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Biological Agriculture and Horticulture, Vol. 7 pp. 117-137 0144-8765/90 $10

1990 A B Academic Publishers Printed in Great Britain

A Review of Non-Chemical Weed Control Techniques

S. Parish

The West of Scotland College, Auchincruive, Ayr, KA6 5HW, Scotland*

ABSTRACT

Weeding has traditionally been a labour intensive operation in crop production. The use of herbicides was rapidly accepted by many farmers and became an accepted part of crop husbandry, although a few farmers always questioned the widespread use of chemicals in farming, and the concept of organic farming necessitated a non-chemical approach to weed control.

The recent upsurge in environmental awareness of the public, interest in organic food production and some problems with herbicide use, has led to a range of techniques and machines being developed for non-chemical weed control. Thermal and mechanical techniques are reviewed for cereal and row crop production.

INTRODUCTION

Ever since the first cultivation systems were developed for food production farmers of all generations and areas have been faced with the problems of noncrop plants growing amongst the crops. These non-crop plants, which compete with the crops for moisture, light, nutrients and space, have long been known as weeds.

The problems which these non-crop plants have caused to farmers have led to the term weed being used as an insult to other humans, often inferring lack of courage or strength. Yet weeds which are thin, spindly and pale are often so because of their resilience and ability to compete with the crop plants. The trade names of herbicides developed to control weeds imply that they are a challenge worth combating, or names such as Avenge, Crusader, Harrier and Stomp would not be used.

With the much greater public awareness of food and environmental issues which has developed rapidly in recent years, it is probably worth looking at weed control from a wider perspective, and particularly at methods of weed control in systems where herbicides are too expensive or ineffective to use and in systems in which they are not permitted for use.

A weed can be thought of as any plant growing in the wrong place at the wrong time. In crops weeds can cause problems of severely reduced yields and also affect the efficient use of machinery, for example in harvesting and crop storage. Effective weed control is therefore an important part of crop husbandry, and has traditionally been a labour intensive operation. In the 1880's a large farm of 100ha near Glasgow had a regular staff of 3 or 4 ploughmen and 2 female servants. In April 20-24 extra women were hired for the planting of potatoes and in late May 30-40 workers were taken on to weed the potato crop (Orr, 1984). In less developed countries the situation still exists where the peak labour requirement is often for hand weeding (and still done by women (Rogers, 1979)). If this labour demand cannot be met, then the crop must be grown on a smaller area than would otherwise be economically viable.

WEED CONTROL AND ENVIRONMENTAL ISSUES

As agriculture became mechanised cultivation techniques for weed control were developed, particularly for inter-row work in widely spaced row-crops. A significant amount of manual work was still required for the weeds in the crop rows, although steerage hoes were used for the inter-row areas. To control couch (Agropyron repens) and other rhizomatous weeds, intensive cultivations were practiced, particularly using "L" blade rotary cultivators (Fail, 1956). Inter-row cultivations for weed control in potatoes can cause problems of clod formation and variations on the traditional equipment design were developed (Green, 1962).

The huge reserve of weed seeds in the soil means that any cultivation operation will stimulate another flush of weeds to germinate. The nature of the growth of the crop plants then becomes an important factor. Leafy growth which spreads to cover the ground can effectively smother the weeds but crops such as onions, which have thin leaves tending to stay well above the soil surface, are prone to severe weed competition.

It is therefore hardly surprising that the development of chemicals to act as herbicides gained rapid acceptance among farmers, as successful weed control is a major contribution to a successful crop. Extremely toxic substances, such as sodium arsenite and DNOC, one of the first selective herbicides, were succeeded by translocated and growth affecting chemicals like MCPA, which is less toxic to animals. Persistent and residual herbicides, such as simazine and the longer lasting linuron, are now available for appropriate applications. However, in the 1960's concern was already being shown over the environmental effects of pesticides in general, and insecticides in particular. Herbicides were also investigated, and studies in the USA as long ago as 1959 showed aminotriazole, a translocated weedkiller, to be carcinogenic (Carson, 1965).

The use of arsenical weedkillers to destroy potato haulm ceased in the UK after 1961 but even herbicides which are less toxic to animals continue to cause concern. The wide use of MCPA since the 1940's has led to a significant decline in numbers of wild flowers (Mellanby, 1969), and although many will regard the term "wild flower" as a euphemism for "weed", even these have their uses, particularly from a medicinal or dietary point of view. For example, stinging nettle (Urtica dioica) can be used as a diuretic and to improve the blood supply; common fumitory (Fumaria officinalis) can aid digestion and scented mayweed (Matricaria chamomilla) is used for skin conditions (Kresanek, 1982). Where floating weeds, such as the water hyacinth (Eichhornia crassipes) have become a nuisance, they can be harvested and used as a fertiliser source or pig feed (Pirie, 1978).

