How to use pre-emergent herbicides. 11

What is a pre-emergent herbicide?

A pre-emergent herbicide prevents weeds from gaining a foothold. A post emergent herbicide i.e. sulfonylureas kills weeds when they are already present. The key to using pre-emergents is knowing how they work and behave in the soil. Failing to do this this will lead to poor results.

In the above image, ryegrass is being affected by the pre-emergent diflufenican which is still active in the soil. Our recent turf blog covers split applications of pre-emergents and reasons for pre-emergent failure. 

How do pre-emergent herbicides work?

Pre-emergents have zero effect on weed seeds. They act by preventing germinated weed seedlings from becoming established by forming a barrier at the soil surface. The majority of pre-emergents act on weed roots. In contrast, oxadiazon is the only shoot active pre-emergent for use in turf and this is the active in Echelon® and Echelon Duo®. 

To work properly you must wash the herbicide into the soil. It must also be present when the weed seeds are germinating so soil moisture is vital to allow these to work. 

Are pre emergents safe?

Table showing the modes of action of turf registered pre emergents.

When not to use pre-emergent herbicides.

Factors affecting the successful use of pre-emergent herbicides:

Volatilization.

Table showing the vapour pressure of common pre-emergents.

The persistence of pre-emergent herbicides.

Table showing the Average DT50* values for common pre-emergents.

Not Watering in the Product.

Table showing the solubility of pre-emergent turf herbicides.

Soil properties.

• High sand content soils have little ability to hold onto any chemicals;
• In low organic matter soils less herbicide is likely to become “tied up”;
• Physical movement occurs on slopes and areas of thin turf cover where water movement occurs.

The sorption coefficient (K_oc) is a measure of how much a chemical binds to soils. Chemicals that have a high Koc tend to bind tightly to soil and organic matter i.e. pendimethalin. Low K_oc herbicides i.e. metalochlor are less likley to bind to clay or high organic matter soils and are likely to be mobile.   

So if we rank from those least likely to bind to those with the highest binding: Metalochlor < Dithiopyr < Oxadiazon < Pendimethalin = Prodiamine.

Table showing the average adsorption coefficients for common pre-emergent turf herbicides.

Chemical Breakdown of pre-emergent herbicides.

Incorrect pre-emergent herbicide timing.

Poor timing has a major impact on any results and in my opinion this is the main reason for product failures. So, lets clear something up. If you can see weeds, most of the time its going to be a waste of money applying a pre-emergent.

The table below shows critical soil temperatures for common grass weeds (measured at 10cm depth) courtesy of North Carolina State University.

Because crowsfoot germinates at a higher temperature than summergrass it tends to germinate 2-8 weeks later in the spring and knotweed germinates even earlier. This means that if you are planning make a pre emergent application for crowsfoot a smart move is to make this around 2 weeks after you see summergrass.

Photodegradation

This is the breakdown of chemicals due to the action of sunlight at the soil surface. Yellow coloured herbicides like prodiamine and pendimethalin, can all decay in sunlight if you do not water them in after application.

Getting the best results:

The following are some general tips to give you a better chance of getting great results:

Make sure your equipment is properly calibrated.

Correct calibration is important if you use a granular or liquid as failing to do this means it will be applied unevenly and you’ll get poor results.

Choosing pre-emergents on price.

Buying something purely on price never guarantees good results. Instead, you should select the right product for your situation. There are five factors to consider:

• Firstly what weeds are you trying to control? Different products work better on different weeds and the cheapest option might not even work on your target weed;
• Secondly what is the true cost of application? You should consider factors such as rate, length of control and bag size;
• Thirdly what is your turf type? Not all of these are safe on all turf types. There isn’t much point in buying the cheapest product on the market if its going to kill your turf;
• Next when you make a purchase are you buying it from a supplier who can give you good agronomic advice on how to get the best results?
• Lastly how long do you want it to last? Longer control isn’t always the best if your example you want to seed later etc.

