Combine cultural and mechanical practices for season-long weed control.
Weed eradication doesn’t come with a simple recipe. No longer can growers reach in the toolbox to pull out just one tool that will keep fields clean from start to finish. Today, weed eradication calls for a diversified approach.
Herbicide resistance continues to plague crop farmers as Palmer amaranth and waterhemp continue to develop resistance to more herbicide sites of action. In fact, one population of Palmer amaranth has been found resistant to five different sites of action, leaving few viable herbicide options to control it. As one industry expert puts it, “Pigweed is simply a beast, and the most troublesome weed in the United States.”
But it doesn’t have to be that way. By taking on a multi-faceted approach to weed control, growers can get a leg up on the journey toward a weed-free future.
Consider cultural and mechanical practices as key weapons to have in your weed eradication arsenal.
According to Dr. Prashant Jha, professor and lead specialist in the weed science program at Iowa State University (ISU) Extension, conventional and conservation tillage as well as narrow-row spacing and harvest weed seed control measures can impact weed emergence and viability, respectively, helping growers take positive strides on the track to eliminating pigweed.
While not always viewed as the best practice from a soil conservation standpoint, conventional tillage can have a positive impact on pigweed eradication. Dr. Jha says research conducted in the South and Southeast, deep tillage using a moldboard plow has been shown to have a powerful effect in reducing pigweed emergence.
While pigweed emergence is possible after deep tillage, the practice severely reduces the ability of the seed to germinate and emerge from the soil, according to Jha.
In the Midwest, shallow or conventional tillage has seen mixed results in pigweed eradication efforts. Typically, fall tillage after corn harvest will be followed with another pass of light tillage in the spring, often using a cultivator.
While conventional tillage generally helps to reduce the density of pigweed seeds that could emerge during the next growing season, the final impact the practice has on pigweed emergence ultimately depends on the level of infestation in the weed seed bank, according to Jha.
From a conservation standpoint, growers using conventional tillage practices also compromise soil infiltration as well as organic matter that might see more favorable results from conservation or no-till practices.
Jha suggests deep tillage for pigweed eradication efforts in years when all other modes of control have failed. “Then you do deep tillage and don’t disturb the soil for another four or five years because that would bring seeds back to the surface and then there’s no advantage to it,” he says.
In no-till systems, Jha says seeds align with the soil surface and are either subject to predation in the fall or microbial decay. However, at the same time he says most of those seeds will sprout the next spring.
Over the last three years, researchers at ISU have been studying the effects of row spacing on weed emergence.
According to Jha, when comparing 30" row soybeans versus their 15" row counterparts, a drastic reduction in the emergence of waterhemp was seen.
“We have seen close to 30-40% reduction in the emergence of waterhemp, the most troublesome pigweed species in the Midwest,” Jha says.
According to Jha, narrower rows close the canopy at least 10 to 14 days sooner than wider rows, which makes a huge difference for a weed species that can emerge throughout the growing season.
Additional research conducted by Jha in Arkansas and South Carolina also resulted in a significant reduction in Palmer amaranth presence in narrow-row soybeans.
“I see the narrow-row soybeans or narrow-row spacing as one of the complimentary strategies in [eradicating pigweed] because now you are reducing the density or the proportion of the seed bank that is emerging,” Jha explains. “Plus, we are seeing reduction in the size of emerged plants because of the early canopy closure. So, that’s a win-win situation.”
In the battle against herbicide resistance, adding non-chemical measures to the toolbox beefs up the journey toward pigweed eradication.
As a mechanical strategy, harvest weed seed control involves attaching different types of equipment to the combine to destroy weed seeds at the time of harvest. Seed destructors/impact mills and chaff lining are two examples.
High-impact mills, which destroy weed seeds picked up by a combine, have been shown to destroy between 70% - 90% of weed seeds in soybeans, wheat and other small-statured crops.
Recent studies conducted by Jha’s team at ISU show that approximately 95% of the waterhemp seeds were mechanically damaged by the Redekop™ seed destructor attached to a John Deere® S680 combine, an impact mill developed in Saskatoon, Canada, at the time of soybean harvest. Furthermore, there was 95% reduction in viability of the damaged seeds, suggesting a drastic decline in pigweed emergence in the subsequent growing seasons.
While researchers aren’t certain growers can expect nearly zero weed seed survival when using impact mills in the field, based on five-year U.S. field trials, they might be able to count on typical results of a 70% to 80% reduction in weed seed deposit to the soil.
Another harvest option for growers to consider in their weed eradication efforts is chaff lining, a technique which separates the straw from the chaff inside the combine.
Jha’s research at ISU suggests that chaff lining at the time of soybean harvest can result in at least 95% reduction in the spread of pigweed seeds in the field. Additionally, the chaff lining technique resulted in a delay in emergence and consequently a reduction in size of the emerged pigweeds at the time of post herbicide application, again a complimentary strategy to reduce selection pressure on herbicides.
“These two technologies are going to be key in weed eradication,” Jha says.
From mechanical and tillage strategies to narrow-row spacing and harvest weed seed control, no single approach to pigweed eradication can save the day. Pigweed eradication can be best achieved through a combination of measures that work in tandem to control weeds all season long.
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