How pre-emergent herbicides work

Author: Deanna Lush | Date: 11 Apr 2014

The trial plot shows crop damage with pre-emergent herbicides due to poor separation of herbicide and crop seed. Photo: Christopher Preston

Water solubility key to success, or failure, with pre-emergents

EVER put out a pre-emergent herbicide, looked at the final result and wondered what went wrong? University of Adelaide’s Christopher Preston says this can be an all-too-common occurrence – either weeds still germinate or crops end up damaged.

Dr Preston says there are several rules of thumb in understanding pre-emergent herbicides:

  • Water solubility

The more water-soluble herbicides will move more readily through the soil profile and are better suited to post-sowing, pre-emergent applications than the less water soluble herbicides. They are also more likely to produce crop damage after heavy rain (see Table 1).

“The key facet to getting pre-emergent herbicides to work is to understand their solubility in water,” Dr Preston said. “Greater water solubility also means more mobility in the soil and higher risk of crop damage with heavy rain after sowing.”

Trifluralin and pendimethalin (Stomp®) are the least water soluble chemicals while Boxer Gold®, particularly its S-metolachlor component, is one of the most soluble. This means less moisture is required for activation of Boxer Gold than for Sakura®.

“Our rule of thumb is that 5 to 10 millimetres of rainfall in the 10 days after sowing is fine for Boxer Gold but 10-15mm is required for Sakura.”

  • Binding to soil components

Dr Preston says herbicide movement into the soil profile is strongly influenced by their ability to bind to soil organic matter. Light soils with low organic matter are more likely to have herbicide washing into crop rows, causing damage to seedlings.

“Trifluralin and pendimethalin are strongly bound to organic matter in soil. This means they will generally not move far from where they are applied,” he said.

“In contrast, Sakura® and S-metolachlor in Dual Gold® and Boxer Gold® are not bound tightly and are prone to movement in soils with low organic matter. Using the lower label rate in these soils will reduce the risk of crop damage.

  • Moisture availability

If the soil is dry on the surface, but moist underneath there may be sufficient moisture to germinate the weed seeds, but not enough to activate the herbicide. Poor weed control is likely under these circumstances. The more water soluble herbicides will work more effectively under these conditions.

  • Position of weed and crop seed

Pre-emergent herbicides need to be at a sufficient concentration and at or below the weed seed for effective control. This is except for Avadex®Xtra which needs to be kept above weed seeds. Keeping weed seeds on the soil surface will improve control by pre-emergent herbicides.

“Since many pre-emergent herbicides can cause crop damage, separation of the product from the crop seed is essential,” Dr Preston said. “Some people have experienced issues with pre-emergent herbicide use and disc seeders but the two rules about disc seeders is you need to move some of that herbicide-treated soil out of that crop row to separate it from the crop seed, and you must get your seeding depth right.

“The biggest thing that influences shallow seeding disc-seeders is speed of travel. We do all our work at 12 kilometres an hour and we get beautiful, even sowing depth with that. Some growers may need to slow down.”

  • Effects of crop residue on the soil surface

High crop residue loads on the soil surface mean pre-emergent herbicides are not likely to work as well because they prevent herbicide contact with the seed. More water soluble herbicides cope better with crop residue, but the best solution is to manage crop residue so that at least 50pc of the soil surface is exposed at the time of application

More information: Christopher Preston, 08 8313 7237,


Table 1. Water solubility and binding to soil organic matter for some common pre-emergent herbicides.


Trade name

Water solubility

(at 20°C and neutral pH)

Capacity to bind to organic matter

(in typical neutral soils)



Very low

Very high



Very low

Very high






Avadex Xtra




Boxer Gold (also contains S-metolachlor)












Dual Gold









Very high



Region South, North