The Competition Between Weed and Plant/Crop

A.    INTRODUCTION

Competition between plants for the capture of the essential resources for plant growth (light, water, and nutrients) is a critical process in natural, semi-natural, and agricultural ecosystems. (Agro)ecologists have studied interplant competition intensively. However, because of the complex nature of interplant competition, it has been difficult to develop generalizing concepts and theories.

In general, a weed is a plant that is considered by the user of the term to be a nuisance. The word commonly is applied to unwanted plants in human-controlled settings, especially farm fields and gardens, but also lawns, parks, woods, and other areas. More vaguely, "weed" is applied to any plants that grow and reproduce aggressively and invasively. In weed ecology some authorities speak of the relationship between "the three Ps": plant, place, perception.

Weeds may be unwanted for a number of reasons. The most important one is that they interfere with food and fiber production in agriculture, where in they must be controlled in order to prevent lost or diminished crop yields. The next most important reason is that they interfere with other cosmetic, decorative, or recreational goals, such as in lawns, landscape architecture, playing fields, and golf courses.

Although farmers must have recognized competition effects in their systems as soon as they started to shape ecosystems to meet their needs. Competition has been regarded as one of the major forces behind the appearance and life history of plants and the structure and dynamics of plant communities. The term weed in its general sense is a subjective one, without any classification value, since a plant that is a weed in one context is not a weed when growing where it belongs or is wanted. Indeed, a number of "weeds" have been used in gardens or other cultivated-plant settings. 'Volunteer weeds' are crop plants from one year which are growing in the subsequent crop. Many studies of plant competition have been directed towards understanding how plants respond to density in monocultures and how the presence of weeds affects yield in crops.

B.     DISCUSSION

Weeds generally share similar adaptations that give them advantages and allow them to proliferate in disturbed environments whose soil or natural vegetative cover has been damaged. Naturally occurring disturbed environments include dunes and other windswept areas with shifting soils, alluvial flood plains, river banks and deltas, and areas that are often burned. Since human agricultural practices often mimic these natural environments where weedy species have evolved, weeds have adapted to grow and proliferate in human-disturbed areas such as agricultural fields, lawns, roadsides, and construction sites. The weedy nature of these species often gives them an advantage over more desirable crop species because they often grow quickly and reproduce quickly, have seeds that persist in the soil seed bank for many years, or have short lifespans with multiple generations in the same growing season. Perennial weeds often have underground stems that spread out under the soil surface or, like ground ivy (Glechoma hederacea), have creeping stems that root and spread out over the ground.

Some plants become dominant when introduced into new environments because they are freed from specialist consumers; in what is sometimes called the “natural enemies hypothesis,” plants freed from these specialist consumers may increase their competitive ability. In locations were predation and mutual competitive relationships no longer exist, some plants are able to increase allocation of resources into growth or reproduction. The weediness of some species that are introduced into new environments can be caused by the introduction of new chemicals; sometimes called the "novel weapons hypothesis," these introduced allelopathyic chemicals, which indigenous plants are not yet adapted to, may limit the growth of established plants or the germination and growth of seeds and seedlings. Weeds interfere by:

1.     competing with the desired plants for the resources that a plant typically needs, namely, direct sunlight, soil nutrients, water, and (to a lesser extent) space for growth;

2.     providing hosts and vectors for plant pathogens, giving them greater opportunity to infect and degrade the quality of the desired plants;

3.     offering irritation to the skin or digestive tracts of people or animals, either physical irritation via thorns, prickles, or burs, or chemical irritation via natural poisons or irritants in the weed.

The process by which a plant acquires more of the available resources (such as nutrients, water or light) from the environment without any chemical action on the surrounding plants is called resource competition. A number of eco-physiological models of competition between weeds and crops have now been produced. Once a crop: weed competition model has been developed to a level where there is confidence in its predictions, it can be used in different environments to interpret differences in yield loss due to weeds, to explore the interactions between crop, weeds, environment and management factors, and to extrapolate to situations in which there are, so far, no experimental data. Using sensitivity analyses of the models, we can identify plant traits that confer greater competitiveness, thus giving direction to crop breeding programmes.

Competition for light is simulated on the basis of the leaf area of the competing species and its distribution over the height of the canopy. The absorbed radiation by the species in relation to plant height is calculated first. Using the CO 2 assimilation light response of individual leaves, the profile of CO 2 assimilation in the canopy is calculated. Integration over height and the day gives the daily rate of CO 2 assimilation of the species. After subtraction of losses for maintenance and growth, the daily growth rate in dry matter of the species is obtained. Effects of drought and nutrients are taken into account by a simple water and nutrient balance in which the available amounts of soil moisture and nutrients during the growing Season are tracked. Soil moisture and nutrients are allocated to the competing species mainly proportional to their demands.

