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Scientists expect that climate change will significantly reduce corn yields by the end of this century. Some estimates even suggest losses as high as 28% However, these calculations overlook a key factor that could further reduce corn yields: Weeds.
In the Corn Belt, hotter and drier summers are becoming more common. This puts stress on corn during key reproductive phases, such as silking and fill. However, these same conditions can be beneficial to scrappy weeds which thrive in harsh environments.
"Adverse weather conditions and weeds can both be stressors for crop production. However, very little research has been done to determine how they interact with each other. Computer models that project corn yields into the future assume weed-free conditions," Marty Williams, ecologist at USDA-Agricultural Research Service, associate professor at the Department of Crop Sciences at Illinois and coauthor of a new study published in Global Change Biology, says. Without major changes in how we manage weeds, it's unlikely that this will be the case."
In practice, complete weed control is rare. This is especially true when herbicides-the most commonly used tool to kill weeds-are losing ground to resistant ones. Many important weed species such as waterhemp, Palmer amaranth and others can withstand multiple herbicide modes. The prospects of chemical control for resistant weeds are dimming as no new herbicide classes are commercialized in corn.
Late-season control of weeds like waterhemp was more important than any other management practice or weather-related factor.
The research team, including U of I crop scientists Christopher Landau, Aaron Hager and Aaron Hager, analysed 27 years of herbicide evaluations across Illinois, representing more than 200 different weather conditions.
"When researchers in agriculture want to study weather variability and crop yield, it's usually one experiment in two or more environments. Williams states that six to eight environments might be required for large studies. "Our analysis allowed us to examine a historical data set that contained hundreds of environments. This is the true beauty of it."
The researchers used machine-learning algorithms to make sense of the complex data. The researchers looked at crop management factors, such as planting date, hybrid choice and planting density; percent of weed control for multiple species; weather data at key stages in the corn lifecycle; and yield.
An average loss of 50% was seen when late-season weeds weren't controlled. Even with a relatively strong late-season weed control (upto 93%), weeds can exacerbate crop losses in hot and dry conditions.
"We see crop losses that are much greater than those caused by poor weather and weed control. It is difficult to achieve 94% weed management late in the season. Williams states that it would be surprising if more fields could achieve 94% weed control on an ongoing basis.
Researchers discovered that corn is less competitive against weeds when it experiences excessive heat or drought in the middle of summer. However, this is not the only way that climate change can impact corn yield. Weather can have adverse effects on field work conditions and herbicide effectiveness. Pre-emergence herbicides won't work if there is a drought just before they are applied. Emerging corn may be affected by the weeds.
However, farmers who were forced to plant later because of wet spring conditions could have some luck. An analysis revealed that corn planted after April 29 had 18% lower yields.
Landau states that later planting had the advantage of better weed control. Early weeds have more time to grow and can be killed before planting. But that doesn't mean it's the best for the crop. You're more likely to get a window when the corn is too hot or dry for flowering, the later you plant it. Although late-planting can be beneficial for weed management, it could also expose the crop to higher heat and drought stress during reproduction.
This analysis shows that climate change is forcing us to abandon simplistic weed control methods. Williams states that weeds adapt to existing herbicides and that a new product will not be the answer. No matter how innovative the technology, Williams says that any tool would work better than any other.
"History has taught us that if we depend too heavily on a tool, it doesn't make sense to invent a new tool. We do need new tools. It doesn't matter if you want to use harvest weed seed control or genetic engineering methods, robotic weeders, and other advancements. Although there are many advancements, it is important to diversify the way weeds managed. Williams states that not only should registered herbicides be used, but other options are also available.
More information: Christopher A. Landau and colleagues, Diminishing Weed Control exacerbates maize yield losses to adverse weather, Global Change Biology (2021). Information from Global Change Biology Christopher A. Landau and colleagues, Diminishing Weed Control exacerbates maize loss to adverse weather (2021). DOI: 10.111/gcb.15857
