1、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 nem
2、atodes. 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 t
3、heir 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
4、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
5、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
6、 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
7、 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 winte
8、r 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 a
9、ttention 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 b
10、iotype 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,
11、 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 identificat
12、ion 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,Man
13、y 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,
14、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 ea
15、rly. 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 plan
16、t 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 pr
17、actice 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
18、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.,Cultural Weed Control,Mechanical weed co
19、ntrol describes any physical weed-control measure. Prior to the 1940s, done by hand removal or tillage.,A variety of rotary hoes, cultivators, disks & harrows are available to perform weed control in crop fields. Concerns about soil erosion & compaction have reduced use of tillage in recent decades.
20、,Mowing is another important method of weed control in many situations. goals: It eliminates production of seed. If timed correctly, depletes stores of carbohydrates in the roots and vegetative propagules of weedy plants.,Cultural Weed Control,Mulching is a common method of weed suppression in flowe
21、r beds, vegetable crops, and nurseries. Essentially any substance that can be used to cover the ground and smother weeds can be considered mulch.,As organic mulches decay, they can serve as a fertile seedbed for annual weed seeds. New layers of organic mulch must be put down every year. Plastic & fa
22、bric mulches suppress more aggressive creeping perennials species. Preventing establishment of newly deposited annual seed.,Cultural Weed Control,Fire has been tried as weed-control in almost every environment humans have attempted to manage.,Controlled burning has been value in removing unwanted br
23、ush from forest areas, prairie management/restoration, warm-season grassland, and rangeland management. Specialized flame weeders are sometimes used to burn off small annual weeds in organic vegetable production.,Biological Weed Control,Biological weed control involves introduction of a living organ
24、ism to control a weed species. Often a natural pest or disease.,Two approaches to biological weed control: Classical (or inoculative) approacha one-time introduction of an organism to manage weeds. The organism must establish itself in the ecosystem and reproduce. Inundative (or augmentative) approa
25、chapplication of the biological organism on an as-needed basis. Expected to provide short-term control but not to establish itself.,Weed control in this system is typically neither instantaneous nor complete. Cost-effective if the control species establishes itself.,Chemical Weed Control,The basis f
26、or modern chemical weed control was laid out with the initial studies of auxins and plant hormone physiology in the 1930s. Culminating with discovery of the herbicidal properties of 2,4-dichlorophenoxyacetic acid (2,4-D) in 1942.,Chemical weed control helped revolutionize crop production in the seco
27、nd half of the 20th century. Herbicides have several characteristics that are important to understand, to be used effectively. Selectivity, timing of application, use rate, mode of action. Recently, crops have been introduced that are bred or genetically engineered for herbicide resistance. Greatly
28、expanding utility of several common herbicides.,Chemical Weed Control - Selectivity,Herbicides are often broadly classified as grass- or broad-leaf killers, or nonselectivewhich kills both. Some grass herbicides possess a certain degree of broad-leaf activity, and vice-versa.,Development of herbicid
29、e-resistant crops represents the latest important advance in herbicide selectivity.,Chemical Weed Control - Application Timing,Herbicides can be classified according to timing.,Equipment used for application of herbicides should not be used for other purposesto avoid possibility of accidentally harm
30、ing plants.,Burn-down herbicides are applied to remove existing weeds in the field & prepare a clean seedbed for planting. Preplant herbicides are applied before a crop is planted. Often require incorporation into soil. Preemergence herbicides are applied after crop planting but prior to emergence o
31、f the weeds, the crops, or both. Many require rainfall for maximum efficacy. Postemergence herbicides are applied directly to foliage after weed emergence.,Chemical Weed Control - Application Timing,Utilization of satellite positioning technology and advanced application controls allow growers to ap
32、ply herbicides with pinpoint accuracy. And to vary application rate based on the history of weed pressure or on observations of high-density weed patches.,Chemical Weed Control - Use Rates,Utilization of the proper herbicide rate is key to effective weed control. Smaller weeds of a given species can
33、 be controlled with a lower use rate than larger weeds of the same species.,Growers typically favor low use rates because it lowers costs while reducing any potential negative environmental impacts from leaching or runoff. The difficulty with low rates is that larger weeds in may survive, mature, an
34、d produce seedrequiring more herbicide applications in the future. Optimum use rate requires the grower to have a thorough knowledge of the size and species of weeds present in the field.,Chemical Weed Control - Mode of Action,Herbicides can be classed in groups based on plant biochemical processes
35、with which they interfere.,Chemical Weed Control - Mode of Action,Understanding the mode of action of an herbicide is important for three main reasons.,Knowing how the herbicide kills allows predictions of how particular herbicides will perform in different conditions. Understanding mode of action h
36、elps growers identify and assess injury symptoms on desired & nontarget plants. Knowledge of mode of action is an important asset in helping producers manage herbicide-resistant weeds. Rotating modes of action reduces probability that a resistant biotype will be selected for a given field. A few spe
37、cies of weeds have developed resistance to multiple modes of action. Relying strictly on herbicides is not a viable long-term weed-control strategy for some species.,Chemical Weed Control - Resistant Crops,One of the latest advances has been development of crops that are resistant to broad-spectrum,
38、 commonly used herbicides. Using both conventional plant-breeding methods and genetic engineering technology.,The level of weed control obtained can be quite impressive & has fueled rapid adoption of these crops by many producers throughout the world. Some countries have restricted or banned the use
39、 of herbicide-resistant crops or stipulated that grain from these crops be used only for animal feed.,Chemical Weed Control - Resistant Crops,Some countries have restricted or banned the use of herbicide-resistant crops or stipulated that grain from these crops be used only for animal feed. Oppositi
40、on from groups concerned about releasing new crop biotypes in the environment has hindered utilization.,Proponents point to the many potential benefits from these crops. Reduced herbicide use, use of less toxic herbicides, and the opportunity to increase crop yields.,Chemical Weed Control - Resistan
41、t Crops,Many different preventive measures are effective at keeping weeds from entering a field.,Cleaning planting, tillage, and harvesting equipment when moving from one area to the next is essential. Using weed-free crop seed is another method. Preventing seed from moving into a field from surroun
42、ding areas such as roadsides, ditches & fencerows also helps. Preventing weed reproduction in crop fields is an important preventive task, particularly for control of annual species Many US federal, state, and local laws require landowners to control certain aggressive weed species on their property
43、.,Weed Control in Organic Crop Production Systems,Successful weed control in organic systems requires careful integration of preventive, mechanical, and cultural weed-control methods.,Organic producers will normally have higher weed-management costs for less effective overall weed control than their
44、 conventional counterparts. They must continue to receive premiums for their crops to compensate them for the increased production costs. Increasing interest has spurred an increase in weed-management research in organic systems. If consumer demand continues to increase, so will the need for more effective weed-control strategies.,
