Can I Apply Post-Emergent Herbicides to Soybeans before the 1st Trifoliate Stage?

When weed emergence is quicker than soybean emergence it is typical to find weeds at or near the maximum leaf-stage for control, yet the soybean crop is not at the growth stage specified on the product label. Does one focus on targeting the weeds when at their labeled growth stage or wait until the crop reaches the labeled growth stage?

The Department of Plant Agriculture (University of Guelph) in partnership with OMAFRA conducted a series of field trials with the following objectives:

1. To identify if there is an increased risk of crop injury when applying post-emergent broadleaf soybean herbicides prior to the 1st trifoliate growth stage.
2. To identify if there is a negative yield impact by applying post-emergent broadleaf soybean herbicides prior to the 1st trifoliate growth stage.

Methodology

Trials were conducted at the Elora Research Station in 2006 and 2007. Pinnacle, Classic and Reflex were applied at 3 different soybean growth stages (cotyledon, unifoliate and 2nd trifoliate) and at two different application rates (normal and twice the normal field rate). Visual crop injury, plant population and yield data were taken and compared to an untreated, weed-free control.

Results

• 
  Soybean tolerance to all three herbicides was general greater when applications were made prior to the 1st trifoliate stage (See Figure 1).
• When all herbicide treatments were averaged, crop injury at the cotyledon, unifoliate and 2nd trifoliate stage was 3%, 4% and 11% respectively.
• Visual injury was temporary and no longer apparent by the end of July.
• Soybean yields were unaffected by herbicide applications made prior to the 1st trifoliate stage of soybean.
  

Figure 1. Visual crop injury of soybean at two weeks after application when herbicides where applied at the cotyledon, unifoliate and 2nd trifoliate stage of soybean.

Conclusion

Weeds emerging before or with the crop are extremely competitive and should be controlled when they are small (prior to the 6 leaf stage). The experience of two years of field data demonstrate that when a herbicide is applied prior to the labeled crop growth stage there is no increased risk of crop injury and no impact on soybean yields.