If the actives are the same, look at how easy is it to apply and handle. For instance, both Barricade® herbicide and Onset 10GR® contain the same active but one is a liquid and the other a granule. In the next section there are pros and cons to both of these.

Are Granular or liquid applications best?

Influence of granule size on summergrass control 6 months after treating with pre-emergent herbicides. (Kelly and Coats, 1999)

Successful use of pre-emergent herbicides; Generic vs branded?

​A generic product performs as well as other registered products and if you don’t believe me look at the labels for generic and branded products.

Isomers.

Your left and right hands are isomers as they are the same but one is the mirror image of the other. During the manufacture of metalochlor it creates a blend of four isomers. These all have the same chemical formula but have a different molecular make up.

Pennmag® contains 88% of the S and 12% of the R isomers; Meteor® contains roughly equal amounts of the S and R isomers. Research shows that a higher amount of the S isomer works better. This doesn’t mean that the R isomer doesn’t work but it may not work as well. In fact both the R and combination of R and S formulations have around 50% less activity. Interestingly, on the launch of S metolachlor in the USA, the application rate was 30% less than other metalochlor products. 

Timing their use for spring applications

Microbial activity is at its lowest in the spring due to low cool soil temperatures that exist. This means that ideally applying pre-emergent herbicides earlier than you would generally think will still give excellent results. The reason for this is due to the low level of microbial activity that breaks down these herbicides.

As discussed previously summergrass germinates when soil temperatures at a depth of 100mm are between 12-15°C whilst crowsfoot germinates later when soil temperatures are between 16-18°C. This means that in regions such as Canberra, the Southern Highlands and Melbourne that these can be applied in the autumn at the high rate and will provide season long control of summergrass the next year. This is especially the case if this is followed by a split-application in the spring. 

Mixing pre and post emergent herbicides to delay resistance

A mix of both a pre and a post emergent works better than simply rotating chemistry for slowing down the development of resistance. This approach also:

• Has a larger application window which allows for changes in the timing of winter grass germination due to weather variations;
• Produces a better playing surface and turf quality than applying either alone;
• Increases application flexibility and allows for weather extremes such as :
  • Heavy rainfall that can leach and breakdown herbicides;

Delay weed resistance by changing the chemistry

To avoid the development of herbicide weed resistance you should rotate with products with different modes of action every third or fourth year.

Mixing pre and post emergent herbicides to delay resistance

A mix of both a pre and a post emergent works better than simply rotating chemistry for slowing down the development of resistance. This approach also:

• Has a larger application window which allows for changes in the timing of winter grass germination due to weather variations;
• Produces a better playing surface and turf quality than applying either alone;
• Increases application flexibility and allows for weather extremes such as :
  • Heavy rainfall that can leach and breakdown herbicides;

Using pre-emergents safely.

​Turf safety and seeding interval vary dependant on which pre emergent you use. If the chemical is still active in the soil, any seeding work will fail, so make sure that you carry out any seeding work after the time period listed on the label. This means you need to plan ahead and be aware of any future use.

Table showing pre-emergent safety on turf.

Root pruning

Many pre-emergents have negative effects on root growth which is called “root pruning”. Good examples of these are Barricade® and Specticle®. So if your turf is under stressor it already has a poor root system avoid the use of these as they will only send your turf backwards.

The amount of root pruning that occurs is a direct result of the particular chemical and the turf type.

2018 work into how pre-emergents affect roots on couch grass establishment.

• Specticle® did not achieve a 50% couch cover by the end of the trial;
• Only Ronstar® did not increase the number of days required to reach 50% cover;
• All treatments reduced root mass 6 weeks after treatment (WAT);
• By 10 WAT, Barricade®, pendimethalin, and Specticle® had less root mass;
• At 4 WAT, all treatments other than Ronstar® had shorter roots;
• 10 WAT, only Dimension®, S-metolachlor, and Specticle® had shorter roots;
• The conclusion was that Specticle®, Dimension®, and S-metolachlor are not safe on newly established hybrid couchgrass and should be avoided during establishment.