Therefore, to control effectively we should ask: why do weeds emerge; and what factors limit their development?. There is abundant evidence that the presence of weeds reduce crop yields; weeds compete for environmental resources, especially water, light and soil nutrients, resulting in decreased crop yield or reducing the crops quality by contaminating the commodity, interfering with harvest, serving as hosts for crop diseases or providing shelter for insects to overwinter, limiting the choice

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... f crop rotation sequences and cultural practices. The most important parameters that characterize the infestation of weeds in a crop and that determine the competitive relationships between them are the density and time of weed competition. Their competitive ability is associated with the establishment of a dense infestation, and is caused by the different habits of growth of weeds and crops. Weeds have developed a number of features that allow them to survive and even dominate in adverse environmental conditions. Also, to learn more about competition exerted by weeds is necessary to know their life cycle, and we can observe three major life cycle groups in cereal arable fields: Annuals Summer annuals germinate in the spring, mature, produce seed, and die in one growing season. Winter annuals germinate in late summer or fall, mature, produce seed, and then die the following spring or summer. Biennials Weeds grow from seed anytime during the growing season. They normally produce a rosette of leaves close to the soil surface the first year, then flower, mature, and die during the second year. A true biennial never produces flowers or seeds the first year. There are relatively few biennial weeds. Perennials Simple perennials form a deep taproot and spread primarily by seed dispersal. Creeping perennials may be either herbaceous or woody and can spread by both vegetative structures as well as by seed. When we study the competition process between species, we must consider what resources are limiting in the environment, which will account for more competition. Since weeds are so prevalent in many areas of the landscape, management techniques are necessary to maintain order. Weed management is most successful when it involves an integrated approach using a variety of methods. The common methods used to manage weeds include prevention and cultural, mechanical, biological, and chemical means. Herbicides remain the predominant weed management tool with the greatest influence on weed selection over the last 60 years [3] . Reliance on chemicals for weed control has increased significantly in the last decades [4] . However, herbicide use also carries risks that include environmental, ecological, and human health effects. It is important to understand both the benefits and disadvantages associated with chemical weed control before selecting the appropriate control. Many factors determine when, where, and how a particular herbicide can