Weed control in rapeseed cultivation is one of the important factors determining the achievement of high yields with good quality parameters. Winter rapeseed is one of the crops that are exposed to strong competitive effects from weeds. Popular and massively occurring weeds in rapeseed include as many as a dozen species. However, the most troublesome are those characterized by intensive growth at the beginning of the vegetation of the crop (from the emergence phase to the stem elongation phase). During this period, winter rapeseed plants must compete with weeds for space and access to light, which consequently affects their proper growth (so-called plant running out), weakens their condition and increases their sensitivity to freezing (high growth cone above the soil surface).
Many weed species emerge in autumn
In addition to harmful perennial weed species (such as couch grass or field thistle) and annual winter weeds, it is also worth mentioning spring species emerging in autumn. These are so-called wintering weeds, which are characterized by high resistance to low temperatures and pose a serious threat to winter rape throughout the growing season. Wintering forms develop very quickly after the start of spring vegetation and compete more strongly with rape plants than their spring-emerging forms. Examples of wintering weeds include: cleavers, cornflower, field poppy (which are classified as weeds of the high zone) and common shepherd's purse, field violet, common chickweed and cranesbill from the low zone. Winter rape plantations are also threatened by self-seeding cereals and, increasingly, monocotyledonous weeds due to their developing resistance. Weeds can also indirectly contribute to losses in rapeseed yields by creating a microclimate in the field that is conducive to the development of slugs and diseases (seedling blight, blackleg, sclerotia, black crucifer, grey mould). They can also be hosts for pathogens and transmit them (e.g. clubroot). In addition, they constitute so-called green bridges as reservoirs for the development and feeding of pests (e.g. four-toothed weevil and similariformes, cabbage midge, rapeseed pollen beetle, cabbage rootworm).
Prevention as a support for chemical treatments
Therefore, autumn regulation of winter rapeseed weeds is a must. It is worth preceded by preventive measures, such as selecting the right site for cultivation, proper crop rotation, careful agrotechnics, sowing of qualified and dressed seed material free from weed seeds. However, the use of the mechanical method is very limited due to the risk of damaging the root system through autumn harrowing, which is why chemical weeding is used in the vast majority of cases, which is the most cost-effective. Currently, the range includes herbicides intended for soil treatments, directly after rapeseed sowing and applied foliarly.
When there is moisture, it is worth opting for soil application.
The recommendations focus on pre-emergence weeding of winter rape, which controls weeds in early development stages – up to 2 true leaves. This solution is economically justified and recommended due to the early elimination of weeds (especially from the cabbage family).
The effectiveness of preparations taken up by weed root systems is strongly dependent on weather conditions, or more precisely on soil moisture. Their effectiveness drops dramatically on dry soils. On the other hand, heavy rains can cause active substances to be washed deep into the soil profile, to the rapeseed root zone, causing their phytotoxic effect. Another factor reducing the effectiveness of soil preparations may be their immobilization by the sorption complex – hence the need to use them in the highest recommended doses on heavier soils. In such situations, it is necessary to consider using herbicides together with soil adjuvants, such as Atpolan Soil Maxx, Resume, Gleber, Agrigent, Adsoil, Grounded or RSM fertilizer, which allow for uniform coverage of the soil surface by the working liquid and ensure that the active substances reach the weed germination zone to a depth of up to 5 cm. They also limit the movement of active substances to deeper soil layers and groundwater.
Single-component herbicides based on 5 active substances (aminopyralid, clomazone, dimethachlor, metazachlor, napropamide), 9 two-component mixtures and 6 mixtures containing as many as 3 active substances are currently used for pre-emergence weeding of winter rape.
