C H A P T E R 6 WEED MANAGEMENT LEARNING OBJECTIVES DEVELOPMENT STAGES All crop plants and weeds have four stages of devel- opment: After completely studying this chapter, you should: I SEEDLING—small, delicate, newly emerged plants. I Be able to define a weed and its four stages of I VEGETATIVE—plant grows quickly producing development. stems, roots, and leaves. I Understand the differences between annual, biennial, I SEED PRODUCTION—plant’s energy is directed and perennial weeds. into producing flowers and seeds. I Be able to give examples of cultural weed controls. I MATURITY—plant produces little or no energy. Some plants begin to dry out or desiccate. I Know the advantages and disadvantages of the vari- ous methods of herbicide applications. I Understand herbicide carryover and how to prevent LIFE CYCLES OF WEEDS it. Weeds can also be classified according to how long it I Know what herbicide adjuvants are. takes them to complete their development or life cycle. I The three types of plant life cycles for weeds are annual, Understand the weed control problems in conserva- biennial, and perennial. tion tillage systems. ANNUAL Plants that complete their life cycle in one year are Weeds are plants growing where they are not wanted. annuals. They germinate from seed, grow, mature, pro- They can reduce field crop yields by competing for water, duce seed, and die in one year or less. Annuals reproduce nutrients, and light. Some weeds release toxins that by seed only and do not have any vegetative reproduc- inhibit crop growth, and others may harbor insects, dis- tive parts. Summer annuals eases, or nematodes that attack crops. Weeds often inter- germinate from seed in the fere with harvesting operations, and at times contamina- spring, flower and produce tion with weed seeds or other plant parts may render a seed during the summer, and crop unfit for market. Profitable crop production die in the summer or fall. depends on effective weed control. These are the most common annual weeds found in field crops. Winter annuals germi- nate from seed in the fall and die in late spring or early summer the following year. Annual weeds are easiest to control at the seedling stage. Cocklebur. Field Crop Pest Management 49 Chapter 6 BIENNIAL I Fall panicum Biennials are plants I Wild-proso millet that complete their I growth in two years. Witchgrass The first year, the plant produces leaves and Perennials stores food. The second I year, it produces fruits Johnsongrass and seeds. Biennial I Yellow nutsedge weeds are most com- I monly found in no-till Quackgrass fields, pastures, and fencerows that are not Yellow Nutsedge. mowed. They are easi- BROADLEAF WEEDS est to control in the Bull thistle is a biennial. seedling stage. Annuals PERENNIAL I Ladysthumb Perennials are plants that live I Pennsylvania smartweed for two or more years. Perennials I can reproduce by seed or vegeta- Wild buckwheat tively. The plant parts that allow I Common lambsquarters perennials to spread without pro- I ducing seeds include stolons Redroot pigweed (creeping aboveground stems— I Eastern black nightshade Redroot Pigweed. e.g., white clover and strawber- I ries), rhizomes (creeping below- Common cocklebur ground stems —e.g., milkweed, I Jimsonweed quackgrass), tubers (enlarged underground stems—e.g., potato, I Common purslane yellow nutsedge), and bulbs I Common ragweed (underground stem covered by Johnsongrass is a I fleshy leaves—e.g., tulip). Because creeping perennial. Giant ragweed perennial weeds can propagate I Velvetleaf (spread) underground, they can be the most difficult weeds to control. Removing the aboveground vegetation will not I Common chickweed stop the weed from spreading. I Shepherd’s-purse Annuals, biennials, and perennials can reproduce I from seed. Many weeds produce large quantities of Horseweed (Marestail) seeds. Seeds are easily dispersed across a field by wind, I Prickly lettuce rain, machinery, animals, and people. Weed seeds can germinate after being dormant for long periods of time. I Wild mustard They can also tolerate extremes in weather such as tem- I Yellow rocket perature and moisture. To prevent seed dispersal, you Velvetleaf. should control weeds before they produce seeds. Biennials I White campion COMMON WEEDS IN MICHIGAN I Wild carrot GRASS AND GRASSLIKE WEEDS I Bull thistle Annuals Perennials I Milkweed I Barnyardgrass I Hemp dogbane I Large crabgrass I Canadian thistle I Smooth crabgrass I Dandelion I Giant foxtail I Field bindweed I Yellow foxtail I Perennial sow thistle I Green foxtail Grassplant. Chapter 6 50 Field Crop Pest Management WEED CONTROL The first step in successful chemical weed control is the correct identification of the weeds. Annual weeds are CULTURAL CONTROL easier to kill when they are small seedlings and when conditions favor rapid growth. However, crop plants are Crop competition is a very useful method of weed also easily injured under these conditions. Selective her- control. Maintaining production practices that optimize bicides should control the weeds with little or no injury crop growth means the crop plants