The Agronomists, Ep 18: Rob Miller and Jason Voogt on pre-emerge and early weed control


Start clean, stay clean is the motto when it comes to weed control. Super dry conditions, however, can throw a wrench in the best laid plans, as soil-applied herbicides may be less effective and timing a first pass is tricker as weeds are slow to emerge.

To discuss per-emerge and early season weed control on this episode of The Agronomists, we go to Rob Miller of BASF and Jason Voogt of Field 2 Field Agronomy, for insight, some banter, and great discussions on antagonism, resistance management, and more!

Catch a new episode of The Agronomists every Monday night at 8 pm E!


  • Dry, dry, dry where Voogt is near Carman, Man.
  • In Ontario, it’s a bit of a different story, especially where Miller is near Guelph, Ont.
  • There are many different strategies for pre-emergent weed control (Oh the joys of rural internet)
  • Every season is different — no one solution fits all.
  • Don’t underestimate the value of good crop canopy.
  • Do you a) plant when there aren’t any weeds up and then figure it out later or b) wait until some weeds come up and then implement a control, then seed.
  • When it’s dry, seed first, utilize that moisture. The weeds aren’t going to emerge if there isn’t any moisture; same as the crop
  • Not all pre-emergent herbicide residuals are created equal; pay attention to your labels. Moisture for activation. Soil type, organic matter, the hydrology
  • Critical weed free period chart and economic thresholds of some weeds chart from University of Manitoba and Manitoba Agriculture and Food
  • Corn plants sensing weeds growing beside it (Allelopathy y’all)
  • Clip 1: Corn School: Do pre-emerge herbicides pay in dry years?
  • With research, a lot of things can happen by accident
  • Some pre-emerge products are broken down by sunlight, but most aren’t, they’ll still be there when those first rains come
  • Don’t wait until after three leaf stage for corn — but that goes for any crop, refer back to that critical weed free period chart!
  • Waiting to kill the weeds in-crop? Could be a recipe for disaster. Weed seeds are small, they don’t need a lot of moisture to start inbibation
  • Clip 2: Soybean School: Making the most of your soil applied herbicide
  • Another benefit of a pre-emerge with residual? Time management
  • Hang-up on residue is also a consideration
  • Sulfentrazone in a higher pH situation can have a “reach-back” effect and control smaller weeds
  • Does soil temperature affect weed control with soil applied herbicides?
  • Are there any herbicides with residual that can control buckwheat? Always having to make an extra pass of glyphosate in corn, after buckwheat as a cover crop
  • Cold nighttime temps, warm daytime temps? Go or no go?
  • A ticking clock… Buying time to get to canopy closure
  • Antagonism: applying a herbicide, adding something into it, decreasing the efficacy, not all the way to zero, but the efficacy will be less
  • Combining modes of action to control different weeds, you still need to apply at the full rate
  • Clip 3: Soybean School: Comparing pre- and post-emerge weed control strategies
  • Look at this as an overall systems approach
  • Products like Edge or Treflan are starting to make comebacks
  • An agronomist from Brazil is watching, and wonders what are the worst weeds that we have to deal with in Canada. For them, it’s Conyza bonariensis (Hairy fleabane)
  • Nature finds a way
  • Clip 4: Pulse School: Evaluating different approaches to weed control in peas
  • Pulse crops are poor competitors. Pre-emerge products with residual will be important. Two weeks is a long time to keep a crop weed-free to optimize yield
  • When you’re targeting broadleaf weeds, what are you doing to target grassy species? Relying on glyphosate is only going to get us so far
  • Nobody mentioned tillage as an option. Think about it in the right perspective warns Voogt. No recreational tillage
  • Utilize something that’s very targeted, but remember that it will have an expiry date. And plants are plastic, they adapt
  • Start clean, stay clean

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