Indirect weed control

Management of drainage and irrigation systems

Careful choice and maintenance of drainage and irrigation systems is an important preventive measure to reduce on-field weed infestation. Periodical clearance of weed vegetation established along ditches prevents it from invading the field. Where it is economically feasible, substitution of ditches with subterranean drains eliminates a potential source of weed infestation. Use of localized (e.g. trickle) irrigation systems favour crop development to the detriment of weeds. In contrast, broadcast irrigation systems often favour weeds because most of them have a higher water use efficiency (dry biomass production per unit water used for evapotranspiration) than the crop.

Tillage

One of the most important goals of all tillage processes, among other beneficial effects, has always been to reduce the stock of weed seeds in the soil and to deplete the reserve nutrient reserves of underground vegetative reproductive organs in perennial species. The weed seeds in the soil are placed in more favorable layers close to the soil for germination as a result of the disturbance, and the seedlings can be easily destroyed during a repeated tillage. The use of conventional tillage systems is of great importance in organic farming. It consists primarily of an autumn deep plowing or stubble plowing and then, in the spring of the following year, of the tillage procedures in preparation for sowing (disc, cultivator, harrow, combine, etc.). Later, in the vegetation, several inter-row additional mechanical weed control may become necessary (cultivator, weed comb, weed brush, spoke hoe, etc.). Soil cultivation or tillage, as an effective method has long been involved into control weed management. Various factors, like depth, timing and frequency of cultivation can influence different parameters of the weed population (composition, density and long-term persistence). However, similarly to other weed management methods, tillage also have conflicts. Finer seedbeds produce more weed seedlings but a smooth surface makes the direct weed control easier. Larger clods of soil produce fewer weed seedlings but the rough surface gives emerged weeds protection against direct weeding methods. Soil structure can be damaged by excessive cultivation that leads to erosion in longer term. Although, reduced tillage results in better control of soil erosion, conservation of soil moisture and more efficient use of fossil fuel, but not all soils are suitable for reduced tillage. Tillage is often divided into three forms primary, secondary and tertiary, but there are other cultivations that do not fall into these categories.

- Primary tillage. Primary tillage is the principal method chosen for cultivation prior to crop establishment. It is the first soil-working operation in cropping systems that is performed for preparing the soil for planting. Primary tillage is always aggressive and carried out at a considerable depth in order to control annual and/or perennial weeds by burying a portion of germinable seeds and/or propagules at depths at which weed seeds are not able to emerge. The main tools used to perform primary tillage are mould-board ploughs, disc ploughs, diggers, and chisel ploughs.

- Secondary tillage. Secondary tillage is used to prepare seedbeds and leave a level surface for drilling, thus the soil is not worked aggressively or deeply. The aim is to prepare the soil for planting or transplanting or it is used for carrying out the false seedbed. The equipment for secondary tillage are cultivators, harrows (disc, spring tine, radial blade, and rolling) and power take-off machines applied to a depth of 10 cm. In conservation tillage this equipment could be used as a substitute for ploughs in primary tillage. Conservation tillage is useful for conserving or increasing the organic matter content in the soil and for saving time, fuel and. Although, reduced tillage techniques could cause some problems with weeds, farmers can optimally alternate primary and secondary tillage in order to optimize soil management by changing mechanical actions year after year and thus improving annual and perennial weed species control. The timing of seedbed preparation affects weed populations considerably and is an opportunity to reduce weed numbers that emerge in the growing crop. One traditional method of weed control is the stale or false seedbed technique. Cultivation for seed bed preparation has two contrasting effects on weeds: (i) elimination the emerged vegetation resulting from after primary tillage, and (ii) stimulation of weed seed germination and consequent seedling emergence. Utilize these two effects can be achieved by false (stale) seed bed technique. A stale seedbed is a technique where a seedbed is prepared several days/weeks/moths before planting or transplanting crops in order to stimulate the emergence of weeds prior to sowing. The success of a stale seedbed depends on the length of time before planting and on weed spectrum. Late-emerging weeds will still be a potential problem. Application of the false seed bed technique can reduce weed emergence > 80% compared to standard seed bed preparation. The most important factor beside the temperature is the moisture of soil. In dry years the stale seedbe)d method does not serve as a good method of weed control without the intervention of irrigation. A novel method of reducing seedling emergence is to carry out the seedbed preparations in the dark to avoid stimulating weed seed germination, however this technique does not provide consequent results.