The diversification of weed communities has recently been the subject of several studies, particularly those comparing weed species between organic and conventional fields and the field edge and field interior. Van Elsen (1990) found that biodynamically cultivated grain fields show a wider diversity of species with less contrast between the interior and edge, whereas the grain fields treated with herbicides have more weed species at the edge (Table 1). Plakolm (1990) also found that in conventional field boundaries there are more species only present there, which are not present in the field interior.

In 1988 the British Crop Protection Council initiated the organisation of a symposium in order to bring together ideas on how to develop reliable methods for identifying and evaluating the effects of pesticides on the environment. In the paper presented by Booij & Noorlander (1988), it was suggested that the effect of herbicides on fauna is primarily influenced by the availability of soil cover and therefore lack of prey. However, it was acknowledged that further investigations are needed to study the effect of herbicides on the ecology of cropping areas.

Environmental issues, therefore, are a significant influence on the development of non-chemical weed control techniques. There are, however, more mercenary aspects to consider. The high cost of herbicide treatment in crops such as sugar beet and fodder beet have led to investigations into reduced herbicide use, such as band spraying along the crop rows combined with inter-row cultivations. Some farmers or growers may be tempted by high prices to consider organic production, in which no herbicides are permitted (UKROFS, 1989) and the reliance for weed control reverts to mechanical, thermal, cultural and management practices. Other horticultural crop producers may, through poor management, find themselves with a weed spectrum for which there is no effective herbicide permitted for application to the crops being grown (Haas & Streibig, 1982).

WEED COMPETITION

At this stage it is worth considering some basic aspects of weed management, before looking in detail at the techniques available for non-chemical weed control. An awareness of the common weeds in the different fields is important, so operations such as cultivations, sowing and weeding can be timed according to the peak germination periods of the predominant species (Figure 1). In continuous cereals the range of weeds is often reduced to those whose cycles fit with that of the cereal crop. For example, if cleavers (Galium aparine) are a major problem, then the sowing of winter cereals should be delayed until after their peak time for germination. Crop rotations should be designed so that the differences in the timing of seedbed cultivations prevents one weed species becoming dominant (Lockhart et al., 1990).

The total absence of weeds has only become a possibility following the introduction of herbicides. However, the complete removal of weeds from within a crop may itself cause other problems. Insects then have no alternative but to attack the crop itself and there is no suitable cover for the predators of crop pests (Altieri & Letourneau, 1982). Agronomists and statisticians have yet to agree on the effect on crop yield, or the cost/benefit analysis, of the presence of low weed densities, as is indicated by the conflicting curves shown in Figure 2 (Cousens, 1987). The situation in organic crops has yet to be researched and the beneficial effect of weed presence is a topic outside the mainstream of agricultural research and development. No doubt the 1990's will see more investigations in this area.

Combined with the knowledge of weeds themselves, an understanding of the tolerance of crops to weed competition can be critical in establishing the timing of weeding operations. The timing of weeding in vegetables has been shown by Weaver (1984) to be critical in order to avoid yield reduction, and the duration of weed competition is also influential. For most crops, a critical period exists during which weeds must be controlled to maintain yields. The results of experiments in onions show that, during the first four weeks after 50~0 emergence, the weeds had little effect on yield arid weeds allowed to grow on after ten weeks also did not reduce crop yield (Figure 3). The critical period of weed competition for this crop is therefore from four to ten weeks. The onion crop is particularly difficult to keep weed free, other than by hand or with herbicides, although thermal techniques for the crop are outlined later in this paper.

Tables on other crops, for the toleration of early weed competition without yield loss and for the weed-free period required to prevent yield loss, are published in an international review by Zimdahl (1980).

NON-CHEMICAL WEED CONTROL

It is extremely difficult to put a price on the research and development costs of the herbicides used in agriculture today. It would not be a gross exaggeration to state that less than 1% of that cost has been spent on the development of non-chemical weed control methods and yet the major food retailers expect their sales of organically produced vegetables to be at least No of total sales by the mid-1990's. Non-chemical weed control does not necessarily imply reverting to outdated techniques and an impressive array of modern machinery already exists, some of which are new ideas and others developments of more traditional implements. The role of these machines for effective weed control should now be examined as part of a weed management strategy.

Weed control in organic cereals

A good rotation is needed for pest and disease control, the maintenance of soil fertility, as well as for weed control. An example of a mixed farm 8 year rotation is:

 

The 3 year ley is to control annual weeds, while undersowing helps to smother weeds and provide soil cover in the winter.

