Weed management continues at harvest

 Dave Nicolai, Extension educator - crops, Tom Peters, Extension sugarbeet agronomist, Liz Stahl, Extension educator - crops, and Debalin Sarangi, Extension weed scientist

Image 1. Grass and broadleaf weeds in wheat in Wilkin County.  

Small grains harvest is in full swing in northwestern Minnesota and corn and soybean harvest is right around the corner across the state. What are some tactics you can employ around harvest time to help prevent weeds from replenishing the weed seedbank and giving you more headaches for years to come?

First do some detective work

If weed control was less than optimal in your field, do some detective work to figure out why this might have happened. 

• Was there an issue with the application? 
• Did you use the right herbicides, rates and proper adjuvants to control the weeds present? 
• If you used a tank-mix, were there any antagonism issues between tank-mix partners? 
• How big were the weeds at the time of application (remember, the goal is to target weeds less than 4 inches in height when making postemergence applications) and were weeds actively growing? 
• Did you use the right carrier (mostly water) volume, nozzles and spray pressure? 
• Was your sprayer properly calibrated, or did you have any clogged nozzles? 
• Do you suspect the presence of any herbicide-resistant weeds? 

Poor weed control can be a symptom of many different issues, so proper diagnosis of the problem(s) can help prevent issues in the future.

Managing weeds after small grains harvest

Image 2. Waterhemp regrowth in wheat stubble. 

Many weeds coexisted with small grains in 2022 due to later planting dates, and unfortunately small grains were not able to out-compete the broadleaf weeds that emerged the same time as the crop (Image 1). 

Although small grain fields can look pretty clean after harvest, weeds clipped off by the combine may still branch out and eventually produce seeds. There may also be small weeds hiding in the stubble, particularly later-emerging weeds like waterhemp (Image 2). Post-harvest weed control strategies can help prevent these weeds from replenishing the seedbank.

In harvested fields, one can either conduct tillage or use a herbicide to control weeds. If you intend to control weeds with herbicides, target applications when weeds are less than 4-inches tall. Herbicide options could include glyphosate, 2,4-D, dicamba, paraquat (Gramoxone), or saflufenacil (Sharpen).

Remember that glyphosate alone will not control glyphosate-resistant waterhemp or kochia. Field research has shown that glyphosate mixed with Sharpen, Valor, and/or 2,4-D can control glyphosate-resistant waterhemp. However, be aware of the crop rotation restriction (months including unfrozen soil) between application and sensitive crops, including dry bean, potato and sugarbeet. 

Gramoxone can be used as an alternative to glyphosate. Apply Gramoxone SL with nonionic surfactant in at least 15 gallons/acre water carrier when broadleaf weeds are less than 4-6 inches tall. 

Managing weeds around soybean harvest time

Most weeds in soybean fields are at minimum in the flowering and seed development stages of their life cycle, which means there is still time to control some weeds before they produce viable seeds. Viable seeds may have already been produced by early maturing broadleaf weeds like common lambsquarters, kochia, redroot pigweed and waterhemp. Later-maturing weeds like giant and common ragweed, however, are still pollinating in many areas of Minnesota.

Waterhemp and other pigweed species can produce viable seed within 10 days of pollination, according to University of Illinois research. So go rouge if you find any pigweeds now on your farm. It's important to take the rouged pigweed plants away from the field rather than simply dropping them on the soil surface when hand-pulling weeds. 

Your work will be worthwhile as waterhemp is a prolific seed producer, generating up to 250,000 seeds per plant. 

If you have access to the right equipment, electricity is another way to reduce seed production. A recent publication from the University of Missouri reported the WeedZapper reduced the viability of common and giant ragweed and waterhemp by greater than 67%.

Manage fencerows and field edges now

Field edges are often where weed infestations start (image 3). Now is the time to mow on fence lines or field edges to prevent or minimize seed production. Left untouched, these weeds can be picked up by the combine, resulting in the spreading of weed seed well into the field. Tillage can then spread weed seeds even further across the field. Giant ragweed and waterhemp in particular can invade corn and soybean fields from fencerows and field edges. Also, discuss ideas for controlling weeds along field edges that you share with your neighbors (image 4).

Image 3. Giant ragweed along the field edge in McLeod County.

Image 4. Weeds along edges of neighboring fields, Norman County.

Scout for escaped weeds and maintain records

Make maps of weeds along roadsides, in shelterbelts and waterways, and in areas of fields that are subject to flooding or where wildlife may congregate or travel. Maps of troublesome spots can be helpful when planning weed management strategies for next year.

Combines are an effective dispersal mechanism, so check field entrances for new weeds and be on the lookout for noxious weeds like  Palmer amaranth. Segregate weedy areas of fields and harvest these spots last to help prevent spreading weed seed into cleaner areas. Now is a good time to detect new weed infestations before seeds are moved further into fields with equipment.

Consider harvest weed seed control (HWSC) as an option for the future.

The HWSC can be included in the weed management toolbox without increasing the labor requirements for harvest-time weed management. In this method, the weed seeds are collected at harvest and they are either destroyed or deposited in a known location where they are controlled in subsequent season. A number of HWSC systems are developed over the years, and narrow windrow burning, chaff lining, and integrated impact mills are the popular methods and can be adopted in our cropping systems in MN. More information about the HWSC can be found here: https://growiwm.org/how-harvest-weed-seed-control/

Summary 

Planting delays gave an incredible advantage for weeds in some fields in 2022, enabling weeds to germinate and emerge at the same time as crops in 2022. Weed escapes can be found in fields across the state and are also apparent along field edges, fencelines, and borders between neighbors. It is important that farmers do what they can to limit weed seed production and movement to help prevent weed infestations in the future.

References

Hill EC, Renner KA, VanGessel MJ, Bellinder RR, Scott BA (2016) Late-Season Weed Management to Stop Viable Weed Seed Production. Weed Sci 64:112–118

Bell MS, Tranel PJ (2010) Time Requirement from Pollination to Seed Maturity in Waterhemp. Weed Sci 58:167–173

Peters TJ and Lystad AL (2021) Waterhemp control in small grains stubble. Sugarbeet Res and Ext Rep 51:38–39