- Cultivation tillage. Cultivating tillage is performed after crop planting in order to achieve a shallow tillage which loosens the soil and controls weeds. For this purpose, cultivators are used which can control weeds in different ways. The complete or partial burial of weeds and their seeds can be an important cause of mortality. Another mode of action is by uprooting and breakage of the weed root contact with the soil. It is preferable to carry out cultivation tillage when the soil is not too wet because it can damage the soil structure and favor the spread of perennial weeds. Cultivators are generally classified according to their application in a crop: broadcast cultivators could be used both on and between the crop rows; inter-row cultivators are used only between crop rows; and intra-row cultivators which are used for removing weeds from the crop rows. For example, the methods against Cirsium arvense: With wire rope method the field is mounded up by using mounding equipment in place of plow. For tillage, the mounds or ridges are dragged down to a greater or lesser extent, depending on the crop, and sown with cereals, for example, or planted with field vegetables. While the seed is now emerging, but the roots of the crops are still short, the tilled ridges are undercut at the boundary between topsoil and subsoil with a wire rope stretched across the hill implement, thus cutting off the thistle shoots. Undercutting with the wire rope can be done both in the fall and in the spring.

Crop rotation

Crop rotation is a basic technique in organic farming to help pest and disease control and to provide optimum soil fertility, moreover weed control is achieved effectively by combining crop rotation with other cultural treatments. Crop rotation involves alternating different crops in a systematic sequence on the same land (Figure 5.27). Monoculture or high proportion of similar crops results in a weed species composition that are adapted to the growing conditions of the crop (for limiting the field thistle, the cereal content should be limited to a maximum of 50%). Rotating crops at different life cycles can disrupt the development of weed-crop associations, through different planting and harvest dates preventing weed establishment and therefore seed production. Since different crops favour different types of weed species, it is important to change between annual and perennial crops in the crop rotation. Autumn- and spring-sown annual crops also favour different types of weed species, which makes it important to rotate between such crops within a crop rotation. Traditionally, potato (Solanum tuberosum) is included in the rotation to reduce weed problems before a less competitive crop is grown. For an organic farmer, consideration of soil fertility level and including fertility building periods in rotation complicate the crop choice. The inclusion of a fallow period in the rotation in known to reduce perennial weeds. It is best to alternate legumes with grasses, spring planted crops with fall planted crops, row crops with close planted crops and heavy feeders with light feeders. Despite the use of rotations, some weeds have been identified as particular problems in organic farming systems. Couch grass (Elymus repens) and other creeping perennial grasses, and creeping thistle (Cirsium arvense) are often declared as the main problem weeds in all organic systems. Blackgrass (Alopecurus myosuroides) and Cirsium arvense can become more frequent when cereals form a significant part of the rotation. Docks (Rumex spp.) are a particular problem in grassland and bracken (Pteridium aquilinum), has become a severe problem in upland areas of pasture.

Figure 5.24 Possibilities for crop rotation (E. Takács)

Cultivar

It is not simply the choice of crop that influences weed development within a rotation, the characteristics of the cultivar such as morphology and growth rate can have a significant effect on both crop and weed development. Cultivar choice and crop seed rate can be effective in suppressing weeds and hence minimising weed control inputs, as well. For example, spring barley cv. Atem has taller development than cv. Triumph and has a major influence in its greater weed suppression. Similarly, number of weed species found on the plots were significantly reduced in the presence of the traditional longer strawed Maris Huntsman winter weed cultivar in contrast with Mercia cultivar. Morphological traits can influence the competitive ability of crops over weeds. For example, earliness of crop ground cover is vital in weed suppression, and research has indicated that larger initial crop seed size can significantly improve early crop establishment and hence increase the competitive ability of winter wheat cultivars. Identifying and quantifying the traits associated with competitive ability against weeds is indeed complicated by the fact that, although different cultivars have unique characteristics, many of these traits can change over development stage. However, differential rooting patterns, early vigour, leaf size and allelochemical properties may influence the ability of a cultivar to suppress weeds and be successfully selected in breeding programmes.

