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Research: Preemergence (PRE) herbicides

What you need to know

  • Using a preemergence (PRE) herbicide led to greater early-season weed control, compared to postemergence (POST) glyphosate-only applications.
  • Using PRE herbicides didn’t affect soybean plant stands.
  • A couple sites that didn’t use a PRE herbicide experienced weed escapes at harvest.

Using a preemergence (PRE) herbicide is a key strategy for preventing and managing resistant weed populations. University of Minnesota research shows using a PRE herbicide in soybean led to greater weed control without affecting yield, compared to solely relying on postemergence (POST) applications.

Understanding preemergence (PRE) herbicides

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Research: Effects of including a PRE herbicide application in soybean

To help demonstrate the benefits of using a PRE herbicide in soybean, the University of Minnesota initiated the PRE Challenge in 2012. The research and demonstration project aimed to illustrate how applying PRE herbicide in soybean affected weed control, yield and economics, when compared to a POST, glyphosate-only program.

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Research results

Soybean stands and soybean population loss

At each site, applying a PRE herbicide had no impact on soybean stand or stand loss.

Early in the season, soybean stands ranged from 90,000 to 141,200 plants per acre (Table 1). By the end of the season, soybean stands ranged from 86,900 to 136,000 plants per acre. Percent population loss ranged from a low of 1.7 percent to a high of 9.4 percent, but was similar across treated and untreated plots at each site.

Weed populations

Common lambsquarters and waterhemp/pigweed species were the most common weeds at each site.

Although weed populations were relatively low overall, weed populations were greater where no PRE herbicide was applied (Table 2). Weed patches also often were observed where no PRE was used, such as at the Pipestone location in 2012 (Figure 1).

Pipestone location on June 18, 2012
Figure 1: Pipestone location on June 18, 2012. Increased early-season weed control in the PRE treatment (Prefix at 2 pints per acre) was particularly noticeable here, with its patch of grassy weeds.

Although POST applications controlled most weeds, waterhemp survivors were observed at a couple locations. The Nerstrand site, for example, didn’t apply a PRE herbicide and had waterhemp plants at harvest. No escapes were observed in the PRE treatments (Figure 2).

While resistance to glyphosate in waterhemp hadn’t officially been confirmed at any sites, glyphosate-resistance in waterhemp is an expanding problem in Minnesota.

Weed escapes at harvest were observed at a couple of sites that didn’t use a PRE herbicide. Seed production from these plants can help replenish the weed seedbank, which can lead to long-term weed control challenges. Preventing the weed seedbank from replenishing also is a key resistance management strategy.

PRE herbicides
Figure 2: PRE herbicides (Verdict at 5 ounces per acre + Outlook at 10 ounces per acre) lowered weed populations at harvest in Nerstrand. Escapes produced seed, which could increase weed densities.

Yield

At all locations, yields weren’t affected by the inclusion of a PRE herbicide in the weed management program. Yield ranged from 45.7 to 67.3 bushels per acre, with drought conditions contributing to lower yields at the Pipestone location in 2012 (Table 1).

While yields weren’t greater where PRE herbicides were used in these trials, yield advantages have been observed in previous University of Minnesota research.

If weather or other complications delayed POST applications, early-season weed competition would’ve been more likely to impact yield. PRE herbicides may not result in a yield benefit in a given year, but using them provides other benefits that enhance weed control in the long-run.

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Lizabeth Stahl, Extension educator; Ryan Miller, Extension educator and Dave Nicolai, Extension educator

Acknowledgements

This project was made possible by financial support from the Minnesota Soybean Research and Promotion Council. This project also wouldn’t have been possible without the much-appreciated cooperation and support of our farmer/plot cooperators.

Reviewed in 2018

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