A-B-C of Integrated Weed Management

A. Know the enemy

  • Know what problematic weeds are in your area (e.g. herbicide resistant Palmer amaranth).

    According to a recent study more than seventy percent of growers are implementing changes to their weed management programs to combat weed herbicide resistance. With this growing problem, innumerable efforts are underway to fight resistance and control weeds. Weed identification is key to starting any effective weed control program.

    The first step in weed control is to know your enemy, For doing that different options are available, such as using Apps, books, weed ID extension material, on-line websites and professional advice among others.

    For more information, see our resistant weeds and WEED ID pages.

    Weed identification and understanding of which weeds are dominating a field is important. Photo Credit: Barb Scott, University of Delaware
  • Scout routinely and identify your weeds

    Timely weed scouting makes it possible to know

    1. what weed species are in a field
    2. where they are, and
    3. how severe the infestation is – all of which are very important factors in designing a management program that fits the specific needs of that field.

    Scouting should happen early and often. Weeds are easier to control when they are smaller. After application, scouting is important to know if the target weeds were killed and if a following herbicide application is necessary. If uncontrolled weeds are allowed to set seed and contribute to the soil seed bank, they will become a problem year after year.

    For more information go to our scouting for weeds page.

    Uncontrolled weeds, such as the morning glory and pigweeds pictured here in soybeans, will result in a yield penalty. Credit: Claudio Rubione, University of Delaware.
  • Understand the biology of your weeds

    Weed biology refers to those attributes that may be associated with survival and dispersal of the species. Some of these attributes include life-cycle, competitiveness, reproductive biology, and seed bank dynamics (seed germination, dormancy, and length of survival in the soil). “Understanding weed biology is essential for development of integrated weed management systems because it is the basis for weed management programs”. Understanding weed biology can be useful to predict how successful a weed management tactic will be for specific species.

    For more information on weed biology, go to our page on resistant weeds.

    Palmer amaranth dropped by a combine or hand-pulled but not removed completely from the field often regrows as a survival strategy. Knowing weed biology will decrease weed seed production. Credit: Claudio Rubione, University of Delaware.

     

    Hordeum glaucum (smooth barley) sticks to animal hair and clothes, traveling long distances from where it grew and potentially spreading the infestation. Credit: Lovreet S Shergill, USDA-ARS & University of Delaware

B. Plant into weed-free fields and prevent weeds from producing seed

Introduction

The first major step in weed management is to kill all of the vegetation in a field prior to cash crop planting. Weeds present at planting can interfere with the planting operation by binding up equipment and preventing good seed-to-soil contact. Once the crop emerges, the weeds can compete with the new crop seedlings.

Weeds are easier to control prior to planting when there are more options for control. There are set steps to avoid a control failure prior to planting:

  1. Scout early to determine what weeds are present and the growth stage of these weeds.
  2. Terminate weeds when they are small and susceptible.
  3. Allow adequate time for weeds to die before planting.
  4. Scout again prior to planting to ensure that all weeds are dead prior to planting. If further control is necessary, plan to do this prior to cash crop planting.

There are two major methods for preplant weed control: herbicides and tillage.

Tillage will destroy aboveground plant tissues and bury weed seeds that are on the soil surface. However, tillage might not control some weeds and can stimulate weed germination.

Herbicides need to be carefully selected based on the weed species present, specifically the hard-to-control weed species, but there could be plant-back restrictions based on the cash crop being planted.

Planting into a weed-free field is only the first step in weed management. Starting clean does not mean that the field will be free of weeds all season long. Weeds will continue to emerge and will have to be managed throughout the season. For more information go to our page: Plant into Weed Free Fields

a. Diversify crop rotation

Crop rotation is a cultural practice that can suppress diseases, insects, nematodes, and weeds. A diverse crop rotation suppresses weeds because of both the competitive nature of the different cash crops themselves and more importantly, the opportunity to use different management strategies in each cash crop. These strategies include different planting times, row spacings, fertility practices, and weed management options such as herbicides, use of tillage or cultivation, and the ability to use a cover crop or harvest weed seed control.

