The Reality of Herbicide Resistance

Written By: Brandi Buhr, Agronomy Lead (East)

Dealing with herbicide resistance can be an incredibly daunting task and many farmers in Saskatchewan, unfortunately, have experience with it first-hand. If there are lessons to be learned from fellow growers in other countries such as the UK, the US and Australia, it's that the thought process around herbicide resistant weeds needs to shift from strictly "dealing with them" to "preventing them". The state of herbicide resistance in Canada has already advanced to the point that we have 60 unique cases across the country. The cost to the agriculture industry in the form of lost yield due to decreased control and greater herbicide use is thought to be well over $1 million yearly. In Saskatchewan alone, we have over 15 unique cases of resistance, most prominently to the Group 1s, 2s and 9s. Two of the most concerning weeds in Saskatchewan, wild oats and kochia, have been found to have multiple resistance across different herbicide groups. In addition to these, growers are battling with Group 2 resistance in cleavers, wild mustard and stinkweed.

The resistance of wild oats to our commonly used herbicides is increasing at a drastic rate across Saskatchewan. Wild oats have shown resistance to Group 1, Group 2 and also multiple resistnce to Group 1 and 2 in the same population. Once a population of wild oats confers resistance to both Group 1 and 2, control options become very limited and are often very expensive.

Kochia is another very concerning case in Saskatchewan agriculture. Kochia has the ability to multiply quickly and spread its seed across a vast area. Across the prairies, Kochia has been found to be resistant to Group 2, 4, 5 and 9, and shows multiple resistance to Groups 2 and 4 as well as 2 and 9. It is often the case where resistance in kochia (or any other resistant weed) tends to sneak up on a grower and by the time it is realized, it is often too late and the yield has been greatly affected. Chart 1, below, illustrates how weed resistance can build up without any alarms being set off and then can instantly become a very real issue. In this particular example, the seedbank of the resistancce biotype multiplies by 14.3x every year. It is not until the fifth year that this actually results in a weed control failure by that particular herbicide. By this point in time, the resistance biotype is already at an alarming 60% of the population. Because of this, it is extremely important to scout fields vigilantly and take record of any weed escapes. 
 

Chart 1: Progression of weed resistance as a logarithmic function.

Treatment	% of Resistant Weeds in Population	Weed Control
0 Application	0.0001	Excellent
1st Application	0.00143	Excellent
2nd Application	0.0205	Excellent
3rd Application	0.294	Excellent
4th Application	4.22	Excellent
5th Application	60.5	Failure

At the moment, the issue of herbicide resistance seems like a manageable problem, and it is! But, we are not that far from a situation where some weeds may become such an issue that management solutions could become impractical or overly expensive. If we look to our Australian counterparts and their fight in resistance in Ryegrass, it should be a wake-up call for us to take preventative action. Growers in Australia are trying to manage fields with Ryegrass that has resistance to so many groups of herbicides that it now costs them upwards of $60/ac every year to control one single weed. There are many research professionals that predict that wild oats in Canada could become the next Ryegrass if management practices are not altered. Imagine what $60/ac to control wild oats every year does to your bottom line. We, as growers and the industry, need to focus on spending a little more effort and money right now to prevent spending a lot more time and money down the road. 

The cold, hard facts on resistance:

• Herbicide resistance is not going away, not in Saskatchewan and not worldwide.

• Resistance cannot be cured, only managed.

• There has not been a new mode of action developed in over 20 years.

• If a new mode of action is ever developed, it may not be economical or even commercially available due to tightening regulations. 

• No one is going to save us with a box of silver bullets!

• The presence of weeds resistant to multiple modes of action could very well result in the inability to grow certain crops (imagine not having lentils as a cropping option in your rotation).

What can we do?​

There are many management practices that we can adopt to ensure the sustainability of our farming operations. Some of the most important ones are:

• Tank-mix two or more effective modes of action for the target weed. Having two effective modes of action is essential in that each separate product should have the ability to control the target weed on its own. Mixing these two products together reduces the selection pressure on a specific weed because it is being controlled in two different ways.

• Do not spray glyphosate alone wherever possible.

• Have as diverse of a crop rotation as possible.

• Use different modes of action throughout the season and from year to year.

• Implement the use of fall-applied residual wild oat products such as Focus and Avadex.

• Scout fields vigilantly and take note of any weed escapes.

• Keep excellent records and look back on those records!

• Use herbicides correctly! It is well known that spraying herbicides at cut rates leads to the selection of herbicide resistant weeds. Cutting rates may not always be an "acres/jug" choice; it may be due to spraying with incorrect water volumes, travelling at too great of speeds, using a poor water source, or spraying in poor environmental conditions. All of these factors can essentially cut the rate of your applied herbicide and lead to greater resistance.

• Treat any unusual survivng weeds as resistance. Get rid of them by any means possible to prevent the addition of those seeds to the seedbank.

• Have a solid agronomic crop plan to grow a strong, healthy, high yielding crop. The BEST herbicide available is a competitve crop.

Herbicide resistance needs to be one of the most important aspects in our overall farm management. The specific weed species discussed here are only a drop in the hat in regards to the vast amount of other weeds that are of concern and need to be managed on a yearly basis. Being proactive and stopping resistant weeds in their tracks should be a priority. This means having knowledge on why certain weeds seem to adapt very quickly to herbicide use and why certain modes of action you are using and how often you are using them. It means having an understanding of how the different modes of action work on a target weed and how that relates to the various herbicide options. Please don't hesitate to contact your local G-Mac's AgTeam ACE or Agronomist to find out how you can implement a herbicide resistance management program on your farm.