World wide a 10% loss of agricultural production can be attributed to the competitive effect of weeds, in spite of intensive control of weeds in most agricultural systems. Without weed control, yield losses range from 10 - 100%, depending on the competitive ability of the crop. Therefore, weed management is one of the key elements of most agricultural systems. The use and application of herbicides was one of the main factors enabling intensification of agriculture in the past decades. However, increasing herbicide resistance in weeds, the necessity to reduce cost of inputs, and widespread concern about environmental side effects of herbicides, have resulted in great pressure on farmers to reduce the use of herbicides. This led to the development of strategies for integrated weed management based on the use of alternative methods for weed control and rationalization of herbicide use. Rather than trying to eradicate weeds from a field, emphasis is on the management of weed populations. It has been shown that weed control in some crops (like winter wheat) is generally not needed to reduce yield loss in the current crop, but only to avoid problems in future crops. The development of such weed management systems requires thorough quantitative insight in the behaviour of weeds in agroecosystems and their effects. This involves both insight in crop-weed interactions within the growing season as well as the dynamics of weed populations over growing seasons.

Several attempts have been made to develop weed control advisory systems, using thresholds for weed control, i.e. the level of weed infestation which can be tolerated based on specified criteria which are generally based on. A number of concepts for thresholds for tactical (within season) and strategic (long-term) decision-making in weed management have been developed. However, the approach has hardly been used in practice. Besides problems related to accuracy in yield loss predictions, good quantitative data on the effects of specific weeds in specific crops are sparse as well as reliable simple assessment methodologies.

The eficacy of using agronomic practices to manage weed populations will be improved by a comprehensive understanding of the mechanisms of competition. Mathematical models to describe the process of plant competition have developed concurrently with our increasing empirical understanding. The structure of models has reflected the prevailing approach to weed management. Earlier research was focused on the calculation of yield loss as a result of weed competition and an empirical modelling approach. A more recent interest in managing competition, through increased knowledge of the ecology and biology of competing species, has resulted in an increase in the use and development of more mechanistic-based and dynamic population models for weeds. Used as either a tool for research or as a method for prediction, the mathematical model is an essential and integral part of the study of plant competition. Crop-weed competition models have been used extensively for determining the yield loss of crops that result from varying densities of weeds. In one of the simplest extensions of this approach, knowledge of crop-weed competition has been combined with herbicide±weed resource curves to simulate the effects of herbicide use on crop yield and provide a rudimentary economic evaluation of herbicide treatments.

Aggressive weeds and grasses can invade the lawn and garden, certain methods to get rid of them may harm the cultivated plants as well. Herbicides, commonly called weed killers, contain ingredients formulated to kill vegetation. While some herbicides are safe to use among flowers, vegetables, and grasses, others may damage the desirable plants and affect their growth. Weed killers (herbicides) can save considerable labor in the yard and garden. Some of these kill plants selectively, so the manager can control weeds but not injure desirable plants. Others are not selective and may kill all plants in an area. They must be applied directly to weeds carefully to avoid damaging nearby plants.

E. CONCLUSION

Crop-weed models incorporating competition have had considerable success in describing how the process of competition affects crop yield and how strategic weed management decisions impact on weed numbers for a limited range of economically important species. There is, however, a need to increase our understanding of the spatial and temporal variability in model parameters if they are to be used more in a predictive context and to pull together data for a wide range of weeds and crops.

Exploration of integrated weed management requires that we understand how weed management decisions within the crop growing-season affect: (a) the yield of the crop through competition for resources, and (b) the biodiversity and numbers of weeds in the current and future crops. Both mechanistic and phenomenological models have a role to play here. The former include suficient detail of the relationships between plant traits and the environment to allow exploration of within-season management decisions on crop yield, while the latter, although not including such intricate detail, allow exploration of strategic management decisions on the abundance of weeds through the crop rotation.

Different herbicides affect different plant systems, resulting in a range of symptoms from discolored or distorted leaves and stems to a lack of seedling emergence. When landscape plants come into contact with herbicides, major problems can ensue. Effects may be mistaken for indications of insect infestation, disease, nutritional deficiency or environmental disorder. Identify the plant problem before putting on any pesticide.  Herbicides should not be applied at high temperatures nor on windy days, as these increase the risk of drift. While there are techniques to alleviate the damage caused by accidental contact with herbicides, it is best to avoid incorrect application.

BIBLIOGRAPHY

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Park, et al. 2003. The Theory and Application of Plant Competition Models: an Agronomic Perspective. Annals of Botany 92: 741-748.