, although, 10% reduction was achieved in the first year in the UK, the effect across the European Union as a whole was negligible. The scheme has been subsequently modified in successive years so that several options are now available and participation is compulsory for all but the smallest holdings in order to qualify for Arable Area Payments on the remainder of the farm.

Although the primary purpose of set-aside is to reduce production, there are considerable secondary benefits which are very much in line with the overall aims of ICM. Rotational set-aside, in particular, has provided an ideal opportunity to control annual and perennial weeds with non-selective herbicides. Set-aside is also an opportunity to reduce inputs and enhance biodiversity on the farm and there is already evidence that this is happening with bird populations. In choosing which fields are best suited for set-aside, the aims should be to minimise weed seed production and to control perennial weeds and slugs. Top growth needs to be kept to manageable proportions and it is particularly important to avoid harbouring volunteer crops and weeds which act as alternative hosts to diseases and insect pests.

Harvesting and storage

Planning the crop rotation includes considering the requirements for harvesting, storage and marketing the produce. Apart from the obvious need for the correct harvesting machinery and its maintenance, the timing of harvest of the various crops must be matched with available labour and storage capacity. Some of these factors are very weather dependant and even the best plans can falter, for example in a wet autumn. The plan should allow for this possibility and make alternative provision if necessary.

Pest attack does not stop when the crop is removed from the field and some of the biggest losses to pests occur in the store. Whilst there are pesticides available, those used after harvest are the most likely to leave residues on the produce. It is therefore important to include, as part of the overall crop management, measures which will minimise the need for further pesticide use at this stage. This starts with attention to general hygiene in stores and frequent checking of the crops and produce kept in them. Control of ventilation and humidity are essential. Low temperature storage for potatoes and fresh produce helps maintain it in good condition. In-store traps, such as pheromone traps for saw-toothed grain beetles, gives early indications of pest invasion.

Conclusion

The farmers of the late nineteenth century enjoyed the benefits of a managed crop rotation. Integrated Crop Management seeks to combine the best of those traditional practices with modern technology to enhance the advantages for the benefit of farmers and growers, consumers and the environment. By reducing the impacts of pests and improving soil health, crop quality is improved, labour management is simplified and cash flow assured. In addition careful planning helps to conserve wildlife havens and enrich general biodiversity on the farm.

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