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Adverse weather-weed combination slams yields

Adverse weather-weed combination slams yields

Marty Williams and Aaron Hager

Marty Williams, left, is an ecologist with the U.S. Department of Agriculture’s Agricultural Research Service and an affiliate professor in the department of crop sciences at the University of Illinois. He works on an analysis of corn herbicides with Aaron Hager, an associate professor and University of Illinois-Extension crop specialist.

URBANA, Ill. – By the end of the century scientists expect climate change to significantly reduce corn yield. Some predict losses of as much as 28 percent. But those calculations are missing a key factor that could further reduce corn yields – weeds.

Wetter springs and hotter, drier summers already are becoming the norm in the Corn Belt. That type of weather stresses corn during silking and grain fill. But weeds can thrive in those same weather conditions.

“Adverse weather and weeds are two stressors to crop production, but there's been little research into how the combination of the two factors influence crop yield,” said Marty Williams, an ecologist at the U.S. Department of Agriculture’s Agricultural Research Service ecologist. He also is an affiliate professor in the department of crop sciences at the University of Illinois. “Computer models projecting corn yields into the future are assuming weed-free conditions. That's unlikely to be the case without a major transformation in the way we manage weeds.”

Complete weed control is rarely achieved in practice, especially considering herbicides – the single most common tool used to destroy weeds – are losing ground to resistant weeds. Several important weed species – such as waterhemp and Palmer amaranth – resist multiple herbicide modes of action. With no new classes of herbicides nearing commercialization in corn, the prospects for chemical control continue to dim for resistant weeds.

Yet late-season control of weeds such as waterhemp was the most important factor impacting corn yield. It was bigger than any management practice or weather-related factor.

To arrive at that conclusion a research team, which includes University of Illinois crop scientists Christopher Landau and Aaron Hager, analyzed 27 years of herbicide-evaluation trials representing more than 200 unique weather environments throughout Illinois.

“When researchers want to look at weather variation and crop yield in a controlled manner, generally that’s one experiment in two or three environments,” Williams said. “If it's a big study that might amount to six or eight environments. Our analysis enabled us to look at a historic dataset where there were hundreds of environments.”

Machine-learning algorithms helped the researchers make sense of the large, complex dataset. They looked at crop-management considerations such as planting date, hybrid choice, and planting density. They also looked at percent weed control for multiple weed species, weather data at key growth stages throughout the corn life cycle, and yield.

The analysis showed an average of 50 percent loss when late-season weeds were minimally controlled. Even with relatively robust late-season weed control – as much as 93 percent – weeds exacerbated crop losses in hot or dry conditions.

“The combination of less-than-complete weed control and the weather events is where we see crop losses much larger than from poor weather alone,” Williams said. “Achieving 94 percent weed control late into the season is a high bar. I'd be surprised if many fields hit that mark for weed control on a regular basis.”

The researchers know excessive mid-summer heat and-or drought stresses corn and makes it less competitive against weeds. But that’s not the only way climate change interacts with weeds to affect corn yield. Adverse weather affects field-working conditions and herbicide efficacy. For example if a period of drought sets in just after pre-emergence herbicides are applied, the chemical won’t work as well and emerging corn could be engulfed by early weeds.

But farmers forced to plant later due to wet conditions in the spring could be in luck. The analysis showed 18 percent less yield loss when corn was planted after Apr. 29.

“The advantage of later planting was related to improved weed control, with early weeds having time to emerge and be killed prior to planting,” Landau said. “But that doesn't necessarily mean it's best for the crop. The later corn is planted the more likely you'll catch a window of time when it's excessively hot or dry during flowering. Late-planting may benefit weed management, but it may expose the crop to greater risk of heat stress or drought stress during reproduction.”

The analysis highlights the need to move from reliance on simplistic weed control systems in climate change. Weeds are adapting to existing herbicides, and a new product won’t be a silver bullet, Williams said.

“History has shown us that it won't do any good to innovate some brand new tool if we rely too heavily on it,” he said. “We do need new tools. Whether that's harvest weed-seed control, genetic-engineering approaches, robotic weeders or another advancement. Progress is being made in many areas. But as new tools become available, we need to diversify how weeds are managed – not just with registered herbicides, but all available tactics.”

Lauren Quinn is a media-communications specialist for the University of Illinois-College of Agricultural, Consumer and Environmental Sciences. The Department of Crop Sciences is in that college. Visit for more information.

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