Intercropping

Intercropping process means to grow smother crop between rows of the main crop (Figure 5.25). Increased yield, not improved weed control, is probably the main benefit expected from intercropping. It is declared, that intercrops are able to suppress weeds, however it should be carefully applied. Without any attentiveness, intercrops can greatly reduce the yields of the main crop if competition for water or nutrients occurs. Similar to cover crops, intercrops increase the ecological diversity and use of natural resources by canopying, moreover compete better with weeds for light, water and nutrients. For example, a leek-celery intercrops sown in a row-by-row layout decrease relative soil cover of weeds by 41%, reduce the density and biomass of Senecio vulgaris by 58% and 98% respectively, and increase total crop yield by 10% compared to solo cropping. Increased weed suppression and crop yield has also been demonstrated in many environments for cereal-legume intercrops. As in the case of living mulching, the success of intercropping relies on the best match between the requirements of component species for light, water and nutrients, which increases resource use complementarity and reduces competition between the intercrops. In practice, this means optimizing intercrop spatial arrangement, relative plant densities and crop relative growth over time in any given environment.

Figure 5.25 Sugarcane intercropping with cabbage or cauliflower. (shutterstock.com)

Fertilization

Nutrient level of soil in agro-ecosystem is altered by application of fertilizers, thus they directly affect weed population dynamics and crop-weed competitions. Numerous weeds are high consumers of nitrogen and therefore able to reduce the availability of nitrogen for crop growth. Strong effects in weed control can be detected by timing, dosage and placement of fertilizer. Organic farming uses organic manure and compost to replenish nutrients, which, as a consequence of improper treatment, have a “weed-growing” effect on the viable weed seeds in it. It is known that weeds absorb nutrients earlier and in greater amounts than their associated crops, so they need to be treated very carefully with nutrient replenishment.

Cover crops

Cover crops include a wide range of plants grown for various ecological reasons and cover the soil. Cover crops (Figure 5.26) suppress weeds by competing for resources, moreover their residues laying on the surface of the soil inhibit weeds through physical (barrier to weed emergence and establishment, increase of space for normal development of weeds), biotic (blocking of light, avoidance of temperature fluctuation, alteration of moisture conditions necessary for germination) and allelopathic interactions (compound released from living or decaying plant tissue). In general, the larger the cover crop and greater the biomass or dry matter production, the greater the impact on weeds. Despite these potential benefits, physical and biochemical effects from cover crops may not provide adequate weed control. Weed suppression by cover crop residue can vary from negligible to highly effective for anywhere from two weeks to several months, depending on cover crop biomass and nitrogen (N) content, season, weather, and soil conditions. Warm, moist weather combined with high soil biological activity accelerates decomposition of cover crop residues and their allelochemicals, thus shortening the weed control period. Strawy, low N residues last longer than succulent, high-N residues. Use mechanical control tactics and cultural controls to complement cover crops for weed management. The inclusion of cover crops such as rye, red clover, buckwheat and oilseed radish, over wintering crops (i.e. winter wheat) or forages in the cropping system can suppress weed growth. Highly competitive crops may be grown as short duration 'smother' crops within the rotation. When choosing a cover crop, consideration should always be given to how the cover crop will affect the succeeding crop. Examples of highly weed suppressive cover crops are rye, sorghum, kale, rocket and mustard. In contrast, although direct weed suppression by legumes can be significant, their residual weed control effect is usually lower because the high quantity of N released from their residues after cover crop destruction stimulates weed emergence, especially when legumes are used as a green manure.

Figure 5.26 Benefit of cover crops (E. Takács)

Sanitation

It is possible to prevent many new weeds from being introduced onto the farm and to prevent existing weeds from producing large quantities of seed. The use of clean seed, mowing weeds around the edges of fields or after harvest to prevent weeds from going to seed, and thoroughly composting manure before application can greatly reduce the introduction of weed seeds and difficult weed species. It is even possible to selectively hand-eradicate isolated outbreaks of new weeds, effectively avoiding future infestations. Planting clean, high-quality seed is essential to crop success. Other sanitation factors to consider would include thorough cleaning of any machinery which might have been used in weedy fields or washing stations, and the establishment of hedgerows to limit windblown seeds.