Planting only one or a few cash crops in a rotation selects for weed species that thrive in conditions similar to the crop. These species dominate and are often difficult to control in these production systems because they can survive the conditions within this production system. Expanding the diversity of the rotation allows for greater changes in management, which leads to increased weed diversity and lower weed densities.

The greater the diversity of the rotation, the more opportunities available for weed suppression that will create an environment where weeds will not be able to thrive. While rotating between summer annual crop species allows for different control options, growers should consider rotations that include winter annual or perennial crops to provide additional control measures for difficult-to-manage weeds. For more information go to our page: crop rotation

b. Follow herbicide best management practices

A lot needs to happen to make an herbicide kill a weed. It has to get from the jug into the spray mixture and then onto the weed. The right herbicide(s) must be applied at the right rate, spray coverage, timing, and in the right weather. Issues like tank-mix compatibility, spray water quality, adjuvants or surfactants, rainfastness, and others must be addressed. Luckily, herbicide formulations are pretty robust, but we need to do our homework for each application.

  • Select the right herbicide
    Make sure the herbicides you choose are effective on the weeds you have at the growth stage the weeds are in. Proper weed identification is essential. Also carefully consider crop tolerance and carryover concerns.
  • Apply multiple, effective sites of action (SOA) herbicides
    Apply multiple, effective SOA in every application, to the greatest extent possible. For more information go to our page: Mix Effective SOA To Decrease Herbicide Resistance Development
  • Respect herbicide labeled rates and apply at specified weed sizes
    Full label rates applied to proper weed sizes deliver effective herbicide doses. Effective doses kill the weeds and protect against herbicide resistance development. Applying reduced herbicide rates or spraying weeds that are too large reduces the effective dose, which decreases weed control and increases the risk of herbicide resistance.
  • Calibrate your sprayer and select the appropriate nozzles
    To deliver effective doses, a sprayer must be calibrated. Additionally, contact herbicides need adequate coverage to maximize their effectiveness. Coverage requires an adequate volume of water (15 GPA or more is recommended for contact herbicides) and appropriate nozzles. Make sure to follow these label recommendations.
  • Add the adjuvants
    Make sure to include adjuvants according to the product label, whether it is drift reductions agents (DRA), non-ionic surfactants (NIS), ammonium sulfate (AMS), or other additives. If your spray water needs a water conditioning agent or you are mixing many different products, make sure they are compatible and you follow the correct mixing order.
  • Respect weather conditions needed for successful applications
    Some herbicides, like Liberty, need the sun out and the temperature up. Others, not so much. Residual herbicides need rainfall to activate them; postemergence herbicides need time to dry before rain (rainfast time). Stresses, such as drought, make weeds more difficult to kill. Make sure to get these right.
  • Read, understand, and follow the label. The label is the law.

c. Include supplementary weed management tactics

  • Use cover crops for weed suppression
    Cover crops offer weed control opportunities at several points in time. A burndown application or tillage prior to cover crop planting kills weeds. While growing, cover crops suppress weeds by competing for space, light, water, and nutrients. Cover crop termination methods, such as mowing, tillage, and herbicide application, can also simultaneously kill weeds. Cover crop mulch after termination suppresses weeds by discouraging weed seed germination and smothering weed seedlings. For more information go to our page: Cover Cropping

