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Weed Resistance: Looking Back on 2016

November 9, 2016

This spring, researchers warned that weed presence and herbicide resistance were likely to be strong in 2016. With heavy rainfall in 2015 across the Midwest and some parts of the South, herbicide applications were often delayed or abandoned, and weed seeds were expected to spread to new areas as a result of flooding.

The impending threat of herbicide-resistant weeds required an integrated management approach. This approach included employing both cultural practices and multiple herbicide modes of action to minimize the spread of weeds and resistance development.

Trey Koger, a grower from Melzoni, MS, compares soybean production system to a 10-story building. Without a good pre-emergence herbicide to control weeds, he says growers will get stuck on the first or second floor. Koger was able to find success this summer despite the threat of glyphosate-resistant Palmer amaranth invading his fields. When Palmer amaranth came along, he started using Boundary® herbicide as a pre-emergence herbicide and has been doing so for the last three years because of its good Palmer amaranth and annual grass control.


Trey Koger on his farm in Belzoni, MS.

This past season, it was vital for growers to stay vigilant by applying properly timed post-emergence herbicides to keep weed populations low. These proactive steps reduced weed seed bank buildup and delayed the onset of herbicide resistance for many growers across the Midwest and the South.

Since waterhemp grows so fast, and it’s gained so much resistance, it’s hard to stop it once it’s taller than 3 to 4 inches. The residual component in Flexstar® GT 3.5 herbicide helps hold waterhemp down. Using Boundary followed by Flexstar GT 3.5 delivers multiple modes of action for effective waterhemp management.

Because herbicide-resistant weeds can become a serious problem soon after arriving in a field, an integrated weed resistance management approach is key. For next season, Syngenta and the United Soybean Board recommend:

  • Planting soybeans in narrow rows with higher seeding rates to help spread canopy coverage and shade out weeds.
  • Applying a pre-emergence residual herbicide. This helps young soybean plants to establish their full yield potential in a weed-free environment.
  • Applying a herbicide with two different modes of action that target the same species to help combat resistance and effectively manage weeds.
  • Being timely with post-emergence herbicide applications – weeds like Palmer amaranth grow rapidly, so there is a narrow window of opportunity to effectively treat the weed with a post-emergence herbicides.
  • Cleaning farm machinery to prevent the dispersal of weed seeds.
  • Minimizing the seed bank by keeping fields, ditches and turn rows clear of weeds that could go to seed.
  • Planting cover crops after harvest and use tillage, if necessary.

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