Weeds

1. Weeds • A weed can be any plant that… • 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. • Study of weeds is generally divided into two weed biology & ecology and weed control & management.

2. Classification • Weed scientists have found that classification by life cycle—annuals, biennials, perennials—is useful. • The term invasive weed species has recently been added—used to classify species that are particularly aggressive, regardless of their life cycle. • Invasive species can be plants, animals, and other organisms—and human actions are the primary means of invasive species introductions.

3. Competition The time a weed spends in contact with a cropplant can have a dramatic effect on yield.

4. Competition The time a weed spends in contact with a cropplant can have a dramatic effect on yield. The corn plot to the left had weeds removed early in thegrowing season; while in the plot to the right, the weedswere allowed to persist until much later in the growingseason, which greatly restricted growth of the corn crop.

5. Competition • Weeds emerging early in the season have a much greater potential impact on crop yield than thosethat emerge later in the season.

6. Reproduction Annual weeds rely on high seed production, seed longevity, seed dormancy, or a combination ofthese three factors for survival.

7. Reproduction • Seed dormancy helps to ensure long-term survival of many weed species. • Larger seeded weeds may produce relatively low numbers of seeds—but 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 plant—but the seed can germinate only from shallow soil depths.

8. 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.

9. D A A B Examples of vegetative propagules. (A)Johnsongrass (Sorghum halepense) rhizomes. (B)White clover (Trifolium repens) stolons. (C)Yellow nutsedge (Cyperus esculentus) tubers. (D)Wild garlic (Allium vineale) bulbs. (E)Honeyvine milk-weed (Ampelamus albidus) creeping roots. C E Figure 15-10 Reproduction

10. Reproduction Carbohydrate storage is maximum in winter months, which provides the plant with sufficient energy to produce new shoots aggressively in the spring.

11. Reproduction Carbohydrate levels are at minimum levels inspring and early summer because they have beenexpended to support early spring shoot growth.

12. Genetics • Weed genetic study has gained attention as casesof 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.

13. 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.

14. 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 speciesis 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.

15. 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.

16. Cultural Weed Control • Flooding is a weed-control practice used with rice. • Allows regulation of germination of annual weed seedsby 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 weeds—allelopathy.

17. Cultural Weed Control • Mechanical weed control 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. • 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.

18. Cultural Weed Control • Mulching is a common method of weed suppression in flower 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 & fabric mulches suppress more aggressive creeping perennials species. • Preventing establishment of newly deposited annual seed.

19. 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 brush from forest areas, prairie management/restoration, warm-season grassland, and rangeland management. • Specialized flame weeders are sometimes used to burnoff small annual weeds in organic vegetable production.

20. Biological Weed Control • Biological weed control involves introduction ofa living organism to control a weed species. • Often a natural pest or disease. • Two approaches to biological weed control: • Classical (or inoculative) approach—a one-time introduction of an organism to manage weeds. • The organism must establish itself in the ecosystemand reproduce. • Inundative (or augmentative) approach—application ofthe 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.

21. Chemical Weed Control • The basis for 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 propertiesof 2,4-dichlorophenoxyacetic acid (2,4-D) in 1942. • Chemical weed control helped revolutionize crop production in the second 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 expanding utility of several common herbicides.

22. Chemical Weed Control - Selectivity • Herbicides are often broadly classified as grass- or broad-leaf killers, or nonselective—which kills both. • Some grass herbicides possess a certain degree ofbroad-leaf activity, and vice-versa. • Development of herbicide-resistant crops represents the latest important advance in herbicide selectivity.

23. Chemical Weed Control - Application Timing • Herbicides can be classified according to timing. • 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 of the weeds, the crops, or both. • Many require rainfall for maximum efficacy. • Postemergence herbicides are applied directly to foliage after weed emergence. • Equipment used for application of herbicides should not be used for other purposes—to avoid possibility of accidentally harming plants.

24. Chemical Weed Control - Application Timing • Utilization of satellite positioning technology and advanced application controls allow growers to apply herbicides with pinpoint accuracy. • And to vary application rate based on the history of weed pressure or on observations of high-density weed patches.

25. Chemical Weed Control - Use Rates • Utilization of the proper herbicide rate is key to effective weed control. • Smaller weeds of a given species can be controlled witha 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, and produce seed—requiring 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.

26. Chemical Weed Control - Mode of Action Herbicides can be classed in groups based on plant biochemical processes with which they interfere.

27. 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 helps 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 species 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.

28. Chemical Weed Control - Resistant Crops • One of the latest advances has been development of crops that are resistant to broad-spectrum, 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 useof herbicide-resistant crops or stipulated that grain from these crops be used only for animal feed.

29. Chemical Weed Control - Resistant Crops • Some countries have restricted or banned the useof herbicide-resistant crops or stipulated that grain from these crops be used only for animal feed. • Opposition 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.

30. Chemical Weed Control - Resistant Crops • Many different preventive measures are effective at keeping weeds from entering a field. • Cleaning planting, tillage, and harvesting equipmentwhen 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 surrounding 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.

31. 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 conventional counterparts. • They must continue to receive premiums for their cropsto 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.