can compete more to the crop. effectively with weeds. Several crop management prac- Timing and rate of herbicide application are very tices can improve the competitive ability of the crop: crop important in chemical weed control. Applying herbicides and variety selection, planting date, population, soil fer- at the wrong time often results in poor weed control and tility, drainage, etc. Recommended crop production prac- crop injury. tices are also beneficial weed control practices. Crop rotation may also be helpful in maintaining ade- quate weed control. Many weeds cannot tolerate crop TYPES OF HERBICIDES rotation. Chemical weed control can be obtained with herbi- cides applied either preplant incorporated, preemer- MECHANICAL CONTROL gence, or postemergence. Many herbicides can be applied by more than one of these methods. Tillage buries weeds or destroys their under- ground plant parts. Small Preplant Herbicide Soil Applications annual and biennial and Incorporation seedlings are more effec- Preplant herbicide soil applications and incorpora- tively controlled with tion are herbicide applications that are applied and incor- tillage. However, disturb- porated into the soil before planting. Incorporation of ing the soil can bring new some herbicides is required to prevent them from weed seeds near the soil volatilizing (becoming a gas) or decomposing in the sun. surface and create another weed problem. Advantages of preplant soil applications and incorporation: CHEMICAL CONTROL I Early weed control reduces weed competition with the crop. I Wet weather will not delay cultivation or herbi- cide application to control weeds. I Preplant soil application and incorporation is less dependent on rainfall for herbicide activa- tion than preemergence herbicide applications. Disadvantages of preplant soil applications and incorporation: I Incorporating the herbicide too deep in the soil can reduce weed control. I A “streaking” pattern of good and poor weed c ontrol can result from incomplete soil incorpora- tion. I Growers apply herbicide without identifiying the weeds. They are preventive applications. I It is incompatible with a no-till system. Preemergence Herbicide Applications Preemergence herbicide applications are applied to the soil surface after the crop has been planted but before the crop or weed seedlings emerge. Typically, preemer- gence herbicide applications require rainfall within one week following the application to ensure that the herbi- cide moves through the soil. Advantages of preemergence applications: I Reduce weed competition for crop with early control of weeds. Field Crop Pest Management 51 Chapter 6 I Can be used in all tillage systems. I Timing of the application is critical for effective I Planting and herbicide application may be done weed control. at the same time. I Postemergent herbicides should not be applied to wet foliage. Disadvantages of preemergence I applications: Weather may not permit a herbicide application at the proper time. I Depend on rainfall; ineffective in dry soil conditions. I On sandy soil, heavy rains may move the herbi- HERBICIDE CARRYOVER cide down in the soil to the germinating A potential problem of herbicide applications is herbi- crop seed and cause injury. cide carryover. This occurs when a herbicide does not I Growers apply herbicide without identifying break down during the season of application and persists the weeds. They are preventive applications. in sufficient quantities to injure succeeding crops. The breakdown of herbicides is a chemical and/or microbial Postemergence Herbicide Applications process. Generally the rate of breakdown increases with soil temperature. Very dry conditions during the summer Postemergence herbicide applications are applied to and early fall often increase the potential for carryover of the foliage of the weed after crop and weed have many herbicides. emerged. There are two types of postemergence herbi- Herbicide carryover is also influenced by the rate of cides: contact and systemic. Contact herbicides kill only application, herbicide distribution across a field, soil the plant parts that they touch. Typically, the above- type, and time. When herbicides are used above the ground parts of a weed, such as the leaves and stems, labeled rate and/or not uniformly distributed, herbicide turn brown and die. Contact herbicides are commonly carryover problems may result. Poor distribution is gen- used to control annuals. erally the result of improper calibration or agitation, Systemic or translocated herbicides are absorbed by sprayer overlapping, or non-uniform soil incorporation. the weed’s roots or leaves and moved throughout the Vegetable and ornamental crops are often more sensi- plant. Translocated herbicides are more effective against tive to herbicide carryover than field crops. To reduce the perennial weeds because the herbicide reaches all parts of potential of herbicide carryover, read and follow all pes- the plant. However, translocated herbicides may take up ticide label directions. Herbicide labels contain restric- to three weeks to kill the weeds.