Weeds,A weed can be any plant that,Study of weeds is generally divided into two weed biology & ecology and weed control & management.,Competes with desired plants for water, nutrients, sunlight. Interferes with harvesting or reduces crop quality/appeal. Harbors undesirable insects, pathogens, or nematodes. Competes with the crop for pollinating insects.,Classification,Weed scientists have found that classification by life cycleannuals, biennials, perennialsis useful.,The term invasive weed species has recently been addedused to classify species that are particularly aggressive, regardless of their life cycle.Invasive species can be plants, animals, and other organismsand human actions are the primary means of invasive species introductions.,Competition,The time a weed spends in contact with a crop plant can have a dramatic effect on yield.,Competition,The corn plot to the left had weeds removed early in the growing season; while in the plot to the right, the weeds were allowed to persist until much later in the growing season, which greatly restricted growth of the corn crop.,The time a weed spends in contact with a crop plant can have a dramatic effect on yield.,Competition,Weeds emerging early in the season have a much greater potential impact on crop yield than those that emerge later in the season.,Reproduction,Annual weeds rely on high seed production, seed longevity, seed dormancy, or a combination of these three factors for survival.,Reproduction,Seed dormancy helps to ensure long-term survival of many weed species.,Larger seeded weeds may produce relatively low numbers of seedsbut these species can rely on large reserves of stored energy to emerge from considerable depths in the soil profile. Smaller seeded weeds may produce many more seeds per plantbut the seed can germinate only from shallow soil depths.,Reproduction,Many biennials possess the capacity to behave as short-lived perennials.,Unlike annuals and biennials, perennials do not rely as heavily on seed production for establishment.,Reproduction,Reproduction,Carbohydrate storage is maximum in winter months, which provides the plant with sufficient energy to produce new shoots aggressively in the spring.,Reproduction,Carbohydrate levels are at minimum levels in spring and early summer because they have been expended to support early spring shoot growth.,Genetics,Weed genetic study has gained attention as cases of herbicide resistance have steadily increased. Cases have increased to 187 species & 323 biotypes since the first reported case, (Senecio vulgaris), in 1968.,A biotype is a subpopulation of plants in a species. Differing only slightly from the general population norm. If a weed biotype resistant to a herbicide (or group of similar herbicides) survives & reproduces, its pollen or seed from may carry the resistant gene. A population of herbicide-resistant biotypes may become prevalent within a relatively short period of time. Integration of several weed-management strategies, is useful for controlling these problematic weeds.,Genetics,Genetic variability within a weed species is not restricted to physiological differences. Many species vary in features like leaf & seed morphology.,Individuals from closely related species may differ so much in morphology that identification is challenging. The variation likely plays a significant role in adaptive survival and evolution of a species.,Ecology,Ecology is the study of a species in both its physical and biological environment. Study of weed ecology may increase our capacity to use biological methods to control weeds,Many weeds have special adaptations allowing them to compete effectively in specific environments. They provide weeds an advantage over other species. Removing the set of conditions to which a particular species is adapted will reduce competitive ability. Numerous organisms use weeds as a food source, for shelter, and for nesting habitat. Practices that increase populations of seed feeders may be an important nonchemical weed-control method.,Cultural Weed Control,Cultural weed control uses production methods that enhance capacity of a crop to compete with weeds.,Many summer annual weeds emerge early. Crop-planting a few weeks later may allow opportunity for disking, harrowing or chemical application to remove them. Modification of crop row spacing is a widespread practice that directly affects weed competitiveness. The goal is to maximize crop yield while reducing time required for the plant canopy to close. Crop rotation can be effective in weed-control. Planting/harvest dates, fertilizer regimes, tillage practice & herbicide usage vary dramatically between different crops. Different weed control measures can be used with each crop.,Cultural Weed Control,Flooding is a weed-control practice used with rice. Allows regulation of germination of annual weed seeds by indirectly managing oxygen available in the soil.,Cover crops can suppress germination and compete effectively with existing weed species. Often planted in fall and killed in the spring prior to the establishment of the crop intended for harvest. Also an effective method of reducing soil erosion. In addition to effects of competition, production of chemicals by plants that are toxic to other plants may play a role in the suppressive effect of cover crops on weedsallelopathy.,