Tidskrift/serie: Alternativ odling (numera Ekologiskt lantbruk)
Utgivare: SLU, Forsknings- och försöksnämnden för alternativ odling (numera SLU, Avdelningen för ekologiskt lantbruk)
Redaktör: Granstedt A.
Utgivningsår: 1990
Nr/avsnitt: 5
Författare: Ascard J.
Adress: Swedish University of Agricultural Sciences Dept. of Agricultural Engineering Box 66 S-23053 Alnarp
Ingår i...: Proceedings of the Ecological Agriculture NJF-seminar 166 - Miljövård
Titel: Weed control in ecological vegetable farming
Huvudspråk: Engelska
Målgrupp: Rådgivare
Nummer (ISBN, ISSN): ISSN-116X, ISBN 91-576-4279-6
OBS! Fel i texten kan ha uppkommit då dokumentet överfördes från papper.
Weeds are one of the most serious problems in organic vegetable production. The problem is most severe in early sown crops with slow early development, e.g. carrots and onions. Mechanical and thermal control are important methods which can considerably reduce the need for hand-weeding, but still there are many weeds left. In carrots, for example, hand-weeding normally requires somewhere between 100-300 manhours per hectare. On the other and, in a few crops such as potatoes, maize and set onions, all or most of the weeds can be controlled without hand-weeding.
The main problem is usually annual weeds within the row. The weeds between the rows can normally be controlled with some liind of mechanical row cultivation. The solution to the weed problem is not only to find rational methods to kill the weeds when the show up in the crop, but also to find suitable preventive measures and modificationsof the cultivation technique.
Since it is difficult to control the weeds after crop emergence, emphasis must be put on preventing the weeds from emerging in the crop. This involves different methods of reducing the soil seed bank by avoiding dissemination of weed seeds in all crops in the crop rotation, and to control weeds effectively before and after the crops. The use of fallow is often a neccesary evil to control weeds in organic farming, but may sometimes be replaced by green manuring.
By delayed sowing (stale seed bed) several weeds can be killed before sowing and crop emergence. Planting instead of seeding, when possible, is another way of reducing the weed problems. Mulching with organic or artificial materials are very useful methods in many crops and will probably be used more in the future.
Flaming pre-emergence of the crop is very labour saving in slowly germinating crops. In some heat-tolerant crops such as maize and onions, use can also be made of selective flaming in growing crops. In some crops and in some situations, harrowing is useful preemergence and in the early stages of the crop.
Inter-row cultivation, e.g. with a hoe or a multiple row-brush hoe, must be done carefully and repeated several times during the cultivation period. Different systems for manual and automatic guidance are on the market and new systems are under development. There is also development in progress on mechanical implements with brushes to control weeds within the row.
Correct timing of the different control inputs is extremely important. It is also important not to use a standard solution, but to be flexible and combine different methods and implements for different crops and different years. Research and development are suggested to be emphasized on preventive measures, new cultivation methods, and equipment for weed control very close to the row and within the row.
Weeds are one of the most serious problems for organic vegetable growers. Many of the vegetable crops, for exam le carrots and onions, have a very poor competitive ability against weeds and the yield is seriously affected by weeds.
The vegetable crop does not need to be weed-free during the whole growing season. For many crops there is a so called "critical period", during which the presence of weeds has the strongest effect on crop yield (Nieto et al. 1968). The critical period is different for different crops. For many crops this eriod starts some weeks after crop-emergence and end sometime during the first half of the growing period (Hewson & Roberts, 1971, 1973, van Heemst, 1985).
However the critical period does not mean that the weeds can develop freely after the critical period. Weeds at the end of the season often affect product quality and cause problems during harvesting. However, the importance of the weeds depends on the crop and the weeds species. In crops like cabbage that are harvested by hand weeds with a creeping growth is of minor importance, ut contamination of a crop of peas or beans for quick freezing by a poisonus plant may be totally unacceptable (Cussans, 1988; El Titi, 1988; Krauthausen, 1988).
Another good reason to keep control of the weeds throughout the season is to avoid building up of the seed bank. If the weeds are allowed to disseminate seeds, it also means future problems with weeds in subsequent crops, confirming the english proverb "One years seeding means seven years weeding".
However some positive effects of weeds should also be considered. Weeds may serve as host plants for beneficial insects and alternative target plants for pests. In some areas weeds also reduce soil erosion (Ammon, 1988). However, in vegetable crops the negative effects of the weeds usually seem to be of major importance.
