By Sierra Becker, Sustainable Agriculture Assistant, Practical Farmers of Iowa
My role here at Practical Farmers of Iowa gives me the opportunity to learn from farmers about what is working and what isn’t. Lately, waterhemp control, or lack thereof, has been a hot topic of discussion around rural coffee shops.
Farmers all over the Midwest have been trying to tame the waterhemp beast for years, but are struggling to win the fight. Managing this weed is clearly not easy, but there are some simple practices you can implement to help manage it, starting with understanding the weeds in your field.
Cover crops have a multitude of benefits including impressive weed control. Better yet, weeds are not resistant to cover crops. According to Anita Dille, a Kansas State University agronomist who spoke at our Annual Conference, a layer of cover crops “will serve to smother and out-compete weeds for light, water and nutrients.”
We’ve seen the success of this in our own farmer community here at Practical Farmers. Dennis McLaughlin, a member from Madison County, is using a rye cover crop for weed suppression on his farm.
In a recent interview, I asked him if he could see a noticeable difference in his fields and he quickly replied with “Absolutely!” He went on to say, “It is fairly easy to see rye dominating the winter annuals, such as marestail (Erigeron canadensis) and pennycress (Thlaspi arvense), 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, by contrast, germinate in or near June and grow through the height of summer. Since the cover crop is almost always terminated by the time of summer annual germination, weed control relies solely on the biomass of a terminated cover crop.
As Dennis has found, the trick is to manage cover crops so they suppress weeds while they’re growing and after they have been terminated and start breaking down.
This starts with choosing the right cover crop. In order to make sure you are picking the best cover, you need to be sure it aligns with the goals for your land. In this case, our goal is weed control. We need a cover that will provide a lot of biomass in June to choke out waterhemp germination and early growth. The obvious and most popular cover, in this case, is cereal rye.
Rye is a low-risk choice that provides the most spring biomass of the cover crop options, and will live through the winter – so it pulls double duty, suppressing winter annuals and controlling summer annuals!
Planting, Termination for Weed Suppression
The next step is actually getting the cover crop in the ground. There are pros and cons to both aerial seeding and drilling, so it depends what’s available and which works best for you and your land.
If you drill the cover in you are able to get an even stand, whereas aerial planting can open spots in the field ripe for opportunistic weeds to plunder. However, aerial seeding can be done in standing soybeans and corn, much earlier than drilling, making it “first to the field” which leads to greater biomass production.
Dennis prefers aerial application. This works best for him because he is able to get the cover in earlier, providing more root growth and biomass on his field in the spring. Both options should be part of the weed control toolbox and can provide a good stand to last you through the spring and more importantly into the summer. In the spring, the cover crop will need to be terminated.
Biomass is the key in weed suppression, so terminating a crop in order to get the maximum amount biomass is important.
There are not a lot of leniencies when planting corn into cover crop residue, considering termination of the cover crop has to occur 10-14 days before planting unless overcoming any deleterious effects with a planter applied nitrogen program.
However, when planting soybeans, termination of the cover crop can occur before or shortly after the beans have been planted with no yield risk. This window allows for a longer growth period and can make a big difference when controlling weeds.
PFI member Jack Boyer can back this up with results found during his research trial on his farm. Jack waited until May 19, the day after seeding soybeans to terminate his rye, and as a result “had upward of 8,000 lbs of dry matter per acre in those strips.”
He later concluded, “the most interesting part of the trial was the improved control of waterhemp in the cover crop areas versus the no-cover areas.” This amount of biomass would be more than enough to suppress most types of weeds.
Andrea Basche, a soil scientist from University of Nebraska Lincoln and PFI member, recently completed a research study on the amount of biomass growth needed for weed control. She found that “between 3,300 – 3,600 lb/ac of rye biomass reduced both weed density and weed biomass by greater than 90%.
Cover crops were planted from mid-September to mid-October in these experiments and were terminated with herbicides in mid-April.” These results were conducted in a continuous corn system.
In another meta-analysis, Basche noticed, “Across the Corn Belt, experiments show that cover crops are generally more effective at reducing weed biomass (size of weeds) compared to weed density (the number of weeds). This suggests that cover crops may be more beneficial from the standpoint of controlling the size of weeds, and therefore the efficacy of a herbicide program.”
Combining cover crops and herbicides can be beneficial against aggressive weeds. When using herbicides, start with a pre-emergent, soil-residual herbicide. Soil residual 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 so that the herbicide will be effective against the targeted weeds at the soil level instead of getting caught up on the cover crop biomass.
If you’re planning on terminating your cover crop later, consider mixing both your pre-emergent and termination method together. Additionally, make sure you are using a full rate when applying to get the most coverage.
An article from the Soybean Checkoff program states that using an effective post-emergent herbicide is extremely important. So to avoid this issue, use best practices when selecting herbicides. This includes varying your mode of action from year to year to maximize the life of the herbicides on your farm before herbicide resistance is developed.
Another option during termination is to avoid herbicides altogether. A roller crimper effectively bends and lays the cover flat on the ground. Check out Practical Farmer research from Scott Shriver who evaluated soybean planting dates compared to date of roller-crimped cover.
Other Tools to Use
Using cover crops is a great, and arguably the best, way to protect your field against hardy weeds like waterhemp. There are other techniques that can be used alongside cover crops to provide the best weed suppression for your land.
Increasing the seeding rate and planting narrower rows has proven to diminish weeds. “My 38-inch rows don’t help matters,” Dennis says. Decreasing row-width reduces the amount of land and nutrients weeds have to grow, creating an effective weed management tool.
If done over time, these simple practices can make a big impact in the fight against waterhemp. They can also be used to suppress other hardy weeds such as palmer amaranth.
Palmer amaranth is a weed in the pigweed family, which has also become problematic for farmers. Although it is not as prevalent in the Midwest, it is never too early to implement effective weed control.
The best way to manage any weed is to understand what it is and where it’s coming from, but overall patience and active monitoring is key when it comes to changing your field.
“Changing weed pressure takes time,” Dennis says. “I still add a pre-emerge (Valor) to the burndown and then see what happens. I too am looking for something other than group 14 or group 15 to rely on.”
Although he hasn’t found anything yet, Dennis still works diligently toward his goal of using cover crops as a major part of his weed control strategy while simultaneously improving the soil on his field.
He understands that investing his time now, will ultimately make the difference long term. Waterhemp is not going to go away and its resistance to herbicides will only get stronger. Reducing a weed’s ability to survive by planting cover and coupling it with other techniques is one of the best ways to have a fighting chance against this tough weed.
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