Category Archives: Sorghum

Time to start planning for horseweed/marestail

Charlie Cahoon and Michael Flessner, Extension Weed Specialists at Virginia Tech

Horseweed or Marestail

Horseweed is a winter or summer annual and a member of the asteraceae family.  In the winter annual cycle, horseweed germinates in the fall and overwinters as a basal rosette. The following spring, the rosette bolts, reaching 1.5 to 6 feet in height.  While most populations of horseweed in the region emerge in the fall, significant spring emergence can occur under certain conditions.  Spring germinating horseweed does not form a rosette.  Leaves are alternate, simple, linear to oblanceolate in shape, and lack petioles.  Leaf margins are either entire or toothed.  Flowers consist of numerous small heads arranged in a panicle with many white ray flowers and 20 to 40 yellow disk flowers.  Seed are small and have a pappus of tan to white bristles (resemble dandelion seed).  Seed are easily dispersed by the wind, allowing it to quickly spread to nearby fields and within fields.

Bolted horseweed. Virginia Tech Weed ID Guide.

Herbicide resistance:  Regionally, glyphosate- and ALS-resistant horseweed are widespread.  If you are unsure of the resistance status in your fields, assume resistance to glyphosate and ALS herbicides. Resistance to Gramoxone (PS I-inhibitor) has been reported in Delaware.

Management:  Horseweed is more prevalent in no-till fields compared to fields prepared conventionally.  Tillage can be useful in the long-term management of horseweed.  Chemical control of horseweed is more consistent when the weed is in the seedling or rosette stage compared to bolting plants.  Traditionally, glyphosate and ALS-inhibiting herbicides effectively controlled horseweed, however, biotypes resistant to these herbicides are wide-spread.  Chemical control of glyphosate- and ALS-resistant horseweed requires a postemergence herbicide to control emerged horseweed and depending on application timing a residual herbicide for horseweed yet to emerge.  Due to the prolonged germination period, horseweed seedlings can emerge 5 to 6 weeks after residual herbicide application, thus fall applications of residual herbicides often have limited effectiveness for spring emergence.

Herbicides used for emerged plants include 2,4-D (1 pt in the fall or 1 qt in the spring), dicamba, Liberty, Sharpen, or Gramoxone plus a triazine herbicide (atrazine or metribuzin).   Preliminary research from Virginia and North Carolina suggest Elevore (a new auxin herbicide from Dow) also controls horseweed well.  Fourteen days are required between Elevore application and corn or soybean planting and 30 days prior to cotton planting.  Herbicides providing residual control include Valor SX, triazine herbicides (atrazine, simazine, and metribuzin), and ALS-inhibiting herbicides (if horseweed is not ALS-resistant).

Horseweed seedlings do not tolerate shade.  Thus a well-established cover crop, or dense crop canopy can be very effective to manage horseweed infested fields.

Horseweed rosette. Virginia Tech Weed ID Guide.

Soybeans: The key to managing horseweed in in soybeans is to control it prior to planting. Horseweed needs to be controlled prior to bolting (grow upright); this may require an application weeks prior to planting.  Glyphosate plus 2,4-D (1 qt/A), glyphosate plus dicamba or glyphosate plus Sharpen will effectively control glyphosate- and ALS-resistant horseweed when they are small.  Pay attention to rotational restrictions of preplant burndown herbicides when planning burndown applications and planting.  If horseweed are susceptible to the ALS-inhibiting herbicides, Classic-containing herbicides (i.e. Canopy, Valor XLT, Envive, Surveil) are effective.  If applications are made early pre-plant, herbicides containing Valor SX or metribuzin or the Authority products can be included to provide residual control, or a second application of a non-selective herbicide may be needed at planting. Liberty or Gramoxone also control small horseweed and can be used as a part of the burndown application.  If horseweed is present at planting, Gramoxone plus a residual product (Valor SX, Authority products, metribuzin, or Sharpen) is suggested in regions with later emerging horseweed.    Foliar applied PPO-inhibiting herbicides (Blazer, Reflex or Flextar, Resource, and Cobra) DO NOT control emerged horseweed.  For Liberty Link varieties, Liberty applied postemergence controls small horseweed.   Classic and FirstRate are postemergence options where horseweed are susceptible to the ALS-inhibiting herbicides and are small.