The weeds cause expensive and labourious hand-weeding, in carrots normally somewhere between 100-300 manhours per hectare. The weed problems are most severe in early sown crops with slow earl development, e.g. root crops and onions. The main problem is usually annual weeds within the row. Perennial weeds may also cause problems but can more easily be controlled mechanically between the crops.
The hand-weeding is mainly a problem of getting labour rather than a economical problem. At least in Sweden, the additional price for alternatively grown products well includes the extra costs for hand-weeding.
Mechanical and thermal control inputs are important control methods which reduce the need for handweeding. However, harrowing, row cultivation and flaming, as they are used today, all have one thing in common; they only have a short term effect and in general they can not control the weeds in the row we enough.
The problems are not as severe in all vegetable crops though. For example, in planted brassica crops and lettuce, the crop competes much better with the weeds compared with root crops. In a few crops such as potatoes, maize and set onions, all or most of the weeds can be managed without hand-weeding by means of mechanical and thermal methods (Ascard 1988, 1989; Geier & Vogtmann 1986; Vester, 1986, 1987ab).
In order to manage the weed problems in a rational way you have to combine different methods. It is important not only to find rational methods to kill the weeds when they show up in the crop, but also to find suitable preventive meassures and modifications of the cultivation tecnique. One of the best preventive measures is a sound crop rotation, which does not favour the building up of weeds. The ideal situation is a crop rotation which includes perennial crops, such as pasture or green manuring.
The use of fallow is often a neccesary evil in organic farming to control weeds. In order to avoid negative effects such as damaging soil structure and causing leakage, green manuring may serve as interesting alternatives, specially with species that compete very well with the weeds, such as Phacelia tanacetifolia (Nygaard Sørensen 1989).
Since it is difficult to control the weeds in the row after crop emergence, emphasis has to be ut on avoiding the weeds to emerge in the crop. This involves different methods of reducing the soil seed bank by avoiding dissemination of weed seeds in all crops in the crorotation. This means that there are reasons to be more careful with the weed control in the agricultural crops if vegetables are grown in the crop rotation. It also means that it is even more important to control weeds effectively before and after the crop.
The seed bank is of considerable importance since it represents the potential weed flora and the weed problems of the future. Very large number of viable seeds are usually present in arable soils,often somewhere in the range of 5,000 to 50,000 seeds/m2, but there are also fields where you find less than 500 and up to 500,000 seeds/m2.Vegetable cropping favours weed species which require only a comparatively short interval between emergence and the start of seed production, and whose seeds can germinate over a wide temperature range. Intensive vegetable production therefore often results in large seed populations of a few species, among them Stellaria media, Poa annua, Urtica urens, Senecio vulgaris and Chenopodium album (Roberts, 1983).
Losses from the seed bank occur as a result of germination, predation, microbial attack and natural death. The major cause of depletion of the seed bank is germination and the major cause of seed death is through germination without emergence of a seedling, i.e. germination in an unfavourable situation. The decline is most rapid when the see s are near the soil surface and when there is frequent soil disturbance. With frequent cultivation the annual losses from the seed bank may be as much as 50% (Roberts, 1983, Milberg, 1990). It is important though that the emerged weeds are effectively controlled, otherwise cultivation can give the opposite effect.
The seed bed reparation is very important. With delayed sowing (stale seed bed) several weeds can killed before establishing the crop. Delayed sowing is often combined with flaming pre-emergence of the crop. The final harrowing is then made one or two weeks before sowing and sometimes followed by a rolling. If many weeds have emerged at the time of sowing one flaming can be done before sowing. With delayed sowing many growers often need to irrigate before sowing to ensure accurate crop germination (Ascard, 1988).
By using a narrow seed coulter (25mm) instead of a wide one (50mm), the hand-weeding work can be done faster. Some growers have experienced, though, that a narrow seed coulter requires a very good seed bed preparation (Brunström, 1990).
By transplanting the crop instead of direct drilling on the field, the weed problems are considerably reduced. Many crops, for example cabbage, lettuce and leeks are already used as transplants to a great extent. There is also a growing interest for transplanting onions but the high costs for seedlings and transplanting are the major problem today.
Mulching with organic or artificial materials are very useful methods in and will probably be used more in the future. With plastic mulch you can cover the soil before plantinand then plant many crops, e.g. strawberrys, maize and cabbage through the material. Organic materials such as straw or cut fresh green manure can be spread out immediately after planting the crop. With this type of mulching the need of handweeding is almost eliminated. There is a great need, though, for equipment for rational spreading of green manure between and under the rows.
Use of cover crops can also be used in vegetables, but because of their cometitive ability towards the crop, they are usually sown later during the growing season, when the critical period ready is over.
