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A guide to cereal herbicides

Herbicides are still the best way to control weeds, but adding integrated 
weed management practices can give you an edge

There are a wide range of herbicides that can for control weeds in cereals. However, most agronomists agree that, in the long run, herbicides alone are not a silver bullet. Ultimately, a proper integrated weed management (IWM) program is key. IWM is a farming system that incorporates various inter-dependent cultural, biological and chemical weed control practices.

“IWM involves an array of tools including the rotation of available herbicide groups, ensuring that weeds are exposed to a diverse range of control mechanisms,” says Nasir Shaikh, a provincial weed specialist with Manitoba Agriculture, Food, and Rural Initiatives’ (MAFRI’s) Crops Knowledge Centre.

“The principal aim of IWM is to improve the health and vigour of crops so they may out-compete weeds emerging in the stand, … helping reduce selection for resistance to any single control agent and to delay or prevent the development of herbicide resistant weeds.”

Shaikh was quick to point out that, “Practicing IWM doesn’t mean abandoning chemical weed control, just relying on it less exclusively.”

Tips for IWM

Shaikh says there are a variety of ways to move your farm to an IWM system. These are some of the tips Shaikh offers to help give your crops a competitive edge.

1. Vigorous, high quality seed, sown shallow, provides better competition for weeds than poor quality seed or deeply-seeded crops.

2. Early seeded crops (like barley) can get a jump on the weeds, allowing farmers to avoid herbicide applications.

3. Banding nitrogen near the seed can give your crop an advantage over weeds.

4. Adding a short-term forage crop to crop rotations, like short-term alfalfa stands, can reduce wild oat and green foxtail populations by up to 80 per cent during the year after breaking.

Best herbicides

Clark Brenzil is Saskatchewan Agriculture’s provincial weed control specialist. “The desired method is varied, as are cereal crop producers, and will depend on what weeds are predominant in the field.”

Brenzil did a quick comparison of cereal herbicides listed in 2003 verses those in 2013. Ten years ago there were 72 branded herbicides. “Today, we have 152, but the number of herbicide compounds (active ingredients) in those products has only increased from 30 to 31 with four ingredients being lost and five being gained,” says Brenzil.

“Of all the new herbicides introduced over the last 10 years, only one new ingredient has emerged with a unique mode of action (herbicide group) from the previous 30 ingredients in 2003, bringing the number of herbicide resistance groups for use in cereals to nine.”

It is important for farmers to know which herbicide group they’re applying, so they can rotate modes of action (herbicide groups) over time. Brenzil sees this increase in the number of branded products as the biggest challenge for farmers, due to the overwhelming amount of available choices.

“Part of the increase is as a result of the development of convenience packaging or co-packs of tank mix combinations (Product A + Product B placed in a box and called Product C), and the rest of the increase is a result of the entry of generic products to the market.

“Generic products have put downward price pressure on many herbicides, but also add to the number of trade names out there.”

One suggestion Brenzil has for farmers is “to become familiar with the active ingredients in their herbicides (and other pesticides for that matter), bringing the number of things there are to worry about for weed control in wheat from 152 to 31.” (That is, rather than focusing on the 152 branded products, pay attention to the 31 active ingredients.)

“Learn what each ingredient brings to the control equation, and select a set of active ingredients that will meet weed control requirements, then select brands based on those active ingredients (on price point or by how they fit into your bundling deals).”

Managing weed resistance

To manage resistance, Brenzil says the first step is to “select herbicide options starting with the crop in their rotation with the fewest herbicide group options available first.” For many farmers, this would be pulse crops.

Then, progress through your rotation and select herbicides for the rest of your crops. Start with the crops that have the least options. As Brenzil says, “All the while trying to prevent duplication of a herbicide group used with one of the previous crops.”

Crops that often provide the most choice are wheat and barley. “With little option but to duplicate because of their weed spectrum, tank mixing is a way to prevent resistance from gaining a foothold,” says Brenzil.

“Another thing for producers to keep in mind is whether the herbicides they select have a risk of residues carrying over into the following year, injuring sensitive crops. Producers may have to juggle their rotation a bit to make all the pieces fit.”

Using herbicides

Brenzil sees herbicides as safe when used as intended, and says they work well relative to other management options. “The introduction of 2,4-D about 70 years ago was a great revelation for farmers who had major problems with weeds like wild mustard.”

Still, he noted that farmers may tend to rely too heavily on herbicides, “depending on them to carry the whole weed management load and assume they’re always going to work perfectly in all situations. But when you place all of your weed control eggs in the herbicide basket, there’s no net should there be a failure for any reason.”

Although there are many alternatives to herbicides, no single alternative measure on its own will be as effective as herbicides, says Brenzil. “

“The term ‘IWM’ is the combining of various methods for weed management, both cultural and chemical, to achieve the most robust and cost effective weed controls. Several weed scientists describe this as the use of ‘many little hammers.’”

These “little hammers” include agronomic factors, like the four tips listed earlier in this article, and anything else farmers can do to increase crop competitiveness.

“Most of these the producer controls already and really only has to tweak them a bit to optimize them for weed suppression,” said Brenzil. “Integrate all these with your herbicides and maybe you won’t need to buy the premium herbicide for a particular weed.”

For more information about available herbicides, including modes of action for each one, there are “Crop Protection Guides” available for farmers in Saskatchewan, Manitoba and Alberta (Blue Book). These books are published by the provincial governments in each province and are available in print and on-line in government offices and on provincial web sites. †

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