Cotton:  Like soybean, horseweed needs to be controlled prior to cotton planting.  Burndown combinations of glyphosate plus 2,4-D/dicamba plus Valor SX is normally in order.  Again, pay attention to plant-back restrictions when timing burndown applications.  Glyphosate plus Sharpen is also effective, but requires a long waiting period between application and cotton planting.

Corn:  In no-till corn, Gramoxone plus triazine, glyphosate plus atrazine plus 2,4-D or dicamba, and Liberty plus atrazine applied burndown of emerged seedlings and residual control of glyphosate- and ALS-resistant horseweed.   Consult labels for waiting interval prior to planting corn.  Atrazine alone will provide good residual control of horseweed.  Dicamba and 2,4-D are the most effective postemergence herbicides.  For small emerged horseweed, foliar applied HPPD-inhibitors or Liberty plus atrazine are effective.

Sorghum:  Glyphosate plus Sharpen or Gramoxone plus a triazine applied burndown.  Atrazine alone or in combination with Gramoxone applied preemergence for residual control and control of small emerged horseweed.  Atrazine can be used postemergence for control of small horseweed.  However, similar to corn, 2,4-D and dicamba are the most effective postemergence options.

Small grains:  2,4-D, dicamba, Quelex, or Huskie will provide effective postemergence control of horseweed in small grains.  Harmony Extra is also very effective on small horseweed that is susceptible to ALS-inhibiting herbicides.

Fall fallow application:  In fields with a history of horseweed, fall applied herbicides can be helpful in managing the weed.  However, a fall herbicide application will not substitute for a spring burndown application.  Target applications for emerged horseweed plants in the late fall after one to two killing frosts.  2,4-D or dicamba should serve as the base for these applications.  Glyphosate is often suggested in combination with 2,4-D and dicamba to control other winter annual weeds.

To see similar information on other problematic weeds consult the Virginia Pest Management Guide: Field Crops at or the Mid-Atlantic Field Crop Weed Management Guide at

As always, don’t hesitate to contact Michael or myself with any questions or concerns.


Dicamba Training Announcement

The federal labels for XtendiMax® herbicide with VaporGrip® Technology (Monsanto), Dow DuPont ® FeXapan® herbicide Plus VaporGrip® Technology, and Engenia® Herbicide (BASF) now require additional training beyond a Pesticide Applicator’s License prior to use of these products “over the top” of dicamba-tolerant soybean or cotton.  Training for 2018 will be provided by the registrants of the products (BASF, Monsanto, and Dow DuPont).

Agents/dealers interested in scheduling a training in their area or having a company representative deliver the training at an already scheduled meeting should contact the following company representatives:







Eastern Shore

Gar Thomas



Rest of Virginia

Kelly Liberator




Jeff Phillips



Southeast Virginia

Ken Lampkin


If you schedule a training with either BASF, Dow DuPont, or Monsanto, I would encourage you to make the other companies aware of the training planned in your area.  That way, the companies can better coordinate their efforts to reach as many applicators as possible.  Also, training by any of the three registrants will cover all dicamba products labeled for in-crop use to dicamba-tolerant soybean or cotton (applicators do not need to take training from the registrant of the specific dicamba product they intend to use).

In lieu of the face-to-face trainings, the companies also plan to have a web-based training that will satisfy applicator training requirements.  Michael and I feel the face-to-face training will better prepare the applicators for the off-target challenges of dicamba.  Web-based training can be used as a last resort if a grower is unable to attend face-to-face training.  The following websites offer more information on web-based training:

There are a few trainings scheduled for the area.  See the  link below for an announcement from Monsanto for two training sessions in Suffolk, VA on Wednesday January 31st.  BASF will be training applicators at the Virginia Grains & Soybean Conference (  The BASF training for this meeting is schedule for Wednesday February 21st at 12:00pm.  I anticipate both companies to have other training;  Michael and I will keep you updated as we receive word.  Please help spread the word on these trainings, as many growers still do not know that training is required.  Also,I would encourage you and your applicators to pre-register for the events so folks can plan accordingly.

With that said, feel free to reach out to Michael or me if you have any questions or concerns.

Monsanto Dicamba Training Announcement

Eastern Shore AREC Field Day CANCELED!

Due to impending rain Tuesday and Wednesday and already saturated soils, the Eastern Shore AREC field day scheduled for Wednesday, September 13, 2017 has been canceled. Let’s hope Hurricane Irma keeps tracking further west. We certainly do not need any more rain!