Flaming pre-emergence of the crop is a very useful and common method in all slowly germinating crops suc as carrots, onions and other root crops. In these crops flaming is the single most important treatment. The result depends on how many weeds that emerge after flaming. Therefore it is very important to prepare the seed bed in a way that as many seed as possible emerge before crop emergence. If successful, the flaming often reduces the required time for hand weeding in carrots by 50%. Flaming has a selective effect and in some crops such as maize, onions and potatoes the flaming can be done when the crop plant is a few cm high. In these tolerant crops with a upright way of growing, selective flaming in the row can also be carried out in the growing crop (Ascard, 1988, 1989, Hoffmann, 1989, Vester, 1986, 1987ab). There is a great need of further improvement of both flaming implements and methods.
In some crops and in some situations harrowing is interesting pre-emergence and in the early stages of the crop. Shallow harrowing can be done pre-emergence in, for example, carrots, onions, beans and peas (Rundgren, 1988; Rydberg, 1985). Harrowing has a selective effect and can therefore under certain conditions be used after crop emergence also. In a young weed stage weed harrows can sometimes be used in, for example, potatoes, beets, set onions, maize and cabbage, as long as the crop plants are firmly anchored in the soil (Jonkers, 1988; Rydberg, 1985). Research is going on at the moment on harrowing in more sensitive crops like carrots and seeded onions (Vester, 1990, Brunström, 1990). Harrowing is very dependent on dry soil conditions and pre-emergence harrowing involves a greater risk for crop damage than flaming. However, with both flaming and harrowing it is often advisable to raise the seed amount to some degree to afford some plant reduction when treating.
The weeds between the rows are usually not a major problems and can normally be managed with a hoe. However, under certain circumstances the inter- row weeds also cause problems. Under wet conditions (wet weather or humid soils as organic soils) the weed reduction is poorer due to a higher regrowth of the uprooted plants and a better ability of the covered plants to penetrate the soil. This problem is more severe for certain species and on big plants (Habel, 1957; Koch, 1964, 1965; Mattsson et al, 1988).
However, these problems can usually be managed by using another type of inter-row weeder. The harrow has a better uprooting effect than a hoe on many weeds with a creeping growth (Schmid & Steiner, 1987). A good solution is therefore to use a finger weeder harrow or another complementary tool after the hoe (Rasmussen, 1989, Nylander, 1989). Another sucessful way is to use a PTO- driven tool like the multiple row-brush hoe (Kress, 1987, Vester & Rasmussen, 1989) or some other kind of rotary cultivator. In some instances flaming can also be used for intra row-weeding when mechanical methods fall (Mattsson et al, 1988).
Inter-row cultivation is not always easily applied in all crops. In narrow row crops like spinach, peas and washed carrots it is less suitable (Jonkers, 1988). In these crops a modification of the cultivation technique has to be considered if they shall be organically grown.
Inter-row cultivation has to be done repeatedly during the growing season. Specially during the early cultivations it is very important to get close to the row. If the rowcultivation leaves a strip of only 6 cm instead of, say, 10-12 cm you save a lot of labour requirement for hand weeding. With an implement with low side pressure, like the multiple row brush hoe, it is possible to cultivate very close to the row without damaging the plants. There are, for example, reports on sucessful treatments with only 4-5 cm wide protecting tunnels (Kress, 1987). If you want to drive that close then it is necessary to have very straight rows, very accurate steering and shallow cultivation.
Different mechanical steering systems are on the market (Mattsson et al, 1988). Electronical dance systems for inter-row weeders are being developed (Nybrant, 1990, Tønnes Pedersen, 1990) and some systems are already on the market (Goldstein, 1990; Zahradnik, 1983). Some systems are made mainly for agricultural crops using a mechanical sensor device that is guided along the sides of the plants in the rows. However, to be able to use an electronic guidance system in the early stages of a vegetable crops you rather need an optical sensor.
There is ongoing development in new mechanical implements to control weeds within the row in growmg crop. One approach is to use rotating brushes on vertical axis for selective weed control in the row. German equipment is already on the market (Paul, 1989), and in Sweden development is proceeding with a similar approach (Rundgren, 1988, Möller, 1990). To achieve a better selective weed effect the German equipment is available with soft brushes in the periphery and stronger brushes or metal tools in the center of the brush unit.
In USA special implements are developed to control weeds in the row of row crops. One system use a combination of so called spring hoes and spyders (Goldstein, 1990; Zahradnik, 1983).
