Published Feb 14, 2019

Taming the Waterhemp Beast With Cover Crops

By Sierra Becker

In the world of herbicide-resistant weeds, waterhemp is surpassed by none. Once little more than a nuisance to farmers, this native broadleaf rapidly gained superweed status during the 1990s and 2000s. In just over 20 years, waterhemp has evolved resistance to six different herbicide groups – and in February, University of Illinois researchers reported the discovery of a small population of waterhemp in Illinois research plots with confirmed resistance to a seventh class, known as Group 15 herbicides. With the revelation, waterhemp gained the dubious distinction of becoming the first broadleaf weed in the world to show resistance to this chemical family.

While the degree of resistance varies from farm to farm, stacked resistance – where a weed is resistant to multiple herbicide groups – is increasingly common. Farmers all over the Midwest have been trying to confront the waterhemp challenge for years, but are struggling to gain the upper hand. Managing such a resilient weed is clearly not easy, but there are some simple practices you can implement to help better manage it, starting with truly understanding the weeds in your field.

What is Waterhemp?

Waterhemp is a species of amaranth that grows rapidly, produces many seeds and has proved to be highly adaptable to modern agricultural practices that rely on chemical weed control and reduced tillage. Its ability to rapidly evolve herbicide resistance means it has the potential to be highly detrimental to corn and soybean yields if not controlled. In soybeans, Purdue University Extension has found that 20 waterhemp plants per square foot can reduce yield by 44 percent. In corn, Purdue researchers have found that a heavy waterhemp infestation early in the season can reduce yields by 15 percent.

To minimize the economic impact of this potent weed, farmers have turned to a variety of expensive chemical cocktails. As waterhemp has developed resistance to new herbicide products, however, the chemical approach to control is becoming costlier, more complex and less effective. One tool to fight waterhemp that deserves a second look is cover crops.

Out-Compete Weeds With Cover Crops

Soybeans growing with rye residue used for weed control

Soybeans growing in a field of terminated rye.

Cover crops have a multitude of benefits, including impressive weed control. Better yet, weeds cannot develop resistance to cover crops. According to Anita Dille, a Kansas State University agronomist who spoke at our annual conference in January, a layer of cover crops “will serve to smother and out-compete weeds for light, water and nutrients.” Dennis McLaughlin, who farms near Cumming in Madison County, is using a rye cover crop for weed suppression on his farm and can attest to its effectiveness. “I can absolutely see a noticeable difference in my fields,” he says. “It is fairly easy to see rye dominating the winter annuals, such as marestail and pennycress, in the field. Summer annuals are always harder to fight, but cover crops do make a difference.”

Winter annual weeds germinate in the fall and winter and then take off early in the spring. Cover crops also germinate in the fall and winter and have a similar life cycle, which is why they are so effective at smothering these winter annuals. Summer annuals like waterhemp, by contrast, germinate around June and grow through the height of summer. By this time, most farmers have already terminated their cover crop – but they can still take advantage of the resulting biomass to control summer annuals.

As Dennis has found, the trick is to manage cover crops so they can suppress weeds both while they’re growing and after they have been terminated and start breaking down. Anytime you’re thinking of planting a cover crop, it’s important to know the goals for your land and what you want to accomplish with the cover crop. In the case of waterhemp, the goal is weed control. That means choosing a cover that can produce a lot of biomass in June to choke out waterhemp germination and early growth. Cereal rye meets these criteria, and is a low-risk and popular cover crop for Iowa. Because rye provides the most spring biomass of the cover crop options, it has the best shot at controlling summer annual weeds. As a bonus, it can live through the winter and pull double-duty helping to suppress winter annuals too.

Planting and termination to increase weed suppression

The next step is actually getting the cover crop planted. There are pros and cons to both aerial seeding and drilling, so the approach you choose depends on what’s available and which works best for you and your land. Drilling the cover in produces a more even stand, while aerial seeding can leave gaps in coverage that opportunistic weeds can exploit. On the flip side, however, farmers can aerial-seed into standing corn and soybeans, while they have to wait to drill until after harvest.

soybeans growing with rye residue controlling weeds

A thick layer of rye biomass controlling weeds at Jeremy Gustafson’s farm in Boone.

The ability to get rye planted sooner gives rye more of a growing window in the fall, and leads to more biomass production in the spring. Dennis prefers aerial application for this reason. “I’m able to get the cover in earlier, which provides more root growth and biomass in the spring,” he says. Both options have merits and can provide a good stand to last you through the spring – and more importantly, into the summer.