    Cover crop options are not limited to one species such as rye, you have different choices depending on your location, time of the year, and goals you need to achieve, amongst other factors. Photo credit: Claudio Rubione, University of Delaware
  • Planting date
    When planning a weed control strategy, planting date can be one of the tools to ensure success of crops and cover crops against weeds. Choosing planting dates conducive to the rapid emergence of crops will result in a dense crop canopy that shades out weeds and gathers sunlight in an efficient manner.See our crop rotation page for further information.
  • Use moldboard plow for weed-seed burial, where feasible
    To improve control of herbicide resistant weeds, tillage is a useful tool which prevents pigweed and waterhemp seeds from germinating. Care should be taken to avoid bringing buried seeds up to where they can germinate.See deep tillage for burying weed seeds for more information.
  • Use in-crop tillage for weed control, where feasible
    While tillage is no longer a popular practice, it is a good tool to control weeds and its use may become necessary to combat herbicide resistant weeds. Every cash crop has a critical weed free period during which weeds should be managed to prevent yield losses. Ideally the critical weed free period for corn is VE-V6; for soybeans it is V1-V3. Be careful to cultivate weeds at the ideal time to kill the weed: it should be used when weeds are less than 2” tall, and before a hot, dry period to prevent weeds from surviving and re-rooting. Cultivation is more effective for annual weed control; in the case of biennial and perennial weeds, it could spread weeds instead.See our mechanical control page as well as our experience with heavy residue cultivator for use with no-till and cover crop video.
  • Nutrient management
    Soil amendments applied in order to produce a healthy crop and achieve excellent yields also give the crop a competitive advantage over weeds. For example, while wheat responds well to nitrogen, does better in soil low in nitrogen. This gives wheat the advantage. Similarly, wheat responds well to phosphorous while weeds do not. Since soybeans produce their own nitrogen, limiting the application of nitrogen causes the soybeans to starve weeds of this nutrient. It is essential to consider soil fertility and the timely application of soil amendments. In order to produce healthy crops with an advantage over weeds, choose the right source (fertilizer), right rate, right time, and right place.

    Figure 10.2. 4Rs: Right source matches fertilizer type to crop needs; Right rate matches the amount of fertilizer to crop needs; Right time makes nutrients available when crops need them; and Right place keeps nutrients where crops can use them (TFI 2017).
  • Row spacing, seeding rate, and leaf architecture
    It is essential to achieve quick closure of the crop canopy in order to give crops an advantage over weeds, and using these strategies allow the crop to efficiently use sunlight and limit the light available to weeds. Crop canopy closure blocks weed access to light, suppressing weed growth and development. An effective way to enhance canopy closure is reduce row spacing and/or increase seeding rates. These strategies allow for the early canopy closure. Leaf architecture and also impact shade development, but with horizontal leaves intercepting more sunlight than vertical ones.

    Row spacing promotes rapid row canopy closure. This picture shows how weeds were minimized when soybeans were planted at 7” and 15” row spacing. Credit: Claudio Rubione, University of Delaware.

    For more information go to our page: Row Spacing.

 

C. Actively manage the soil seed bank

A number of attributes contribute to a plant’s “weedy traits” with high seed production and ability to produce seeds under stressful conditions are often described as key characteristic. Producing a large number of seeds often allows a plant species to overwhelm other species.

Seed production, coupled with other seed characteristics, such as dormancy and longevity, allows a species to survive over multiple years in the soil; this reservoir of seeds is often referred to as the soil seed bank. The seed bank is the main source of weeds that ultimately infests agricultural areas and is strongly influenced by cultural practices. The resulting weed populations are quite variable as a result the differential response of weed species to cultural practices.

 

  • Stop weed-seed set
    Whenever possible, hand pull your escaped weeds before they set seed. As described above, the seed bank size will drastically increase if weeds set seeds.

    Stopping weed seed set sometimes means hand-pulling your weeds and being sure to remove them from the field. In this case Palmer amaranth was removed from the crop field and burned to prevent spreading the seeds to new areas. Credit: Claudio Rubione, University of Delaware.
  • Manage your field borders with different tactics than in the field. Prevent weed seed set and influx from borders.
    In recent years, there have been tremendous effort to prevent weed escaping control from producing seed within a field to avoid rapid replenishment of the seed soil seedbank and the spread of resistance. However, management must go beyond the borders of the field if growers are to be successful long-term in their fight against weeds.
    Stop weed-seed dispersal
  • Be sure to buy weed-free cash crop and cover crop seed
  • If you apply manure or other soil amendments, minimize risk of introducing new weeds (e.g. through appropriate composting or testing of materials)
  • Be sure your soil preparation tools, seeders, and planters are weed-free
  • Start planning your harvesting sequence, leaving weedy fields for last as well as infested areas within a field.
    For more information go to our page: Prevent Weed Seed Dispersal at Harvest, A Simple Plan
  • Plan your harvest sequence and logistics in advance.
    Consider the machinery needed for harvest and plot out movement through your fields from least weed infested to most. Know where you will clean your combine once ready to harvest and allow time to clean it to prevent the spread of weeds.
  • If you plan to hire harvest contractors, be sure they thoroughly clean their machinery before coming in to your fields and when moving between fields. If you buy used combines, be sure to clean them thoroughly.
    When you move your combine and support tools from one field to another, do a thorough cleaning before running them in the next field to be harvested. Watch this video to help deep clean your combine of weed seeds:

    Use a tarp at the back of your combine to see how well your combine gets cleaned of weed seeds after feeding the header with straw bales. Credit: Claudio Rubione, University of Delaware.
  • Along with thoroughly cleaning field machinery, implement Harvest Weed Seed Control
    Weeds that escape control are likely to be mature at the time of crop harvest; the erect seed heads will likely enter the combine harvester. Harvested weed seeds are expelled from the rear of the combine, resulting in their dispersal across the field to become additions to the soil seedbank, a process that increases the risk of herbicide resistance evolution. Seed production of annual weeds persisting through cropping phases replenishes/establishes viable seed banks from which these weeds will continue to interfere with crop production. Harvest weed seed control (HWSC) is an Australian innovation that is now viewed as an effective means of interrupting this process by targeting mature weed seed, preventing seed bank inputs by many major weed species. However, the efficacy of these systems is directly related to the proportion of total seed production that the targeted weed species retains (seed retention) at crop maturity. Weed species with a level of seed retention are good candidates for HWSC. HWSC methods include
    a) chaff Carts to remove weed seeds contained in the chaff fraction from the fields in bales;
    b) narrow windrow burning, which consists of making narrow chaff rows behind the combine then burning them to eliminate weed seeds, and
    c) seed destroyers, which are built-in machines that fit inside the combine and destroy weed seeds during crop harvest.

    Windrow burning is a highly effective harvest weed seed control tool as long as the chaff reaches a high temperature when burned. Credit: Claudio Rubione, University of Delaware

    Find more detailed information on our page: Harvest Weed Seed Control

    Weed seed heads are often seen poking through the top of the crop canopy and will enter through the combine at harvest, making the combine the #1 weed seed spreader. Credit: Claudio Rubione, University of Delaware.


Get Rid of Weeds: Act proactively

Note: Consult your local Extension Agent, Certified Crop Advisor (CCA), or other specialized professional for customized advice specific to your needs and goals.

 

Literature cited:

Lazaro, L., Norsworthy, J.K.,Walsh, M.J. and Bagavathiannan, M.V. Efficcy of the Integrated Harrington Seed Destructor on Weeds of Soybean and Rice Production Systems in the Southern United States
Doucet C, Weaver SE, Hamill AS, Zhang J (1999) Separating the effects of crop rotation from weed management on weed density and diversity. Weed Science 47:729-735.
Liebman M and Dyck E (1993) Crop Rotation and Intercropping Strategies for Weed Management. Ecological Applications. 3:92-122.
Mohler, Charles L. The Role of Crop Rotation in Weed Management. Sustainable Agriculture Research and Education.
Norsworthy, J.K., Korres, N.E., Walsh, M.J., and Powles, S. Integrating Herbicide Programs with Harvest Weed Seed Control and Other Fall Management Practices for the Control of Glyphosate-Resistant Palmer Amaranth (Amaranthus palmeri)

VanGessel, M.J., Wallace, J., Besancon, T., Flessner, M. and Chandran, R. Mid-Atlantic Field Crop Weed Management Guide, 2019.
Walsh, M.J., Aves, C., and Powles, S. Harvest Weed Seed Control Systems are Similar Effective on Rigid Ryegrass
Walsh, M.J., Powles, S.B. High Seed Retention at Maturity of Annual Weeds Infesting Crop Fields Highlights the Potential for Harvest Weed Seed Control

Victoria Ackroyd, Thierry Besançon, Jess Bunchek, Charlie Cahoon, Rakesh Chandran, William Curran, Michael Flessner, Annie Klodd, Dwight Lingenfelter, Steven Mirsky, Matthew Ryan, David Sandy, Mark VanGessel, Kurt Vollmer, Meredith Ward Integrated Weed Management Guide for Mid-Atlantic Grain Crops. (TO BE PUBLISHED)