The timing of sowing is important, and with winter wheat, autumn weed problems can be avoided by delaying sowing until late October or November. A cereal variety with long straw and an initially prostrate growth habit, which covers the ground quickly, enables the crop to smother the weeds. However, in organic production resistance to common diseases is a prerequisite for variety selection (Parish, 1989a). Other techniques to smother weeds include increasing the sowing rate by up to 20%, sowing cereals in bands and, as in parts of mainland Europe, undersowing winter cereals with mustard (Lampkin, 1986).

Higher seed rates are desirable, not only to provide more competition for weeds but also to help compensate for any crop loss due to weed control cultivations after sowing. A thin-tined implement (Figure 4) can be used for operations pre- and post-emergence. Blind harrowing, just before the crop emerges, aims to disturb weeds which have already germinated. Careful examination of the soil to examine the weed seeds is essential, otherwise germination of the weeds will be stimulated instead. Once the crop has reached the 3 leaf stage harrowing can be effective, with chain harrows or striegels being used at speeds up to 8 km/h. The draft requirement is low, and a workrate similar to that for herbicide application can be achieved for a similar effect at a reduced cost (Brautigan, 1990). Some harrows have adjustments for different levels of pressure on the soil, to match the prevailing conditions. Hoeing cereals is possible if the rows are spaced closer than normal drills allow, in bands, and between the bands a wider gap is left for a hoe (Figure 5). The overall sowing rate should be increased by 10% for this system. Mustard sown as a fast growing cover crop to smother weeds in autumn sown cereals is possible if severe frosts are certain to kill the mustard, to prevent it competing with the crop in the spring. It is also thought that mustard may have an allelopathic effect on weeds.

Further investigations into the natural enemies of weeds are likely to identify either insects or diseases which can be used as a form of biological control. Examples here are the effect of ground beetles on weed seeds in USA maize production and the use of the rust Puccinia chondrillina to control the weed Chondrilla juncea L, which had become a pest in Australian wheat and pasture areas (Andres & Clement, 1984).

In a report by Patterson & Bufton (1986), the modification of combine harvesters was recommended in order to separate the weed seeds from the grain, straw and chaff to avoid returning seeds to the soil. The destruction of weed seeds in the soil has been found possible by the use of microwaves which distort the molecules of tissues and cause lethal internal damage (Davis, 1975). However, for the control of wild oat (Avena fatua) seed a similar method was found to be impractical for field use due to high energy costs (Lal & Reed, 1982).

Weed seeds can also be returned to the soil through animal wastes. To reduce the viability of seeds, the wastes need to be composted and to reach a temperature of at least 60C (Soil Association, 1989). In UK conditions aerated slurry lagoons are unlikely to reach this temperature without a cover and insulated sides.

Weed control in organic row crops

Many of the aspects of weed management already discussed are also pertinent for row crops. A wide range of machines exist for weed control in row crops and it is useful to consider the two major types separately.

Thermal techniques for weed control

The control of weeds in the crop row is a major problem in many organic crops. To combat this problem, thermal techniques pre-emergence of the crop are becoming more sophisticated and there are also some crop plants which can tolerate post-emergence treatment at specific growth stages.

Thermal techniques, often called flame weeding, generally use liquified petroleum gas (LPG), mostly propane, but in the 1950's work was also carried out using oil burners to reduce weed competition in bulb production (Wolfe & Horton, 1958). However, some experimental work has been carried out into the use of electricity, both as a heat energy source and for electrical shock treatment. Of the latter, two methods, spark discharge and electrical contact, are under development, both needing voltages of around 20 kV to be effective. An electrical contact machine to control sugar beet bolters has produced a plant kill rate of 40%, compared to 60% for chemical control (Diprose et al., 1985). Only one machine of this type has been produced commercially and its capital cost makes its use uneconomical below 900 ha/annum. However, compared with chemical use there are no dangerous residues following the treatment and therefore no delay in subsequent operations. Also the field efficiency of the operation is high, as there is no requirement to refill sprayer tanks or, in the case of thermal treatments, exchange gas cylinders.

The high voltage required for these machines poses a hazard, which may be less of a problem if lower voltages were used to generate heat to expose weeds to infra-red radiation. Laboratory investigations into the effects of different infra-red wavelengths on plants, identified a medium wave tubular fused quartz emitter to be the most effective of the infra-red emitters tested. Energy intensities between 200 and 400 kJ/m2 were required to severely affect plant growth at the seedling stage (Parish, 1989b), with dicotyledons more susceptible to heat than monocotyledons.