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Active substances in pre-emergence treatments
Among the listed a.c., napropamide is used earliest – immediately before sowing rape. This substance penetrates weeds through the seed coat, roots or cotyledons. It acts systemically and is classified as an inhibitor of long-chain fatty acid synthesis (group 15, formerly K3 according to HRAC classification). Herbicides classified in this group inhibit the synthesis of cutin and cuticular wax, which means that weed sprouts and young seedlings are deprived of a protective layer against external factors. Napropamide is present as a single substance in six preparations, which after application should be immediately mixed with the soil to a depth of approx. 3 cm using a harrow. This is necessary because napropamide is very quickly decomposed by sunlight. Its undoubted advantage is the fact that it requires minimal moisture for its activity, and weeds emerging during dry weather are effectively eliminated after rainfall. In recommended doses (2.4-3 l/ha) it effectively and quickly reduces the occurrence of monocotyledonous weeds (such as barnyard grass and annual meadow grass) and dicotyledonous weeds (geraniums, chamomiles and chamomiles, and cabbage weeds). Some of the above-mentioned preparations can be used together with other herbicides (tank mixtures) to increase the effectiveness of action and broaden the spectrum of weed control. Factory mixtures with quinmerac or clomazone and in combination with: napropamide + quinmerac + metazachlor; napropamide + clomazone + dimethachlor, as well as napropamide + clomazone + metazachlor are also recommended for rapeseed cultivation.
Aminopyralid as a single active substance is available only in the Runway preparation. Aminopyralid is an active substance with systemic action, classified as a synthetic auxin (group 4, formerly O according to HRAC). It is absorbed through the leaves and roots of germinating weeds. It can be applied in two terms. Pre-emergence (BBCH 00-09) is recommended at a dose of 0.2 l/ha for effective reduction of cornflower, field poppy, scentless marigold and common chamomile. It is also present as a component of mixtures. However, in pre-emergence regulation, only the three-component Cleravo Flex preparation can be used, in which aminopyralid is accompanied by quinomerac and imazamoc.
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Since clomazone has been available on the Polish market for 32 years, it is the most commonly used active substance for pre-emergence weed control of rapeseed. It is classified in group 13 (F4) according to HRAC and acts on weeds mainly during their germination, inhibiting the synthesis of carotenoids. As a result, sensitive species do not emerge or, after emergence, those lacking pigments turn white and die quickly. According to the label, the preparations can be used up to five days after sowing rapeseed (BBCH 00-05/07/08/09) on moist, carefully cultivated soil.
When using clomazone, it must be remembered that it can cause phytotoxicity of rapeseed in the form of whitening or yellowing of leaf blades – especially in varieties sensitive to this s.f. This is a temporary condition and the symptoms disappear after a few weeks, without negatively affecting yield. However, in order to reduce the risk of damage to crops, it should not be applied to too wet soil or at temperatures above 20°C. Clomazone is a volatile compound and can be drifted to neighboring crops – especially during weather conditions favoring temperature inversion. In such conditions, it is possible for the liquid containing clomazone to be lifted and for it to fall after cooling the air masses at a certain height onto crops even at a considerable distance from the protected plantation.
Clomazone is a component of many commercial mixtures. It is found, among others, in combination with metazachlor, dimethachlor, napropamide and petcosamide. Three-component solutions are also available: clomazone + quinmerac + metazachlor and clomazone + metazachlor + napropamide.
Another group of herbicides commonly used for weeding rape is represented by preparations containing metazachlor, which, like napropamide and the dimethachlor mentioned below, is classified in group 15 (K3). It acts both in the soil and on the leaves. Applied in the BBCH 00-05/09 phase at a dose of 1.5-2 l/ha (depending on the herbicide), it is taken up by weed roots and effectively eliminates from the plantation, among others, cleavers, chamomiles, field cranesbills, speedwells, common chickweed, sea mayweed, cereal broom, field poppy, speedwells, dog's whip and common shepherd's purse. Metazachlor is also used together with clomazone, quinomerac and dimethenamid-P and is present in 5 out of 6 three-component mixtures.
The last single-use substance is dimethachlor. This compound, like other substances from group 15, acts in the soil and early on the leaves. It is effective in controlling a few monocotyledonous species (e.g. cereal broom), while in the case of dicotyledonous weeds it effectively destroys species that are a strong threat to rapeseed cultivation. Like other substances, it can be used in mixtures. The most popular combinations are dimethachlor + quinomerac; dimethachlor + clomazone and dimethachlor + clomazone + napropamide.
The substances dimethenamid-P and pethoxamid from group 15 and quinmerac from group 4 currently occur only as components of mixtures. ▪