A well-known method for intra-row weed control, which we must not forget, is hoeing with high speed in a way that soil is thrown into the row and covers small weeds (Terpstra & Kouwenhoven, 1981). However, this kind of hoeing can only be done after a certain crop stage. Moulding-up also has the same effect and is very useful in some crops, for example, potatoes, carrots and cabbage (Jonkers, 1988).
For crops with plants widely spaced in the row there is equipment on the market in southern Europe on which the cultivation tools work in and out of the rows. Depending on the transplanter used, one person can operate either one or several rows at the time (Spapperi, undated).
Correct timing of the different control inputs is extremely important since the effect of the mechanical and thermal control is greatly favoured by small weed plants, and the harrowing and hoeing also require dry soil conditions. It is also important not to use a standard solution, but to be flexible and combine different methos and implements for different crops and different years.
It is obvious that weed control in ecological vegetable crops involves good knowledge and a proper combination of different methods an implements. Other methods not mentioned here that may be important in the future is electrical currents and microwave radiation (Ascard, 1988; Diprose & Benson, 1984; Diprose et al, 1984) and biological control methods. Future research and development within weed control in vegetable production are suggested to be emphasized on preventive measures, new cultivation methods and equipment for weed control very close to the row and within the row.
Aminon, H.U. 1988. Do actual tresholds favour the build-up of herbicide resistance in weed populations? In: Weed control in vegetable prouction. Commission of the European Communities. Proceedings of a Meeting of the EC Experts' Group/Stuttgart. 28-31 October 1986. pp. 281-289.
Ascard, J. 1988. Termisk ogräsbekämpning. (Summary: Thermal weed control) Swedish Univ. of Agr. Sciences, Dept Agr. Eng. S-230 53 Alnarp. Report 130. 146 pp.
Ascard, J. 1989. Thermal weed control with flaming in onions. 30th Swedish Crop Protection Conference, Uppsala. Vol. 2. Reports. pp 35-50.
Brunström, P. 1990. Pers. comm. Adviser. Hushållningssällskapet, S-651 13 Karlstad.
Cussans, G.W. 1988. The economics of weed control in vegetable crops. In: Weed control in vegetable production. Commission of the European Communities. Proceedings of a Meeting of the EC Experts' Group/Stuttgart. 28-31 October 1986. pp. 99-105.
Diprose, M.F. & Benson, F.A 1984. Electrical methods of killing plants. J. Agric. Engng. Res. 30, 197-209.
Diprose, M.F.; Benson, F.A & Willis, A.J. 1984. The effect of externally applied electrostatic fields, microwave radiation and electric currents on plants an other organisms, with special reference to weed control. The Botanical Review 50, 171-223.
El Titi, A. Relationship between weed density and yield in peas crppped for canning industries. In: Weed control in vegetable production. Commission of the European Communities. Proceedings of a Meeting of the EC Experts' Group/Stuttgart. 28-31 October 1986. pp. 107-113.
Geier, B. & Vogtmann, H. 1986. Control of Weed in Corn (Maize) without Herbicides. The Sixth International IFOAM Conference. Santa Cruz. 8 pp.
Goldstein, W. 1990. Pers. comm. Research Director. Michael Fields Agr. Inst. Wisconsin, USA.
Habel, W 1957. Über die Wirkungsweise der Eggen gegen Samenunkräuter sowie deren Empfindlichkeit gegen den Eggvorgang. Z. Acker- und Pflanzenbau 104, pp 39- 70.
Hewson, R.T. & Roberts, H.A. 1971. The effects of weed removal at different times on the yield of bulb onions. J. hort. Sci. 46:471-83.
Hewson, R.T. & Roberts, H.A. 1973. Effects of weed competition for different periods on the growth and yield of red beet. J. hort. Sci. 48:281-292.
Hoffmann, M. 1989. Abflammtechnik. KTBL-Schrift 331. Landwirtschafsverlag, Münster-Hiltrup. 104 pp.
Jonkers, J. 1988. Integrated weed control in some vegetable crops. In: Weed control in vegetable production. Commission of the European Communities. Proceedings of a Meeting of the EC Experts' Group/Stuttgart. 28-31 October 1986. pp. 259-264.
Krauthausen, H.J. 1988. Weed control under the practical conditions of a region of intensive vegetable growing. In: Weed control in vegetable production. Commission of the European Communities. Proceedings of Meeting of the EC Experts' Group/Stuttgart. 28-31 October 1986. pp. 273-278.
Kress, W. 1987. Die Reihenhackbürste - eine neue Entwicklung in der mechanischen Beikrautregulierung. In: Beikrautregulierung statt Unkrautbekämpfung (Hoffman, M. & Geier, B. Hrsg.) Verlag C.F. Müller, Karlsruhe, Alternative Konzepte 58, 83-88.