LPG fuelled flame weeders have now become established as part of the organic grower's machinery complement. The aim of a flame weeding operation is not to burn off the weeds but to apply sufficient heat to severely damage the plant cells so the plant will eventually wither and die. The technique involves raising the plant tissue to a temperature of 100C for 0. is, in order to burst the cell membranes. Coagulation of proteins occurs between 50C and 70C (Hoffmann, 1980). The accurate measurement of temperatures of small plants for short time periods is not easy, although some methods have been documented and a workshop to discuss techniques was held in February 1990 in Denmark. Two basic designs of flame weeder are available, flame contact and infra-red. The latter design (Figure 6) involves heating a surface to radiate energy on to the plants being treated. The heated surface is required to be about 1.5 m in length, as exposure of plants to the heat for approximately 2 seconds is required. Infra-red equipment has a much higher purchase price, but was considered to be more economical on gas when compared to the flame-gun type shown in Figure 7. Recent trials have shown the infra-red performance to be inferior to that of flame guns (Ascard, 1990), but the latter design is continually being refined to improve the efficiency of gas use. Shielded burners, which utilise more of the energy of flame combustion, are now being developed and have both a contact and radiant effect on the weeds being treated. Gas energy use has been recorded within the range 8 to 36 kg/ha, according to trials on carrots by Vester ( 1984). Other applications of flaming are for onion and potato haulm destruction.

There is very little evidence of research carried out on the integration of burners into flame weeder design. An experimental approach has been taken to investigate the effect on plants of the burner position, for different flame treatments (Parish 1989c, 1990). The effect of burner design, speed of travel, height above the ground and angle to the ground were also investigated, along with the effect of gas pressure and direction of travel. Some results are presented in Table 2.

The use of LPG to control weeds is seen in some quarters as contrary to one of the basic principles of organic production, in that the reduction in use of fossil fuels is seen as a major objective (IFOAM, 1981). Refining the design of flame weeders, and improving their efficiency in use, goes some way towards achieving this objective. Table 3 shows how the energy intensity of a single burner can vary according to use.

Crops, such as carrots and parsnips, which have a long germination period can be flamed pre-emergence. From the date of sowing to emergence is normally at least 10 days and within this period many weed seedlings can emerge, having been stimulated into germination by the seedbed preparation operations. The timing of the operation is crucial for successful weed control. The ideal time is just prior to the emergence of the carrot crop and the best way to determine this is by very careful examination of the sown seeds. Unless the relative cost of LPG is low, a band operation is usually carried out with the aim being to treat an effective width of at least 100 mm. (Weeds between the rows can be controlled by subsequent inter-row cultivations). It is difficult to visually assess the effect on the weeds immediately after the flame has passed.

A guideline is to press a treated leaf lightly between a finger and thumb, and a lasting imprint on the leaf indicates extensive cell damage. If it is considered that a greater effect is required, this can be achieved by either increasing the gas pressure or reducing the travel speed. Typical operating parameters are: propane gas used between 1 and 2 b pressure, burners 100 to 200 mm above the ground set at an angle of 30-45 to the horizontal (Parish, 1987a). Speed of travel is slow, 2-4 km/in, which gives a work rate range of 0.4 to 0.8 ha/in for a machine matching a 2 m bed system.

Crops such as maize and onions can tolerate flaming post-emergence, at specific growth stages. In the Federal Republic of Germany flaming and inter-row cultivations are techniques widely used in maize production, as there is an area of some 75000 ha where the weeds have become resistant to atrazine and there are traces of the chemical being found in drinking water (Hoffmann 1990). The requirement for hand weeding onions is significantly reduced by flame weeding, particularly in set onions, for which flaming at three stages is possible. At a crop height of 5 cm, the burner was set directly above the plants (Figure 8), whereas at crop heights of 20 and 40 cm, flaming was done with burners set at each side of the row, fitted with leaf lifters and leaf protectors (Ascard, 1990).

Inter-row cultivations

There are a large number of different designs of inter-row cultivators available on the European market, varying from traditional spring-tine cultivators to novel pto-powered brush type machines. The basic requirements for successful inter-row cultivations for weed control are:

i) to cut or uproot weeds, and then either completely bury them or leave them on the soil surface for desiccation;

ii) to protect the crop plants;

iii) to control implement direction; iv) to control implement depth;

v) to maintain or improve soil conditions (Parish, 1987b).

