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URBANA, IL -- Even with all the time and resources that were expended to control weeds in 2015, weeds will once again be a foe in Illinois fields in 2016, said University of Illinois weed scientist Aaron Hager.

As weed management practitioners begin to contemplate plans and programs for next season, Hager recently published a four-part blog series on weed management over the past year that could provide helpful information for the 2016 season.

In the series, Hager points out that weather patterns during portions of the 2015 growing season once again demonstrated the trouble with weed management programs that rely exclusively on one tool or tactic, he said.

"These perils were often highlighted in soybean fields not treated with soil-residual herbicides. Applications of post-emergence herbicides were often delayed by frequent precipitation and persistently wet field conditions until well beyond the point when weed interference began to reduce soybean yield," he explained.

Hager added that this provides a reminder of a very important central tenet of weed management: resources expended to keep weeds under control do not increase crop yield. "Increases in crop yields are accomplished though plant breeding," he said. "Weed management, on the other hand, preserves the genetic yield potential achieved through breeding."

Put another way, Hager said weeds and crop plants require the same resources for growth. Any resource consumed by competing weeds becomes a resource unavailable for the crop to use to express its genetic yield potential. "Once weed interference has persisted long enough to adversely impact crop yields, nothing can restore the lost yield," he added.

Aside from weather challenges, Hager said that lower commodity prices have many contemplating ways to reduce input costs in 2016. He explained that there are several viable options to reduce herbicide costs, but cautioned that hybrids and varieties, even those with the highest yield potential, will not realize their yield potential if weeds are not adequately and timely controlled.

"Keep in mind, especially while planning 2016 weed management programs, that wise investments to manage weeds before interference reduces crop yields will realize a return through more bushels harvested at the end of the growing season," Hager said. "An investment in high-yielding hybrids and varieties should be coupled with an investment in weed management that adequately protects yield potential."

Another continuing problem is the common occurrence of herbicide-resistant weed populations across most areas of Illinois. Waterhemp and horseweed (marestail) are the two most common herbicide-resistant weed species in Illinois, and observations during 2015 suggest these species are likely to remain prevalent in 2016, Hager said.

Approximately 1,700 waterhemp samples (representing 338 fields) were submitted to the University of Illinois Plant Clinic in 2015 for herbicide resistance screening. Although summary data for these samples are not yet available, Hager said the sheer number of samples submitted suggests herbicide-resistant waterhemp continues to be a significant management challenge for farmers.

"Waterhemp plants and/or populations resistant to herbicides from more than one site-of-action group are not uncommon, and we anticipate this phenomenon will continue. Data from 2014 indicated resistance occurred in close to 90 percent of the fields sampled, and multiple resistance to glyphosate and PPO inhibitors was confirmed in 52 percent of the fields sampled," he added.

The continuing challenges on agronomic cropping systems by weed populations resistant to various herbicides has led to renewed interest in utilizing multiple modes of herbicide action in weed management programs.

"Articles written about and advertisements for products that contain multiple modes of action populate many farm media publications," Hager said. "But simply because a herbicide premix or tankmix combination includes herbicides representing more than one mode of action, this doesn't necessarily mean that each component in the premix or tankmix will be effective against the target weed species of greatest concern.

"There are many instances when multiple modes of action and 'effective' modes of action are not synonymous," he added.

Hager expands on these and other issues related to successful weed management in a four-part blog series on the Bulletin, the U of I pest management and crop development website, bulletin.ipm.illinois.edu/

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