Eastern Shore AREC Field DAY: September 13th, 2017

Please join us for Virginia Tech’s Eastern Shore Agricultural Research and Extension Center (ESAREC) 2017 Research Field Day on Wednesday, September 13th. Registration is free, open to the public and will begin at 8:00 AM at the ESAREC complex located at 33446 Research Drive, Painter, Virginia 23420. The field tour will begin at 9:00 AM and conclude with lunch at 12:30 PM.  See the attached flyer for specific projects to be highlighted and more information.

If you would like more information or are interested in sponsoring this event, please contact Lauren Seltzer at 757-414-0724 ext. 11 or email at

2017 ESAREC Field Day Announcement

Section 18 authorized for Transform WG for control of sugarcane aphid on sorghum in Virginia

In accordance with Section 18 of the Federal Insecticide, Fungicide, and Rodenticide Act, the EPA has authorized the Emergency Exemption use of Transform WG (active ingredient = sulfoxaflor) to be used on sorghum to control sugarcane aphid (Melanaphis sacchari) in specified Virginia counties. The authorization will expire November 30, 2017. Foliar applications may be made at 0.75-1.5 oz/acre, with a maximum of 2 applications per acre per year. Please refer to the product’s Section 18 registration for further information, including application directions (at the time of this writing, the 2016 version of the Section 18 is the one available on [the 2016 version expires on April 8]). Micah Raub and others with the Virginia Department of Agriculture and Consumer Services are acknowledged for their assistance.

Scout now for marestail/horseweed

Recent mild temperatures and the mild winter are setting the stage for rapid development of marestail/horseweed (Conyza canadensis) this spring.  Marestail was particularly troublesome last year in soybeans.  Marestail can germinate in both the fall and the spring. It is more likely to overwinter in the rosette stage during mild winters.  If you wait until your typical burndown the marestail may start bolting and therefore be more difficult to control. Adding to this difficulty, many marestail populations are resistant to Roundup (and other glyphosate containing products). You should scout your fields targeted for soybeans now to identify overwintering marestail.  Marestail control can be achieved with 2,4-D  or dicamba now and still offer plenty of time to avoid plant back restrictions (up to 15 days for 2,4-D or up to 28 days for dicamba). Glyphosate resistant weeds and the difficulty in controlling more mature weeds underscore the need to scout fields earlier and use some alternative herbicides in your program.  Always consult the product label for specific instructions.

View the Program: Virginia Eastern Shore Ag Conference and Trade Show

We look forward to seeing you January 25th and 26th at the 27th Annual Eastern Shore Ag conference & Trade Show! You can find the program online at: Virginia pesticide re-certification and Certified Crop Adviser credits will be available. See the program for more information.

The event will be held at the Eastern Shore Community College Workforce Development Center, 29300 Lankford Highway, Melfa, VA 23410. When you enter the driveway to the Community College, we will be meeting in the building to the left.

The Annual Oyster Roast will be held on Wednesday night, January 25th beginning with a social at 6:00 pm and oysters served at 6:30 pm. Along with oysters, there will be all-you-can-eat barbecue, sides and beverages. Tickets will be $35.00 in advance and $40.00 if purchased the day of the oyster roast.

If you have any questions or concerns please contact either Theresa Pittman ( or Ursula Deitch ( for accommodation. Thank you!

Virginia Cooperative Extension logo

Virginia Eastern Shore Ag Conference and Trade Show

Join us in Melfa, VA for the 27th Annual Eastern Shore Agricultural Conference and Trade Show on January 25-26, 2017. This event is free, open to the public, and will be held at the Eastern Shore Community College Workforce Development Center. We will offer Virginia Pesticide Recertification credits for categories 1A, 10, 60, and 90. We will also offer Certified Crop Adviser Credits for nutrient management (2), soil and water (1), integrated pest management (4.5), crop management (6), and professional development (0.5). Click on the following link for topic areas being presented: ag-conf-press-release-2017

Registration is open for the Mid-Atlantic Crop Management School.


November 15-17, 2016
Princess Royale Hotel in Ocean City, MD

Registration is open for the 22nd annual Mid-Atlantic Crop Management School. This year’s school will feature 2 ½ days of timely presentations in the areas of crop management, nutrient management, pest management, soil and water management, and climate. This year, the school will also feature the popular Crop School on Wheels field tour (limited to 50 participants). Nutrient management (VA, MD, DE, PA), pesticide, and certified crop adviser (CCA) credits will be available. Register early for the best selection of sessions.

