The 5 W’s of Granular Herbicides

Sep 04, 2019

By Brandi Buhr

WHY use granular herbicides?
Granular herbicides have slowly become less common over the years due to factors such as minimum tillage practices, the efficacy of new herbicide products on the market, and the ease of use of liquid herbicides.  In spite of this, there are many reasons why growers should consider re- incorporating the use of pre-emergent granular herbicides into their herbicide rotations. 
One of the biggest issues beginning to face farmers is the presence of herbicide resistant weeds.  Wild oat, millet and kochia are all weeds that have been found to show resistance to one or more herbicide groups in west-central Saskatchewan.  The use of granular herbicides that belong to groups 3 and/or 8 can provide assistance in the battle against these herbicide resistant weeds and ease reliance on other heavily used herbicide groups such as 1, 2 and 9.  By removing such weeds as wild oats before they ever emerge, resistance selection pressure is reduced for post-emergent modes of action.  Another reason why growers may consider adopting the use of granular herbicides is that it can allow them to spread out their workload by getting the herbicide applied in the fall, this also allows growers to take advantage of the residual activity on flushing weeds that these herbicides offer.
WHAT are the product options?
The most common products that are utilized in G-Mac’s Country are Ethalfuralin (Edge), Triallate (Avadex), Trifuralin (Rival, Treflan or Bonanza) and a combination Trifuralin/Triallate product (Fortress).
Avadex is a great tool to battle any kind of wild oat problem.  Whether it is a very heavy infestation that keeps on flushing throughout the season or a Group 1 resistance problem, Avadex can help to keep those populations under control.  Being a group 8 herbicide, it is an effective addition to herbicide rotations and can be used before seeding almost any crop. After experiencing such a wet season in 2016, Avadex will be an important product to keep in mind for this coming fall, as wild oat populations will be very heavy going into next year.
Edge is the old go-to product that is commonly applied ahead of lentils, but it can also be used pre-seed canola and peas.  Edge is a group 3 herbicide that offers residual control of such weeds as kochia, wild buckwheat, green foxtail, barnyard grass and also gives suppression of wild oats. Many of these weeds can be difficult to manage with the options currently available for in-crop herbicides, so using Edge can give the additional control that may be required.  Having the group 3 chemistry in the rotation will provide assistance in preventing and managing Group 1 and 2 resistant wild oats and green foxtail and also group 9 resistant kochia.
Trifuralin products are very versatile in that they are registered for use before seeding many different crops, especially if used as a fall application.  These Group 3 herbicide product can be applied where seeding lentils, peas, wheat and durum.  The biggest strength of Trifuralin is its residual control of green foxtail in situations where the populations are extremely high and where it is impossible to control all of the flushes with a single application of an in-crop herbicide.  It also has some control of barnyard grass and Japanese brome as well as suppression of wild oats and several broadleaf weeds.
Fortress is a combination of the active ingredients Triallate and Trifuralin, making it a Group 3 and 8. It can be applied before seeding such crops as wheat, durum, barley, canola and mustard.  Fortress provides the green foxtail control and suppression of several broadleaves that Trifuralin will give, along with the added wild oat control that Triallate offers.
WHEN is the best time to apply?
Fall applications generally provide the best crop safety and weed control.  All of these granular products need warm soil temperatures, along with a moisture event in order to activate.  They can take anywhere from 10 to 14 days after application to be activated (much longer if there is never a rainfall event).  Since we live in an area where early spring rainfall is not a guarantee, it can result in spring applications that are sometimes not as effective as a fall applications.  By applying late in the fall, when soil temperatures are below 5oC to avoid activation in the fall, there is a better chance of the product activating the following spring near the time of weed germination.
WHERE should they be applied?
The goal of using a pre-emergent granular herbicide is to create a solid barrier in the top 1-2 inches of soil which will prevent germinating weeds from emerging. The crop should be seeded below the herbicide layer to prevent any crop injury. It is best to avoid application to soils with very low organic matter or a large amount of eroded hill tops.  Weed control occurs when germinating weeds grow toward the soil surface and come in contact with the active layer of herbicide.  Because of the difficulty of getting a perfect layer with these types of products, weed control is usually in the 70-85% range.

Heavy trash is one of the biggest problems a grower needs to consider in order to get good weed control.  Granular herbicides have the tendency to get hung up in heavy residue and never reach the soil surface.  If the herbicide does not come in contact with the soil particles, it can never be activated.  Harrowing, or some other form of incorporation, is highly recommended to ensure proper herbicide-to-soil contact. The specific recommended application times, incorporation practices and rates for each product vary slightly and will depend on planned crop, amount of residue, organic matter, type of soil and cropping system.  

Figure 1. Wild oat control Avadex vs. UTC (Gowan Canada, 2016)

WHO should be using granular herbicides?
It is important for every grower to consider all possible options to expand their herbicide rotations.  Granular herbicides are a great solution for adding different herbicide groups as well as helping to prevent and manage resistant populations of weeds.  They are just one more tool to add to a complete farm management program.

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