Crops to manage weeds
Differentiation of crops grown over time on the same field is a well-known primary means of preventive weed control. Different crops obviously bring about different cultural practices, which act as a factor in disrupting the growing cycle of weeds and, as such, preventing selection of the flora towards increased abundance of problem species (Karlen, 1994). In contrast, continuous cropping selects the weed flora by favouring those species that are more similar to the crop and tolerant to the direct weed control methods used (e.g. herbicides) via repeated application of the same cultural practices year after year.
In addition, continuous cropping can negatively interact with tillage systems and shift the weed flora towards a troublesome composition. For example, in continuous winter cereal cropping in temperate regions, minimum tillage can cause the dominance of grasses with low-dormant seeds, such as Alopecurus myosuroides and Bromus spp., to occur after a few years. (Froud-Williams, 1983). In these cases, the consequent higher use of graminicides acts as an additional selection factor for the weed flora and can also accelerate the selection of herbicide-resistant biotypes. To recover highly degraded floristic situations such as the one just pictured, it is imperative to rotate cereals with crops having a different growing period, as well as to plough the soil from time to time to disadvantage low-dormant grass species whose seeds are usually unable to emerge from deep soil layers. If there is a long fallow period between the cereal and the next crop, this can be exploited to cultivate the soil to stimulate the emergence of problem weeds, which are then destroyed by additional cultivation or by herbicides.
Cover crops (used as green manures or dead mulches)
Inclusion of cover crops in a rotation in the time frame between two cash crops is another good preventive method to be used in a weed management strategy. Cover crops do not give a marketable yield but, by extending the period in which the soil remains covered by vegetation, exert a series of beneficial effects on the agro-ecosystem, such as optimization of natural resource use (solar radiation, water, soil nutrients), reduced water runoff, nutrient leaching and soil erosion, and, last but not least, weed suppression (Lal et al. 1991). Weed suppression is exerted partly through resource competition (for light, nutrients and water) during the cover crop growing cycle, and partly through physical and chemical effects that occur when cover-crop residues are left on soil surface as a dead mulch or ploughed down and hence used as green manure (Mohler and Teasdale, 1993; Teasdale and Mohler, 1993).
Interference with weeds, including competition, physical, and allelopathic effects, is generally higher when grasses or crucifers are used as cover crops than when legumes are used (Blum et al. 1997). Interference from cover crops and their residues is related to their occupation of ecological niches otherwise available for weeds. This is mostly a result of the sequestration of soil nutrients and to modifications of the soil microenvironment (Gallandt et al. 1999). 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 with legume-rotations (Blum et al. 1997).
The effect of primary tillage on weeds is mainly related to the type of implement used and to tillage depth. These factors considerably influence weed seed and propagule distribution over the soil profile and therefore they directly affect the number of weeds that can emerge in a field. Mouldboard ploughing is very effective in reducing weed density and hence it is an important preventive method where farmers are forced (or are willing) to use partially suppressive direct weed control methods (e.g. mechanical weeding), and reduces the labour needed for subsequent hand-weeding. In contrast, with non-inversion tillage (especially with no tillage) weed seeds are only partially buried and therefore they are mainly distributed in the upper soil layer, from which they can easily germinate and give origin to established plants. Disturbance posed to weeds by tillage is dependent more on the type of implement than on tillage depth. Tools that do not invert the soil (e.g. chisels) increase weed density and shift weed flora composition towards an increased presence of biennials, perennials, and non-seasonal annuals. Most of these species are characterized by wind-dispersed seeds with reduced longevity and dormancy and are unable to emerge from deep soil layers (Zanin et al. 1997).
