Ready or Not: Weeds Are Coming

In today’s world of growing herbicide resistance, farmers can use every weed-control tool they can get. Pre-emergence herbicide application is one such tool, and researchers say that it might be one of the most important.

“The easiest time to control a weed is before it ever comes out of the ground,” says Jason Bond, Ph.D., Mississippi State University research and extension professor. “That’s the simplest explanation for it.”

Timing Matters
Pre-emergence herbicides are applied to the soil either before or directly after planting and prior to weed emergence. Bond says that although farmers often apply pre-emergence products right after planting, it might be more practical to make that application prior to planting for logistical reasons and to lessen the possibility of crop injury.

“For instance, they plant, apply the pre-emergent herbicide and then they receive a big rain,” he said. “If herbicide is combined with rainfall, then that potential for injury increases.”

These herbicides are applied to the soil surface and need at least some soil moisture – or rainfall – following application for incorporation into the soil. Until this occurs, uptake by the weed could be limited, and weed control could be poor.

Pre-emergence herbicides are taken up by either the emerging root, shoot or a combination of both. The specific site of root or shoot uptake varies between each herbicide and mode of action, giving each herbicide group its own unique weed-control attributes. 

Why Pre-Emergence?
Bond says farmers should consider using pre-emergence herbicides for several reasons, including: 

  • They’re cost-effective 
  • They offer different modes of action from post-emergence herbicides 
  • They control weeds early in the crop year, which can help maximize crop yield potential

As farmers consider which pre-emergence product to use, Bond says they should keep a few factors in mind, including performance, targeted-weed efficacy, crop safety and herbicide rate, especially during tough economic years in the ag industry.

“The herbicide rate varies depending on the soil texture,” he said. “We generally use higher rates on fine-textured soil and lower rates on coarser-textured soils. A full rate of some herbicide applied to a sandy loam soil would be lower than a full rate of the same herbicide applied to a clay soil.  Some residual herbicides are more active on coarse-textured soils.”

Diversification is Important
For the past two decades, many farmers have relied solely on post-emergence herbicides, such as glyphosate, to control weeds. Experts strongly encourage farmers to diversify their weed-control program, and Bond said that pre-emergence herbicides are an important part of that diversification.

“For the Delta area, it’s the foundation of our weed-control program,” he said.

Top tips for using pre-emergence herbicides

  • Use them as part of an integrated weed-control plan.
  • Minimize soil disturbance, allowing weed seeds to remain on the soil surface.
  • Ensure the seed furrow is closed to prevent crop injury.
  • Understand herbicide chemistry: Use the right herbicide in the right field at the right rate.