Although inter-row cultivations are normally used to control weeds between the crop rows, some investigations have been carried out to control weeds in the row. Klooster (1982) found that setting the implement to direct soil into the crop row to cover small weeds was as satisfactory as herbicide use. Mechanical gappers and thinners, widely used before herbicide development, have yet to come back into favour for weed control in the rows. In the development of a new hoe ridger, experiments on plants grown in trays showed a 90% kill rate in dry conditions, 57% by incorporation and 33% by desiccation (Kerpstra & Kouwenhoven, 1981). This hoe ridger (Figure 9) was designed for use in the later stages of sugar beet growth, and a similar effect is claimed for the ground driven rolling cultivator (Figure 10). This versatile machine can be operated at speeds up to 12 km/in and set to direct soil into the crop rows, to ridge soil, to cut down ridges, or simply to disturb weeds between the rows. The blade design produces a cutting and mixing effect, but Mattson (1990) found that the depth of work was insufficient to control larger weeds and that the correct setting and operation of all inter-row cultivators influenced the results.

Steerage options are available for rolling cultivators, brush weeders and fined cultivators. The advantage is to be able to work closer to the crop rows without causing plant damage, but the effect of soil disturbance close to the root zone has yet to be quantified in terms of plant growth. The design of guards used to protect the crop leaves can vary from rotating discs to floating shields or tunnels. The crop needs protecting both from soil thrown by the tines and from the tines themselves.

To avoid crop damage, the cultivator must be accurately set to match the drills and to run straight. Rear-mounted machines should be operated with the tractor check chains slack and the top link exactly in line with the direction of travel. Any steering deviation of the tractor is then not passed on to the cultivator. Mid-mounted machines, as attached to tool-carrier tractors follow the direction of steering best, and front mounting is preferable to rear mounting for directional control. The blade depth of a spring tine cultivator is maintained through a depth wheel connected to a parallel linkage mechanism.

These components, together with the steering disc which prevents lateral slippage and helps maintain direction, and crop protectors are clearly shown in Figure 11.

The tine design can either be a rigid sweep-tine, which works below the soil surface and cuts the weed stems away from the roots, or a spring-tine with a goosefoot share. The vibrating action of the spring-tines disturbs the soil, lifting out the weeds. Three tines are usually used, in a delta formation between the crop rows, with the middle share overlapping the outer ones. The recommended speed of work is up to 10 km/in for some models.

The pto-powered brush weeder (Figure 12) consists of flexible polypropylene brush discs, assembled into units of the desired width and spacing for the crop onto a horizontal pto-driven shaft. The brushes are set to work at a depth of 50 mm, and the crop rows are protected by tunnels 600-800 mm long and of the appropriate width, usually 40-200 mm. The effect of the brushing action is to lift the weeds out of the soil, causing damage by stripping leaves and breaking stems. The roots are brushed bare of soil, leaving the weeds far more vulnerable to desiccation. Compared to the other designs of inter-row cultivator the brush weeder can be effective in wet conditions. In dry conditions the other machines would be more appropriate, due to their faster work rates and lower running costs. The effects of these machines on soil surface structure are not well documented.

Pederson ( 1990) has defined the intensity of work of the brush as the ratio of the brush peripheral speed to the forward speed of the machine. The best effect on weeds was obtained with a work intensity of about 6, and the effects of a range of travel speeds, for the same brush speed, were investigated in weeds at different growth stages (Table 4).

Other techniques

Light exclusion techniques are widely used in small scale horticultural organic crop production. Materials used include black plastic, carpet, straw, cardboard, tree bark and wood chippings. The effects of such mulches on weeds, pests and crop yields, have recently been studied by several workers. In their study, Lennartsen et al. (1990) compared different surface mulches for their effect on clearing an established grass pasture and subsequent crop yields for organic horticultural produce. Weibel & Niggli (1990) investigated available nutrient levels in the soil, yields and fruit flavour in apple orchards.

CONCLUSIONS

Techniques for non-chemical weed control have been developed to reduce chemical costs in conventional agriculture, in response to environmental pressures and to provide for the needs of organic food production. A wide range of equipment is available to cover the major crops grown. Successful non-chemical weed control requires a well managed, integrated system and attention to detail.

Future work is required to research the effects of heat from thermal techniques on soil micro organisms, and weed seed germination and viability. The effects of the different soil/weed/tine combinations on the success of the weeding operation and on the soil structure also merit attention.

ACKNOWLEDGEMENTS

The author would like to thank Nic Lampkin for the opportunity to read the manuscript of the Weed Management chapter of his book, Organic Farming, which is due to be published in 1990 by Farming Press.

 

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(Received 22nd February, 1990: accepted 3rd March, 1990)

Copyright 1990. Reprinted with permission. All rights reserved.


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