Cultivation for seed bed preparation has two contrasting effects on weeds: (i) it eliminates the emerged vegetation resulting from after primary tillage; and (ii) it stimulates weed seed germination and consequent seedling emergence, thanks to soil mixing and reallocation of seeds towards shallower soil layers. Together, these two effects can be exploited through application of the false (stale) seed bed technique, a preventive method with the specific aim of reducing weed emergence in the next crop cycle. The false seed bed technique consists in the anticipation of cultivation time for seed bed preparation, in order to stimulate as much as possible the emergence of weeds prior to sowing. Emerged weeds are then destroyed by the next cultivator pass or by application of a total herbicide (e.g. glyphosate), the latter being useful especially where perennial weeds are present. At sowing time, the seed bank of those weed species able to emerge together with the crop is then already partially depleted and their emergence in the crop is reduced.
Weed-risk assessment can be conducted at any stage of a species entry and spread into a country or area. The procedures described here are concerned primarily with preventing the entry and initial spread of plant species that are likely to become pests within a country, or which are there and ought to be considered pests. They describe the responsibilities of all participants in the process of plant importation required to halt the spread of weeds. Procedures involving weed-risk assessment for determining which species are prohibited imports, and which are to permit entry, are described. These procedures are based primarily on the experience of Australia and New Zealand, two countries that have effective quarantine protocols for preventing the introduction of and spread of unwanted plant species. One reason for the effectiveness of weed-risk assessment systems in these countries is a regulatory environment that enables the official plant protection organizations to restrict the movement of plants across the borders and within the countries. Without such national legislation, weed-risk assessment by itself cannot prevent the entry and spread of weeds. To be successful, weed-risk assessment requires an appropriate legal, institutional, and social framework that recognizes weeds are the concern of everyone.
Allelopathy and solarization
Allelopathy is defined as the direct influence from a chemical released from one plant on the development and growth of another. Allelopathic compounds may be released into the environment from plants by means of root exudation, leaching, volatilization and decomposition of plant residues in the soil. Allelopathic substances, if present in crop varieties, may reduce the need for weed management, particularly herbicide use. Allelopathy alone may not be a perfect weed management technology but it may be a supplementary tool for weed control. It is extremely difficult to demonstrate allelopathy in nature because of the complexity of plant interference which includes positive, negative and neutral effects on each other (Christensen, 1993). Interference is a combination of the processes of competition for resources and production of allelopathic compounds which suppress competitors (Duke et al. 2001).
Alleopathy research has been conducted for several decades, but very limited knowledge is still available. An improvement in crop cultivars is the only area that has not been exploited to any great extent as a weed management strategy (Khush, 1996). The possibility of incorporating allelopathic traits into improved rice cultivars, which would reduce the need for applying herbicides to the crop, is worth exploring (Khush, 1996). Of course, thus far, no commercial cultivars carrying allelopathic properties have been developed (Duke et al. 2001). Although a breeding approach alone cannot overcome weed problems, an increase in the allelopathic potential of crop varieties will likely have a great impact on both low- and high-input cropping systems. Moreover, allelopathy-based technology is also more easily transferable to farmers in low-input management systems than those in high-input management systems, which entail a heavy use herbicide.
Soil solarization or “solar heating” is a non-chemical disinfestation practice that may serve as a component of a sustainable IPM programme. Solarization effectively controls a wide range of soil-borne pathogens, insects and weeds. Soil solarization is based on the exploitation the solar energy for heating wet soil mulched with transparent PE sheets to 40–55ºC in the upper soil layer. Thermal killing is the major factor involved in the pest control process, but chemical and biological mechanisms are also involved. The efficacy of the thermal killing is determined by the values of the maximum soil temperature and amount of heat accumulated (duration x temperature). The use of organic amendments (manure, crop residues) together with soil solarization (biofumigation) elevates the soil temperature by 1–3ºC, and improves pest control due to a generation and accumulation of toxic volatiles. Although cheaper than most chemicals used as soil fumigants, not all crops can afford the PE prices, particularly in developing countries. Not all soil-borne pests and weeds are sufficiently controlled. Cheaper and more environmentally accepted mulching technologies are needed before expanding the range of the controlled pests by solarization.