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A New Weed Paradigm

Wednesday, October 25, 2023
filed under: Weeds

David Horvath, Fargo-based USDA-ARS research plant physiologist, provided this summer’s 2023 NSA Summer Seminar attendees with an intriguing discussion titled “Resource-Independent Weed-Induced Crop Yield Loss: A New Paradigm & New Challenges.”
        The thesis of Horvath’s talk was that while weeds reduce crop yield, they do so not by competing for resources, but rather because the weeds cause changes in the crops that prevent them from reaching their yield potential.
        “We were all taught in high school biology that the reason weeds reduce crop yield is because they compete with crops for light, nutrients, water – or perhaps by producing allelopathic chemicals that reduce the growth of the crop,” Horvath noted. “And I’m guessing that most of you [growers] have been relying on that basic paradigm for making decisions as to how you fertilize and treat for weeds.”
        The crux of Horvath’s message to the NSA audience, however, was that there’s actually another mechanism by which weeds reduce crop yield — a new paradigm wherein weeds initiate signaling processes early in a crop’s life cycle that in turn impact the crop’s growth and development well before any nutrient, water or light limitations are perceived.
        Horvath referred to a number of studies that showed final crop yield loss when weeds were present early in the season — even if they were removed before they could strain the field’s nutrient, water and light resources. “Weeds generally cause the greatest yield loss early in the growing season,” he said. If weeds can be controlled early – during what he referred to as the “critical weed period” – there’s little crop yield loss.

David Horvath

        “It is during that critical period – early in the growing season – that the weeds actually cause damage,” Horvath observed — which is surprising, since that’s a time when nutrients, water and light are typically still very available.
        As an example, Horvath referenced a South Dakota study with velvetleaf in corn. Showing a photo where velvetleaf was in a field of corn at the V6 to V7 stage, he noted that the velvetleaf was removed about a week later. “But by that point, even though the field was kept weed-free going forward, this field had already suffered a 10% yield loss.”
        Horvath also referenced studies at University of Agricultural Sciences, Bangalore, India, looking at nitrogen uptake. Those studies showed that “you can’t overcome the presence of weeds by giving the crop more nutrients,” Horvath stated. “If you give the plants more nitrogen, you’ll still have reduced crop yield when weeds are present. Even when you go up to 90 kg/ha (about 80 lbs/ac), the weeds will still have a significant impact on yield.”

        What’s going on, then? “Ecologists have known about this phenomenon for quite a long time,” Horvath said. “And they’ve coined a term for it: ‘Tragedy of the Commons.’ ” In laymen’s terms, this term refers to plants being able to detect weeds, through both below- and above-ground signals, that early in the crop life cycle impact their growth and development well before any nutrient, water or light limitations exist. In effect, crops actively alter their development to reduce their own growth when they detect other plants nearby — even when nutrients are plentiful.
        That signaling behavior allows plants in the wild to anticipate future resource availability – and then adjust their growth accordingly and therefore survive and produce offspring. But resources are not generally limited in well-managed fields.
        So, it follows that if crop plants could be “blind to weeds,” those plants could then allow for a broadening of the critical period for weed control, gain the full benefit of available nutrients – and benefit as well from nutrient replacement after weed removal.
        That said, there’s a lot of research yet to be done before a “workable” strategy exists, Horvath emphasized. “For example, the genes and molecules required for perception and transduction of the weed-generated signals must be identified so we can specifically block these signaling processes only when weeds are present,” he said. “And because crops respond differently to their ‘kin’ plants versus weeds, it’s also essential to make sure a crop plant’s responses to adjacent crop plants are not inhibited. We also need to look for mutations that block these signaling processes.
          “With sunflower specifically, this work will translate into a better understanding of how sunflower responds to neighboring weeds – and increases yields even if weeds are present,” said the ARS plant physiologist. — Don Lilleboe