Pest resistance management has become a more pressing issue over the last 10 years, particularly as weeds like Palmer amaranth, waterhemp, marestail and giant ragweed have been confirmed resistant to different herbicide chemistries in Iowa and other Midwest states.
As the range of effective chemistries in growers' arsenals grows thinner, it's a reminder that weed control — and pest management in general — takes an integrated approach. I've written before about the interest in cover crops in Iowa and the growth in acres required to meet the Iowa Nutrient Reduction Strategy's goals. But I've also written about some of the economic challenges to cover cropping, and how it's easier to realize a profit more quickly with grazing livestock.
However, these economic benefits are mostly referring to direct benefits. When it comes to indirect benefits, the return on investment is more difficult to measure. That's especially true when it comes to benefits like reduced erosion, improved soil water infiltration, and increased organic matter and nutrient cycling. But there's also a benefit to weed control, especially when cover crop establishment and termination are timed right — when planting soybeans into standing cereal rye, for example.
Weed seed control alternatives
Of course, this can be a challenge for growers in the western Corn Belt, where rainfall (especially in the last couple of growing seasons) can be highly variable during the growing season. Still, for those who have managed to "plant green" successfully, there can be a reduction in herbicide costs and field passes to manage weeds. It's not surprising more growers and researchers are showing interest — including the folks involved with the Iowa Pest Resistance Management Program in Harrison County, and recent research at Iowa State University. Levi Lyle, a Washington County grower featured on the July Wallaces Farmer cover and feature story, has managed to reduce his costs per acre on both organic and conventional acres by planting green through his one-pass farming approach.
That's not to say cover crops will, or should, completely replace herbicides for weed control on every farm. As mentioned earlier, a good weed management system often involves an integrated approach, and I've heard more than a couple weed scientists address the need for using different tools in the weed management toolbox to battle herbicide-resistant weeds.
With that said, there's also growing interest in methods to control weed seed at harvest — notably, the Harrington Seed Destructor, which originated in Australia, collects weed seeds at the time of grain harvest. It doesn’t grind or crush weed seeds, but destroys them through a series of high-speed impacts, rendering them impotent. ISU and the Harrison County group are studying the Harrington Seed Destructor, as well as chaff lining, as a weed seed-control option. A study in Illinois has also shown the Harrington Seed Destructor kills nearly 100% of all U.S. agronomic weed seeds.
It's worth noting that weeds are adaptable. As Ian Malcolm, the mathematician and chaos theorist character played by Jeff Goldblum in the movie “Jurassic Park” (1993), says, "Life finds a way." Case in point: Barnyardgrass has been identified in China that resists hand-pulling by mimicking rice until it's too late to pull. This was after 300 years of evolutionary selection, and 300 years of manually pulling barnyardgrass from rice paddies.
Even with mechanical control, certain weeds can develop resistance over time. Take for example wild radish — a problem weed for growers in Western Australia. Research there shows that while harvest weed seed control is an effective method, it depends on the machine’s ability to collect weed seeds at the time of grain harvest. This research shows earlier-flowering biotypes have evolved in some populations of wild radish, which can drop seed earlier, prior to harvest, when machines like the HSD would normally collect those seeds.
But it isn't just herbicide resistance we're dealing with; soybean cyst nematode resistant to soybean varieties with the PI 88788 trait, corn rootworm resistant to Bt corn, and frogeye leaf spot populations resistant to strobilurin fungicides have all been identified in Iowa.
The question is: After dealing with herbicide-resistant weeds, fungicide-resistant pathogens, and insects and nematodes resistant to different traits, why aren't we making a greater effort to prevent resistance from spreading?
It can be all too easy to get caught up with what your neighbors are doing, and fall victim to peer pressure. The more stakeholders making a concerted effort to help slow the spread of resistance, the better. Whatever the case may be, it's been said a number of times that weed control will take an integrated approach moving forward.