Two weeds, in particular, are known to wreak havoc in corn and soybean fields across Iowa. Waterhemp and marestail have become increasingly troublesome in recent years. 

Waterhemp’s longer growing season and herbicide-resistant populations make it difficult to control, says Meaghan Anderson, Iowa State University Extension field agronomist. She notes that this weed is even difficult to identify correctly.  

“Waterhemp has significant variability in color, leaf shape and plant appearance based on genetics and the environment,” Anderson says. “Waterhemp will usually have long, skinny leaves that widen slightly toward the center, and the leaf margins are smooth. The leaves will be hairless. Marestail doesn’t have a lot of variability in leaf shape, plant appearance or color.” 

By the time waterhemp is identified in a field, it is likely the planted crop is already growing. Waterhemp has a long emergence period and is most noticeable late in the growing season as it appears above the soybean canopy. Farmers use management tactics to try to control it, but as it can emerge later in the growing season, waterhemp continues to develop. 

Prolific seed producer 

The weed itself is not necessarily threatening to crops like corn and soybeans, but it’s the large number of weeds this plant produces in a field that is the issue. “When waterhemp survives at the end of a growing season, female plants produce a significant amount of seed,” Anderson says. “While the individual plants are not hugely competitive with our crops, they can overwhelm crops with large numbers.” 

Corn and soybean farmers regularly use herbicides to control weeds. However, waterhemp has developed resistance to an increasing number of herbicides. Anderson says populations are known to be resistant to HG 2 (ALS), HG 5 (PS II), HG 9 (EPSP Synthase), HG 14 (PPO) and HG 27 (HPPD) herbicide products.  

If a waterhemp population demonstrates resistance to a particular herbicide in your field, using a different product might not solve the problem. Waterhemp can be resistant to several types of herbicides at once, so it’s important for farmers to choose effective products and apply at the right time. 

“In the short term, including a residual herbicide in the tankmix with postemergence herbicide applications, along with careful scouting of fields to determine timeliness for the post-application will help,” Anderson says. “In the long term, farmers will need to adapt other tactics to complement herbicides, such as planting soybeans in narrow rows, mechanical weed control, and using cover crops and crop rotation.” 

Cover crops alone might not have the power to control waterhemp. When grown in a corn-bean rotation, cover crops create a mat on the soil surface and inhibit weed growth. But this tactic is less effective for waterhemp since this weed emerges later in the growing season. “Cover crops need to have long-lasting and significant biomass to effectively help control weeds,” Anderson says. “Even with all the steps in place, asking a cover crop to provide significant waterhemp suppression is a lot to ask.” 

Marestail a challenge 

While farmers have to manage waterhemp throughout the growing season, there is another weed waiting to germinate in the fall. Marestail, also known as horseweed, develops early in the growing season and grows rapidly. It can also germinate later in the growing season.  

Marestail will germinate and appear in fields before planting season, most often acting as a winter annual weed. By the time farmers apply burndown herbicides prior to planting their crops, marestail can be too tall to kill. Like waterhemp, marestail is resistant to many herbicides and can stack resistance to multiple herbicides at the same time. For marestail, glyphosate resistance is a big issue. In Iowa, there are fewer issues related to herbicide resistance with marestail than with waterhemp, but resistant marestail populations do exist here.  

No matter what the worst weeds are in your fields, you should weigh the pros and cons of all weed control tools, Anderson says. For marestail, as with waterhemp, strategies to consider are rotating crops, choosing herbicides with effective modes of action, and using tankmix combinations and tillage practices from year to year. 

How can you find out whether or not weeds that survive your weed control program are resistant to the herbicide? Farmers can take samples of weeds and have them tested to know which products will be most effective in controlling waterhemp and marestail in their fields. Anderson says ISU’s weed science lab at Ames can help with herbicide screening of weed samples, but the process currently takes significant time.

For faster results, the University of Illinois has a lab that can do some of this testing in a shorter amount of time. For information, call the Illinois Plant Clinic at 217-333-0519. There is a charge for this service. 

Friedrichsen is a Wallaces Farmer intern. 

Online resources offer weed, herbicide information  

Identify your problem weeds before choosing a management strategy for next season. The 2020 Herbicide Guide for Iowa Corn and Soybean Production, WC 94, can offer guidance.

Also, Iowa State University Extension offers the Weed Identification Field Guide, 2nd Edition. This revised and expanded pocket-sized guide for farmers and agronomists to use for weed identification in Iowa corn and soybean fields includes four new weed species, many new images and updated text.

The booklet has tools to aid in accurate weed identification, as well as weed lifecycle and herbicide management and stewardship information. It has 35 illustrations and more than 250 photographs of weeds found in Iowa.