The session schedule is online at:

Registration information is posted at:

Contact Amy Shober ( or Jarrod Miller ( with questions about the school. We look forward to seeing you there.

The Mid-Atlantic Crop Management School is organized by Extension Specialists from Virginia, West Virginia, Delaware, and Maryland featuring speakers from across the nation.

Sugarcane aphid sorghum update

Brent Bean, Director of Agronomy, United Sorghum Checkoff Program, forwarded the following information on sugarcane aphid (SCA):

As many of you know, the sugarcane aphid has moved north faster than last year.  We now have confirmed reports of SCA in three southern counties of Kansas.  In addition, SCA is present as far north as Kentucky and Virginia.  In these northern sites SCA populations for the most part are low but have approached threshold levels in Virginia.  It is likely that at least low populations will be discovered in many counties within the Sorghum Belt in the coming weeks.

It is important to note that just because a few SCA are present in a field does not mean an insecticide application is or will be justified.  Those who planted hybrids with some tolerance to SCA can expect the populations to build more slowly.  In addition, beneficials in many areas are reported to be in high numbers, this will also help keep SCA populations in check.  Although SCA was present in South Texas this year, only a portion of the fields reached threshold levels.

Something we have clearly learned the last two years is that once SCA is present in an area, scouting of fields becomes critical.  Scouting should occur at least once a week once SCA has been discovered in a region, and in fields where SCA has already been detected, scouting should take place at least twice a week.  Insecticide application should begin as soon as the SCA population is at threshold levels.  Research has shown yields can be drastically reduced if insecticide application is delayed for several days once threshold levels are reached.  States and regions vary slightly in their recommended threshold levels, but in general, an insecticide application is justified when 50 aphids per leaf are present on 25 percent of the plants. Consult your state or regional extension service for specific threshold information.  We are entering a critical time in the Sorghum Belt, and SCA can potentially reach threshold levels very quickly.  Diligence in scouting is an absolute must!

There are two products that should be considered if an insecticide application is warranted.  These are Sivanto prime from Bayer, and Transform WG from Dow AgroSciences.  Both are effective, but good coverage is critical.  Most entomologists recommend 4 – 5 ounces of Sivanto Prime and 1 – 1.5 ounces of Transform WG.  The lower rates are usually recommended to control SCA at or close to threshold levels.  If SCA populations are way above threshold levels, then the higher rates may be justified.

If other insects such as midge or headworms are present with SCA, avoid using pyrethroids for their control.  Pyrethroids can lower beneficial populations significantly and cause a rapid increase in SCA populations.  In the South Plains region of Texas the ‘yellow’ sugarcane aphid is showing up in a few fields, often in the same fields with sugarcane aphid.  Care should be taken in distinguishing between these two aphids.

Although sweet sorghum is a minor U.S. crop, sugarcane aphids can greatly impact yield.  Sivanto prime has received a section 18 Crisis Exemption label in Mississippi, Arkansas, Tennessee and North Carolina for use on sweet sorghum in 2016.

NOTE from Ames Herbert, VT: Note that the ‘yellow’ sugar cane aphid mentioned above is a different species than SCA. Yellow sugarcane aphids are covered with small hairs and the two cornicles (pair of small upright backward-pointing tubes found on the top side of the last segment of the bodies) are small and pale (see attached image).  The SCA is also called ‘white’ sugarcane aphid. It does not have hairs on the body and the two cornicles are dark (see attached image).  Yellow sugarcane aphid is a known pest in Virginia of forage grasses like orchardgrass. Like Texas, we are also finding yellow sugar cane aphid mixed with SCA, and in a few fields also mixed with corn leaf aphid, another known pest of corn and sorghum. Corn leaf aphid is easy to distinguish from the other sorghum aphid species as they have a dark head and legs, a dark green body color, and dark cornicles (see attached image).

This sorghum aphid problem is new for us so we are having to make some ‘educated guesses’ about what constitutes a threat, and what the best control options might be. For now, we suggest that SCA numbers should guide control decisions, that is, abide by SCA thresholds—and stick with the products that are recommended for SCA. Avoid the use of pyrethroids since they will not be effective against SCA, and could flare secondary pests by disrupting natural enemy populations.  Natural enemies including tiny aphid parasitic wasps and a variety of lady beetles can be very effective in helping reduce aphid and other pest populations, so we need to conserve them whenever possible.

white sugarcane aphid yellow sugarcane aphid grantham osu corn leaf aphid