Koch, W. 1964. Unkrautbekämpfung durch Eggen, Hacken und Meisseln in Getreide. (I) Wirkungsweise und Einsatzzeitpunkt von Egge, Hacke und Bodenmeissel. Z. Acker und Pflantzenbau 120, 369-382.
Koch, W. 1965. Unkrautbekämpfung durch Eggen, Hacken und Meisseln in Getreide. (II) Das verhalten der einzelnen Unkrautarten gegenüber Egge, Hacke und Bodenmeissel. Z. Acker und Pflantzenbau 121, 84-96.
Mattson, B., Nylander, C. & Ascard, J. 1988. Undersökning av radrensare i Alnarp. Alternativodlaren 2:10-18.
Milberg, P. 1990. Omsättning av åkerogräsfrön i jorden. (Summary: The Turnover of Weed Seeds in Soil). Nordic post graduate course in plant production science. Eleventh course: Weeds and weed control. Garpenberg, Sweden, 4-9 march, 1990.
Möller, N. 1990. Pers. comm. Professor. Dept. of agr. eng. Uppsala, Sweden.
Nybrant, T. 1990. Pers. comm. Professor. Dept of agr. eng. Uppsala, Sweden.
Nieto, H.J., Brondo, M.A. & Gonzalez J.T. 1968. Critical periods of the crop growth cycle for competition from weeds. Pans (c) 14(2): 159-166.
Nygaard Sørensen, J. 1989. Fangafgrøder efter grønsager. NJF seminarium Nr 159. Gröngödslingsgrödor och/eller fånggrödor. pp. 19:1-12.
Nylander, C. 1989. Nya idéer för mekanisk ogräsbekämpning i grönsaker. Hortica mars, pp. 15-18.
Paul. 1989. Brochure. Dr-system Paul Ausputzer. Bad Wildungen. BRD.
Rasmussen, J. 1989. Forsøg med ukrudtsharvning og radrensning i korn. Nordisk Planteværnskonference, 1989. pp. 345-354
Roberts, H.A. 1983. Weed Seeds in Horticultural Soils. Scientific Horticulture 34:1-11.
Rundgren, G. 1988. Harva morötter - är du inte klok? Alternativodlaren nr 11-12. pp 9-12.
Rydberg, T. Ogräsharvning- en litteraturstudie. (Summy: Weed harrowing - literature survey). Swedish Univ. of Agr. Sciences, Dept of Plant Husbandry, Report 154. Uppsala. 25 pp.
Schmid, O. & Steiner, N. 1987. Erfahrungen mit der mechanishen Unkrautregulierung in Getreide auf Betrieben des biologischen Landbaus. In: Beikrautregulierung statt Unkrautbekämpfung (Hoffman, M. & Geier, B Hrsg.) Verlag C.F. Müller, Karlsruhe, Alternative Konzepte 58, 65-81.
Spapperi, Undated. Brochure. Programmed interplant cultivator. F.Lli. Spapperi, Sansecondo, Citta di Castello, Italy.
Terpstra, R. & Kouwenhoven, J.K. 1981. Inter-row and intra-row weed control with a hoe ridger. J Agr. Engng. Res. 26, 127-134.
Tønnes Pedersen, B. Pers. comm. National Weed Research Institute, Slagelse, Denmark.
van Heemst, H.D.J. 1985. The Influence of Weed Competition on Crop Yield Agricultural Systems 18: 81-93.
Vester, J. 1986. Flammebehandlingseffekt på afgrøder og ukrudt. 3. Danske Planteværnskonference/Ukrudt 1986: 250-261.
Vester, J. 1987a. Flammebehandling til ukrudtsbekæmpelse, 2 år resultater. 4. Danske Planteværnskonference/Ukrudt 1987: 140-153.
Vester, J. 1987b. Flammebehandling til bekaempelse af ukrudt. Slutrapport for 1985-86. Inst. for Ukrudtsbekæmpelse, Slagelse. 31 pp.
Vester, J. 1990. Pers. comm. Forsøgsanlæg Foulum, D-8830 Tjele, Denmark
Vester, J. & Rasmussen, J. 1989. Erfahrungen mit der Reihenhackbürste in vershiedenen Kulturen in Dänemark 1987 bis 1989. III Internationale Konferenze zu Aspekten der nicht-chemischen Beikrautregulierung. 10-12 Oktober 1989. Linz, Österreich. 8p.
Zahradnik, F. 1983. Who Needs Herbicides? The New Farm. January. pp 20-23