Weeds: The Sooner You Kill ‘Em, The Farther Your Fertilizer Goes

Photo: ©Debra L. Ferguson

In our last issue we posted an article about the effect weeds can have on crop yields, particularly in areas or seasons with marginal moisture supplies.

The same can be said for the effect that weeds have on available plant nutrients.

While all that sounds like something for a long-ago Agronomy 101, the timing for weed control makes a big difference in how much effect that vegetation exerts on final crop averages.

And timing is everything.

In a 2-year Wisconsin study, researchers found no yield loss in corn when weeds were controlled at the 4-inch stage. But delaying application on 12-inch weeds resulted in an average 9% yield loss.

Here’s a quick breakout of the results:

  • The economic optimum nitrogen rate was 96 pounds per acre when weeds were controlled at 4 inches tall.
  • When weeds grew to 12 inches before they were taken out, it took 200 pounds of nitrogen to reach the same yield level where weeds were eliminated at 4 inches tall.
  • That’s a 2X difference in nitrogen rates between controlling 4-inch weeds or waiting until they hit a foot tall.

Ironically, herbicide-resistant weed populations are prompting more emphasis on controlling small weeds, which also removes this early competition for nutrients.

Phil Cochran, a soil nutrient consultant with Cochran Agronomics, Inc., in Paris, Illinois, says that the appearance of glyphosate-resistant weeds has prompted his clients to do more in terms of preventing emergence and/or killing escapes when they are small.

Resistant waterhemp and horseweed in his area have fired up efforts to stay on top of early weed management. In those cases, nutrient loss to weeds isn’t as much of a concern as it once was.

Weedy outbreaks still occur now and then, Cochran says, and a heavy infestation will pull nutrients from the soil at the same time the crop is consuming them. But emphasis on catching small weeds has likely lessened the competition for fertilizer.

Call it a positive byproduct in the fight against resistant weeds.

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