In the spring when it’s time to kill the cereal rye, it’s important to do so in a way that results in the most biomass. Remember, biomass is the key to suppressing summer annual weeds like waterhemp. With corn, you don’t have much leeway, since the general recommendation is to terminate the cover crop 10-14 days before planting. Soybeans are more forgiving, however: You can kill the cereal rye before or shortly after planting soybeans with no yield risk. This flexibility allows for a longer growth period and can make a big difference when controlling weeds.

Jack Boyer, who farms near Reinbeck in Grundy County, can vouch for the benefits of waiting to terminate his rye. For the past few years, he has conducted research on his farm looking at different cover crop termination dates ahead of soybeans. In 2015, Jack waited until the day after seeding soybeans (May 19 that year) to terminate his rye. He found no yield drop – and he got nearly 8,000 pounds of dry matter per acre in those research strips. While he was pleased with these results, the research yielded an unexpected discovery. “The most interesting part of the trial was the improved control of waterhemp in the cover crop areas versus the no-cover areas,” Jack says.

The quantity of biomass Jack gleaned that year is more than enough to suppress any type of weed, according to Andrea Basche, a soil scientist from University of Nebraska Lincoln. Andrea recently completed a study that looked at the amount of biomass growth needed for weed control in a continuous corn system. She found that just 3,300-3,600 pounds of rye biomass per acre reduced both weed density and weed biomass by more than 90 percent. The cover crops in her study were planted from mid-September to mid-October and were terminated with herbicides in mid-April. In 2017, Andrea conducted a meta-analysis looking at the impacts of continuous living cover on soil hydrology. In the published research, she observed: “Across the Corn Belt, experiments show that cover crops are generally more effective at reducing weed biomass compared to weed density. This suggests that cover crops may be more beneficial from the standpoint of controlling the size of weeds, and therefore the efficacy of an herbicide program.”

Combining cover crops and herbicides can be an effective strategy against aggressive weeds like waterhemp. When using herbicides, start with a pre-emergent, soil-residual herbicide. These herbicides are important because they remain active in the soil for an extended period of time and are able to act on weeds that may germinate after application. This is especially important when fighting summer annuals in a field with heavy cover crop biomass. With soil-residual herbicides, the chemistry remains effective against the targeted weeds at the soil level instead of getting caught up on the cover crop biomass. If you plan to terminate your cover crop later, consider mixing both your pre-emergent and the herbicide you’ve chosen to kill your cover crop, and applying at the same time. Make sure you are also using a full rate when applying to get the most coverage.

An effective post-emergent herbicide is also important to help manage waterhemp and other challenging weeds. To minimize the risk of encouraging herbicide-resistant weeds, use best practices when selecting herbicides. This includes varying your mode of action from year to year. When terminating your cover crop, you can also consider avoiding herbicides completely: A roller-crimper effectively bends and lays the cover crop flat on the ground. To learn more, read the 2017 PFI research report “Rolling Cover Crops and Soybean Row-Width.” Scott Shriver, of Jefferson, participated in this research, which explored soybean planting dates compared to date of a roller-crimped cover crop.

Other tools used

In the era of herbicide-resistant weeds, cover crops are arguably the best way to manage problematic weeds like waterhemp. But other techniques – like increasing the cash crop seeding rate and planting narrower rows – can be used along with cover crops to provide even better weed suppression. “My wide 38-inch rows don’t help matters,” Dennis says, referring to weed control. Decreasing row width reduces the amount of land and nutrients weeds have to grow, creating an effective weed management tool. Similarly, a higher cash crop seeding rate creates a thicker stand that leaves less room for weeds to take hold.

Over time, these simple practices can make a big impact in the fight against waterhemp. They can also be used to suppress other problematic weeds, such as Palmer amaranth, a more aggressive relative of waterhemp that was first confirmed in Iowa in 2013 and is now believed to be present in all 99 counties. Although still less prevalent in the Midwest than waterhemp, Palmer amaranth is bigger and more competitive than waterhemp, and has the potential to significantly affect crop yields.

The best way to manage any weed is to understand what it is and where it’s coming from. Patience and active weed monitoring are also key. “Changing weed pressure takes time,” Dennis says. “I still add a pre-emerge to the burndown and then see what happens. I, too, am looking for something other than Group 14 or Group 15 herbicides to rely on.” In the meantime, Dennis is working diligently toward his goal of using cover crops as a major part of his weed control strategy while improving his soil.

Since the challenge of managing herbicide-resistant weeds will only increase, reducing a weed’s ability to survive by planting cover crops and coupling that with other techniques is one of the best ways to confront this complex problem. This strategy can not only help break the cycle of resistance, it gives you more control over a reality that’s here to stay.