Stop Weeds from Stealing Nitrogen
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Weeds can do more damage than just robbing a field of yield potential. If weeds get 4 inches and taller, plan on them outcompeting corn by stealing 30 to 40 pounds of nitrogen per acre, according to Fabian Fernandez, University of Minnesota nutrient management specialist. That’s not counting other nutrients and water weeds consume.
Replicated university research in the mid-2000s helped quantify nitrogen and yield loss in corn as glyphosate-resistant weeds intensified during that era. One two-year research project, undertaken at the University of Wisconsin, questioned the illogical “Fertilizing Weeds for a Profit.” While growers don’t intentionally feed weeds, this study helped clarify when a growing number of resistant weeds can steal nutrients and yield.
Early-Season Control Critical
The findings in the university’s 2008 report are still relevant, perhaps more so since herbicide-resistant weeds are more abundant now compared to almost two decades ago. “Information in this report remains very valid today. The amount of nitrogen consumed by weeds in an eight-day period when growing from 4 inches to 12 inches tall was substantial,” Fernandez says.
Weeds accumulated 12 pounds per acre of nitrogen at the 4-inch weed control timing compared to 25 pounds per acre of nitrogen at the 12-inch weed control timing when averaged across all nitrogen application rates in 2006.
“While weed control is not my area of expertise, from a nitrogen standpoint it’s long been evident that early weeds outcompete corn for available nutrients and water,” Fernandez says. “Weeds are very aggressive in taking up nitrogen, so early treatment is critical to maximize nitrogen efficiency.”
Corn Slow To Compete
Weeds can win the competition battle early on as corn doesn’t use much nitrogen until about the V6 growth stage. “Once corn reaches the V6 stage, it will grow fast and shade the soil so weed competition typically becomes less of an issue, assuming they are small during rapid corn growth,” Fernandez says.
Before V6, however, the soil gets a lot of sunlight to help weeds grow quickly by using available nutrients and water. That’s why weed scientists and many postemergence herbicide labels recommend weeds be controlled on or before they are 4 inches tall. Another reason not to delay herbicide application is the risk of wet weather that allows weeds to get out of control and become a huge competitor for corn.
Fernandez understands herbicide control is optimized before this weed size is achieved. “My main takeaway from this research is to control weeds early with a preemergence program,” he says.
The critical stage becomes when a follow-up postemergence application is needed. If it’s delayed beyond 3- to 4-inch weeds, then more nitrogen inputs are consumed by weeds and crop efficiency is reduced.
Yield Loss
Along with increased nitrogen accumulation in older weeds, corn yields were reduced by 7% in 2006 trials and 11% in 2007 when comparing control of 12-inch weeds to the weed-free control.
Additional University of Minnesota studies have shown a 12 to 13 bushel per acre loss in the first week after weeds hit the 4-inch height, and 27 to 29 bushels acre the second week.
A postemergence weed control timing study by Michigan State University researchers showed a 25 bushel per acre yield loss when weeds were not controlled until a 9-inch height.
Delay Nitrogen Application?
“Since corn doesn’t have a big nitrogen demand until about V6, waiting to apply most of your nitrogen until about V6 to V8 can be a good strategy in weedy fields,” Fernandez says. Less nitrogen early on will slow down weed growth and limit how much nitrogen weeds consume, giving farmers a better chance to get weeds under control before applying a large nitrogen investment for the crop.
Fernandez says they’ve researched split nitrogen applications for years in weed-free environments, often finding it doesn’t improve the crop from an agronomic standpoint. However, the practice works well in higher nitrogen loss environments like sandy soils or poorly drained soils during a wet spring.
“We normally talk about management practices that minimize nitrogen loss to the environment and improve nitrogen use efficiency of corn, but the reality is if you have weeds, you are losing nitrogen to a crop of weeds which doesn’t do any good and reduces your nitrogen use efficiency,” Fernandez says. “So, the same principles that we talk about for nitrogen use efficiency would be applicable to weed control.”
Content provided by DTN/The Progressive Farmer
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