A-B-Cs of Integrated Weed Management

Farmers successfully use integrated weed management programs across a wide range of situations. It is hard to create a generic “prescription approach” one-size-fits all IWM program because the tactics used are highly dependent on the specific weeds, crops, and field conditions. Understanding the underlying principles of IWM is important to develop strategies for your specific situation.

There is both a science and an art to integrated weed management. The science is outlined in the principles below, which we call the A-B-Cs of weed management. The art is in applying these principles to your specific fields and weeds to create a weed control program that works for you. The principles below are familiar to any crop consultant or Extension agent; use the resources on this page as a starting point to Getting Rid of Weeds in your fields.

A. Know the enemy

According to a recent study, more than seventy percent of growers are implementing changes to their weed management programs to combat weed herbicide resistance. Due to 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 weed pages.

Weed identification and understanding of which weeds are dominating a field is important. Picture: Barb Scott, University of Delaware

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.

Weed identification and understanding of which weeds are dominating a field is important. Picture: Barb Scott, University of Delaware

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. Picture: Claudio Rubione, University of Delaware.
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 diversify weed management strategies

Starting clean means killing all vegetation prior to planting and represents an important first step for weed management. Most growers take this step for granted until a control failure happens. Weeds present at planting can bind up planting equipment and prevent good seed-to-soil contact, thus reducing crop stand. Additionally, existing weeds will compete with crop seedlings. Furthermore, weeds that are present at planting generally have the competitive advantage over the crop because they are already established.

Horseweed plants recovering following a pre-plant herbicide application. Photo credit: Claudio Rubione, University of Delaware

Prior to cash crop planting, growers have more options to control weeds with tillage or herbicides than once the crop has emerged. Once this opportunity is missed, options for weed control will be limited and implementation more difficult. To avoid this, make sure to follow these steps:

  1. Scout early to determine what weeds are present and what growth stage they are in. This information will dictate which termination method will be most effective and when.
  2. Terminate when weeds are small and susceptible to the termination method.
  3. Allow adequate time for weeds to die before planting. If not completely killed, weeds may begin to recover as early as one week after treatment with paraquat or as long as four weeks after treatment with glyphosate or 2,4-D.
  4. Scout before planting to ensure the treatment was effective and new weeds have not emerged. If further weed control is necessary, do so prior to planting.
Vegetation left on soil surface following disking. Will these plants survive? Photo Credit: Michael Flessner, Virginia Tech

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.

Keeping fields clean prior to planting helps avoiding relying in post emergent herbicides and increasing crop yield. Picture credit: Claudio Rubione, University of Delaware.
  • Tillage destroys aboveground plant tissue, often cutting the plants into small pieces and burying them; depending on the tillage equipment it may also bury weed seeds. However, tillage might not completely control large weeds and can also stimulate weed seed germination. Repeated tillage or plowing and field cultivation over a couple of weeks may help to reduce the number of weeds that emerge. Use the correct tillage implement for the weed species and weed sizes that are present.
  • Herbicides need to be selected carefully based on the weeds present and their size. Hard-to-control weeds or presence of cover crop species should be a major factor in this decision. Keep in mind that there might be plant-back restrictions for some herbicides depending on the cash crop.

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

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

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.

1. 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.

2. 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

3. 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.

4. 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.

5. 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.

6. 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.

7. Read, understand, and follow the label. The label is the law.

1. 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. Picture: Claudio Rubione, University of Delaware

2. 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.

3. 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.

4. 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.

5. 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 phosphorus 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).

6. 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 early canopy closure. Leaf architecture also affects timing of canopy closure because horizontal leaves intercept more sunlight than vertical leaves.

For more information go to our page: Row Spacing.

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

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 characteristics. 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. Picture: 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 efforts 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.

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. Picture: Claudio Rubione, University of Delaware.

If you plan to hire harvest contractors, be sure they thoroughly clean their machinery before coming into 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.

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:
1. Chaff Carts to remove weed seeds contained in the chaff fraction from the fields in bales;
2. Narrow windrow burning, which consists of making narrow chaff rows behind the combine then burning them to eliminate weed seeds, and
3. Impact mills, which are built-in machines that fit inside the combine and destroy weed seeds during crop harvest.
4. Chaff lining and chaff tramliningtakes a chute and diverts only the chaff fraction into a narrow row in the center of the harvester, while the rest of the crop residue is spread evenly behind the combine. While weed seeds are returned to the soil, they are in narrow lines instead of being spread across the entire field. The chaff material is allowed to rot and decay. These lines could be treated differently, using targeted herbicides sprays, or managed with different tools at a site specific level.
5. Bale-direct systemconsists of a large square baler that is attached directly to the harvester which constructs bales from the chaff and straw residue.

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

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. Picture: Claudio Rubione, University of Delaware.

Authors

  • Michael Flessner
  • Kara Pittman
  • Claudio Rubione
  • Mark VanGessel

Contributors

  • Victoria Ackroyd
  • Lovreet Shergill

Resources

  • A Practical Guide for Integrated Weed Management in Mid-Atlantic Grain Crops is a new guide, written by scientists from 5 universities and the USDA, explaining different weed management tactics and includes a scenario of how to implement an IWM program through the lens of grain production in the Mid-Atlantic region. Note: Consult your local Extension Agent, Certified Crop Advisor (CCA), or other specialized professional for customized advice specific to your needs and goals.

Citations

  • Doucet C, Weaver SE, Hamill AS, Zhang J (1999) Separating the effects of crop rotation from weed management on weed density and diversity. Weed Sci 47:729-735.
  • Lazaro L, Norsworthy JK, Walsh MJ, Bagavathiannan MV (2017) Efficacy of the Integrated Harrington Seed Destructor on weeds of soybean and rice production systems in the Southern United States. Crop Sci 57:2812-2818.
  • Liebman M, Dyck E (1993) Crop rotation and intercropping strategies for weed management. Ecol Appl 3:92-122.
  • Mohler CL. The role of crop rotation in weed management. Sustainable Agriculture Research and Education. https://www.sare.org/Learning-Center/Books/Crop-Rotation-on-Organic-Farms/Text-Version/Physical-and-Biological-Processes-In-Crop-Production/The-Role-of-Crop-Rotation-in-Weed-Management.
  • Accessed: October 29, 2019. Norsworthy JK, Korres NE, Walsh MJ, and Powles S (2016) Integrating herbicide programs with harvest weed seed control and other fall management practices for the control of glyphosate-resistant Palmer amaranth ( Amaranthus palmeri ). Weed Sci 64:540-550.
  • Wallace J, Lingenfelter D, VanGessel M, Johnson Q, Vollmer K, Besancon T, Flessner M, Chandran R (2019) 2019 Mid-Atlantic Field Crop Weed Management Guide. University Park, PA: Penn State Ag Communications and Marketing.
  • Walsh MJ, Aves C, Powles S (2017) Harvest weed seed control systems are similarly effective on rigid ryegrass. Weed Technol 31:178-183.
  • Walsh MJ, Powles SB (2014) High seed retention at maturity of annual weeds infesting crop fields highlights the potential for harvest weed seed control. Weed Techol 28:486-493.
  • VanGessel M, ed (2019) A Practical Guide for Integrated Weed Management in Mid-Atlantic Grain Crops. https://growiwm.org/wp-content/uploads/2019/10/IWMguide.pdf?x71059 Accessed: October 29, 2019

This material was written by: Michael Flessner, Kara Pittman, Claudio Rubione, Mark VanGessel, Victoria Ackroyd and Lovreet Shergill.


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.