Water solubility key to effective pre-emergents
Have you ever put out a pre-emergent herbicide, looked at the final result and wondered what went wrong? The University of Adelaide’s Dr 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.
More water-soluble herbicides will move more readily through the soil profile and are better suited to post-sowing/pre-emergent applications than less water-soluble herbicides. They are also more likely to produce crop damage after heavy rain (Table 1).
“The key facet to getting pre-emergent herbicides to work is to understand their solubility in water,” Dr Preston says. “Greater water solubility also means more mobility in the soil and a higher risk of crop damage if there is 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 five to 10 millimetres of rainfall in the 10 days after sowing is fine for Boxer Gold® but 10 to 15mm is required for Sakura®.”
Binding to soil components
Dr Preston says herbicide movement into the soil profile is strongly influenced by a herbicide’s ability to bind to soil organic matter. Light soils with low organic matter are more likely to have herbicide washing into crop rows and damage 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 says.
“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. Reducing rates in these soils will reduce the risk of crop damage.”
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. 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 placed at or below the weed seed for effective control. The is exception is 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 says. “Some people have experienced issues with pre-emergent herbicide use and disc seeders. The two rules about disc seeders are that 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 per hour and we get beautiful, even sowing depth with that. Some growers, however, 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 50 per cent of the soil surface is exposed at the time of application.
Dr Christopher Preston
08 8313 7109
- Soil behaviour of pre-emergent herbicides in Australian farming systems manual
- GRDC Fact Sheet: Pre-emergent herbicide use
GRDC Project Code UA00014