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Mind the pests

Mind the pests

After all you do to choose the right hybrid, wait for prime planting conditions, provide plenty of nutrients and place the seed in the soil just so, it’s heartbreaking to think of all the damage weeds, insects and disease can do to your yields. And it’s shocking to realize how quickly that heartbreak can break the bank.

Early-season weed competition has been documented to reduce corn yields by up to 40 bushels per acre in South Dakota, according to agronomy and weeds field specialist Mark Rosenberg at South Dakota State University. The greatest damage is right at the beginning of the season, he adds. Just one foxtail per foot of row can steal 1% of corn yield per day — per day! — starting one week after corn and weeds emerge.

Daily yield loss

Research in Iowa, Minnesota and Wisconsin illustrates that 3- to 4-inch-tall weeds in corn at the V3 to V4 growth stage cause losses of an average of 3 bushels per acre per day until the end of June. That adds up dramatically, notes University of Minnesota Extension weed scientist Jeff Gunsolus.

“Our studies over three years show corn lost from 12 to 13 bushels per acre within the first week and 27 to 29 bushels per acre within the second week if weeds were allowed to remain in the field after they reached 4 inches in height,” Gunsolus reports.

All that early-season loss shows the importance of an integrated weed control program that starts with a soil-applied, residual herbicide, notes Aaron Hager, Extension weed scientist at the University of Illinois. A split application of early preplant herbicide followed by either a preemergence or postemergence spray could provide more consistent weed control than a single, early preplant application, he adds.

Multiple modes of action and multiple application timings also help address herbicide-resistant weeds.

Early weeds steal N, roots

“Corn is much more sensitive to early-season weed competition than soybeans,” Rosenberg says.

For one thing, early-season weeds can consume 30 to 45 pounds of N per acre between planting time and the end of June, notes Gunsolus.

“You don’t get that N back during the season after your weeds are controlled,” he adds.

Corn also appears to react to weeds by pouring more energy into shoot growth at the expense of root growth. The result is plants of widely varying heights competing with each other for light — the very phenomenon that makes uneven emergence such a yield-buster. That compromised root mass could also end up being inadequate to seek out moisture if the summer turns dry.

Watch for pests

Rootworm power trip

Corn rootworm is known in some circles as the “billion-dollar bug” because estimates of its damage approach $1 billion per year. University of Illinois research shows yield losses of 15% for every node of roots destroyed, and up to 40% loss from rootworm
in drought conditions.

That’s why Bt rootworm hybrids were so eagerly adopted across the Corn Belt. However, the Cry3Bb1 gene that confers rootworm resistance in some hybrids has suffered greater-than-expected losses in some areas. In those situations, Monsanto recommends using pyramided products (like SmartStax) that contain multiple corn rootworm traits or use a soil-applied insecticide at planting.

Remember to scout for adult rootworm beetles at tassel. More than five beetles per plant clipping silks can lead to yield loss significant enough to warrant an insecticide spray. Researchers at Purdue University say a threshold above just one beetle per plant may even be warranted in a corn-on-corn program to protect the following year’s crop from
a big hatch of larvae.

Watch cutworms

Unlike most infestations of corn rootworm, which thrive in continuous corn, black cutworm tends to be a bigger problem in first-year corn, according to Mike Gray, Extension entomologist at the University of Illinois.

Weedy or late-planted fields tend to attract cutworm moths, which swoop in to lay eggs. An individual larva can cut three to four plants in its lifetime, and heavy infestations can be devastating.

Seed treatments can help

Seed treatments remain a question mark from an economic perspective, though it is pretty clear that many products do a good job of protecting seeds from insects and disease.

“It’s still unpredictable how much of a yield growers will obtain with treated seed,” says University of Tennessee Extension entomologist Scott Stewart,” but the 4-5-bu. range is still common among many growers.”  The question is whether that 4-5 bushels is a good economic return on an investment of $15 to $30 per bag of seed.

Emmanuel Byamukama, South Dakota State University Extension plant pathologist, offers a few guidelines on assessing the likely value of seed treatment fungicides, including:

  • Continuous corn can harbor a buildup of pathogens from year to year.
  • No-till or minimum-tilled fields may have a higher risk of seedling diseases.
  • Cool, wet soils favor the buildup of Pythium and Phytophthora, and can stall germination, making seeds even more vulnerable to disease.
  • Early planting increases the odds of exposing seeds to cold conditions, slow germination and disease.
  • If germination rate for the seed lot is low, protecting the seed could help make the difference between a poorer stand and a good one.
  • What is your desired plant population per acre?  If you’re aiming to limit populations and save money on seed, it may pay to protect your seed from loss to seedling disease.
  • How much stand reduction is acceptable?  Corn is less able to compensate for poor stands than crops like soybeans or wheat.


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