The role of pre-emergent herbicides in integrated weed management

Take home messages

• Pre-emergent herbicides are an important part of weed management due to resistance in grass weeds to post-emergent herbicides.
• Pre-emergent herbicides are more difficult to get right with moisture conditions and soil type being important factors to consider.
• Pre-emergent herbicides are more challenging to use with disc seeding systems and some soil displacement from above the crop row is often required for better crop safety.

Herbicide resistance in weeds

Post-emergent herbicides are the easiest option to use for weed control. They can be chosen based on the weeds that are present in the field, can be applied to existing weeds and there are only a few situations that will reduce their performance. As a result, there has been significant reliance on post-emergent herbicides for weed control. Unfortunately, this reliance has resulted in the selection of resistant weed populations. Across much of southern Australia there are now high levels of resistance to Group A and Group B herbicides in annual ryegrass (Table 1). This has resulted in more emphasis on pre-emergent herbicides and other weed management practices for control of this weed.

Table 1. Resistance to various herbicides in annual ryegrass populations from random surveys in Victoria and NSW. Samples were considered resistant to a herbicide if more than 20% of individuals survived the herbicide application.

Getting pre-emergent herbicides to work

Pre-emergent herbicides are a lot more challenging to use than post-emergent herbicides. Firstly, there is a need to predict the weeds that will be present in the paddock when the herbicide is chosen. In addition, numerous factors influence the performance of pre-emergent herbicides. The aim with pre-emergent herbicides is to get them at or below the level of the weed seeds so they can be taken up by the germinating weeds. This is easier with no-till systems where most weed seeds remain close to the soil surface than it is with tilled systems where soil disturbance buries weed seeds. The deeper the soil disturbance, the deeper the weed seeds will be buried. Buried weed seeds also tend to last longer in the soil and can cause problems in future years. Bed or furrow forming operations will bury weed seeds and this must be considered when choosing which pre-emergent herbicides to use.

Two of the key aspects of pre-emergent herbicides that are important for their performance are their water solubility and their ability to bind to organic matter in the soil. Table 2 gives a list of water solubility and binding values for common pre-emergent herbicides. Herbicides range in water solubility from the almost insoluble trifluralin to the very soluble S-metolachlor. Herbicide solubility influences how much the herbicide will move through the soil with rainfall or irrigation, but it is not the only factor of importance.

Table 2. Solubility and binding to organic matter (KOC) of some pre-emergent herbicides used in Australian grain production

As the herbicide moves through the soil, it will interact with soil particles and organic matter. Herbicides that have higher binding to organic matter will move more slowly through soils than those with lower binding to organic matter. Therefore, the ability of the herbicide to bind to organic matter must also be considered. Herbicides that have lower binding to organic matter will tend to move further through the profile than those tightly bound to organic matter. Herbicides will also move more quickly through soils with lower organic matter than those with higher organic matter contents.

Water also moves more quickly through sandy soils than it will through clay soils due to the larger particle size and bigger spaces between particles in sandy soils compared with clay soils (Figure 1). This means pre-emergent herbicides tend to be more mobile in lighter soils than heavier soils.

Figure 1. Diagram of the particle size of sandy soils compared to silt and clay soils. Water and herbicides will move further and faster through sandy soils due to the larger pore sizes.

A final consideration regarding movement of herbicides in soil is that movement can be different depending on whether the soil was previously dry or had some moisture close to the surface. When rain or irrigation occurs on previously dry soil, the water will move more quickly through the surface layer than if there was previously some moisture in the surface layer. This means that the first rainfall/irrigation event on dry soils can solubilise higher amounts of herbicide and move it further in the soil, than if the soil was already moist.

Considering the herbicides listed in Table 2, metazachlor (Butisan®) has high water solubility and low binding to organic matter. This herbicide will move furthest and most quickly through the soil. At the opposite end, trifluralin has very low water solubility and very high binding to organic matter and will have the least movement. The importance of movement in the soil is that a little movement can be good in moving the herbicide into the root zone of the weeds, however a lot of movement can cause problems. If the herbicide is moved below the root zone of the germinating weeds, they may not be controlled. In addition, too much herbicide movement can mean herbicide accumulates in the root zone of the crop causing crop damage.

Pre-emergent herbicides and irrigation

Using pre-emergent herbicides in irrigated crops provides its own special problems. Typically, irrigation systems provide more water at a faster rate than does rainfall, which risks moving the herbicide too far through the soil profile. Typically, 5 to 10mm is required to activate pre-emergent herbicides with higher water solubility and 15 to 20mm for those with lower water solubility. Applying herbicides to dry ground and then watering can increase the risk of crop damage, so it is better to water up before putting the herbicide out and then apply 5 to 10mm of water afterwards to incorporate the herbicide. Overhead sprinkler systems are recommended for herbicide incorporation by irrigation and furrow irrigation should be generally avoided. Water distribution in furrow irrigation systems is uneven and too much wetting can occur in the bottom of the furrows pushing the herbicide deeper into the soil profile and not enough on the tops of the furrows to activate the herbicide.

Pre-emergent herbicides and disc seeders

Some pre-emergent herbicides are not registered for use in disc seeders. This may be due to crop safety concerns or because of herbicide incorporation requirements. Where products are registered for use in disc seeders, there can be variable weed control and crop safety depending on the type of disc seeder used.

Disc seeders that provide lower soil disturbance at seeding will have a higher risk of crop damage as the herbicide remains in close proximity to the crop seed, for example with single disc machines. Depending on how they are set up, it is common for disc seeders to sow the crop relatively shallow, increasing the risk of crop damage from the herbicide. Addition of a wavy coulter or residue manager in front of the seeding disc to move herbicide away from the sowing row is often enough to improve crop safety. Disc seeders are typically poor at incorporating herbicides, so if herbicide incorporation is important for the activity of the herbicide, weed control can be less effective when disc seeders are used.

Pre-emergent herbicides and better weed management

Pre-emergent herbicides can play a key role in weed management. Pre-emergent herbicides reduce weed competition early in the crop when the crop is most susceptible to weed competition. This helps to maximise grain yield.

However, in high rainfall situations pre-emergent herbicides tend to degrade well before the end of the season. This makes relying on pre-emergent herbicides alone for weed control impractical. Other practices must be included to ensure weed populations are controlled.

Crop competition is a valuable partner with pre-emergent herbicides, reducing the growth and seed production of later emerging weeds that have escaped the pre-emergent herbicide treatment. Crop competition can be improved by changing to a more competitive crop type or cultivar, increasing seeding rates, reducing row spacing or sowing earlier. Crop competition works best with cereal crops and canola.

Finally, reducing the seed set of weeds through crop topping or harvest weed seed control is the third component of weed management that ensures weed populations are reduced in the following years. Our research has shown that the combination of three good weed control practices every year will run down populations of grass weeds. Employing pre-emergent herbicides, crop competition and harvest weed seed control will achieve this.

Useful resources

Soil behaviour of pre-emergent herbicides: A reference manual for agronomic advisers

Managing Resistant Ryegrass in Canola with Crop Competition and Pre-emergent Herbicides

Disc Seeding Systems & Pre-emergent Herbicides

Acknowledgements

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC, the author would like to thank them for their continued support. Thanks to John Broster of Charles Sturt University for sharing data on annual ryegrass resistance in NSW.

Contact details

Christopher Preston
University of Adelaide
0488 404 120
christopher.preston@adelaide.edu.au