A Worrisome Weed Problem

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Tall Waterhemp

Your goal at harvest is not only to bring in the corn crop safely but also to minimize adding more herbicide-resistant seeds to the weed seedbank. Even if successful, you’ll still likely have to contend with herbicide-resistant weed species on your farm.   

Weed scientists place the blame on what they call a fitness cost associated with a weed species’ herbicide-resistant mechanisms. That means herbicide resistance in waterhemp, for example, that evolves target-site resistance or metabolic resistance doesn’t pay any penalty for those traits — the waterhemp aren’t weaker, smaller or hampered by lower seed production.   

The resistant genes say to glyphosate, for example, don’t go dormant or disappear in the waterhemp weed population even if you don’t apply glyphosate for multiple years. There is no guarantee glyphosate-resistant traits wouldn’t remain in the farm’s populations of waterhemp, ready to overcome that herbicide if it’s used again.

Weed scientists measure bottom-line fitness costs as the amount of weed seed produced by resistant individuals compared with weeds without resistance to herbicides. If resistant weeds produce 90% of the seed that sensitive weeds produce, that would be a 10% fitness cost to the herbicide-resistant weed.  

Overall, fitness penalties in plants (in the absence of herbicides) can be caused by reduced germination and growth rate, lower photosynthesis and reduced competition for resources. “We think about this as an evolutionary phenomenon, what’s going to happen over time with a weed population,” says Pat Tranel, University of Illinois weed scientist and associate head of the Department of Crop Sciences. “So that is why, at the end of the day, the most important question is how much seed is produced.”

Different Herbicide Mechanisms

Every herbicide resistance mechanism is different. The first example of herbicide resistance to be understood, over 30 years ago, was to atrazine. It involved a target site protein mutation that reduced photosynthesis, which did cause a fitness cost penalty.

“That research laid a precedent, so we thought herbicide resistance would always cause reduced fitness in weeds,” Tranel says. “That led us to believe and recommend the need to rotate herbicides because those resistant plants would be less fit and produce less seed without herbicide.”

Unfortunately, that first case was the exception to the rule. Most cases of different herbicide resistance mechanisms have proven minimal fitness costs in herbicide-resistant weeds. That means rotating herbicides won’t be that effective.

“We’ve basically flipped our recommendations over time as we’ve learned more about fitness costs. Now it’s all about mixing multiple effective sites of action,” Tranel says.

Environment Plays a Role

Occasionally, under the right environment, there can be some subtle fitness costs from year to year. For example, Group 2 herbicides (ALS inhibitors) impact a plant’s ability to make amino acids, shifting the proportions of valine and leucine. Research shows this can cause seeds to germinate faster, which could be detrimental if there was a late-spring frost. But from a farmer’s perspective, it won’t be a noticeable fitness cost.

“We can also see similar differences with glyphosate where one mechanism has a slight fitness cost and the second mechanism does not,” Tranel says. “But I can’t say consistently that there is a difference across herbicides.”

In a University of Illinois greenhouse research study, Tranel and colleagues created a waterhemp population with varying degrees of herbicide resistance. The weeds tolerated different combinations of atrazine, ALS inhibitors, PPO inhibitors, HPPD inhibitors and glyphosate.

“After planting, not spraying and harvesting the seed — repeating that six times to represent six generations — we found that the amount of resistance in the population stayed the same,” Tranel says. “So if you stop using these herbicides for six years, the resistance won’t go away, and only a slight fitness penalty was observed in the greenhouse.”

Scientists are watching for possible higher-level resistance concerns if a plant stacks two mutations together. “Early research shows that stacks of two mutations can make Palmer amaranth even more resistant to PPO inhibitors, but these double mutations may come with a fitness cost.”

Content provided by DTN/Progressive Farmer.


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