Skip to main contentSkip to footer

Weed Management

Bayer Crop Science

Dealing with weed survivors

Article overview

Interview with Stephen Moss about why some weeds survive herbicide applications and how to ensure good control across the programme.


Herbicides rarely achieve 100% control, usually there are some survivors. Controlling these survivors is important for successful long-term weed control. Stephen Moss shared his thoughts on survivors and the practical steps farmers can take to control weeds.

 

1. Why do some weeds survive herbicide applications despite being susceptible?

This is a ‘how long is a piece of string question’ there are all sorts of possible reasons why a weed could survive. Resistance is of course one possibility, but farmers and agronomists need to look at all reasons why some weeds have survived.

In post-em herbicides, incorrect conditions for uptake, poor application technique and interception by the crop or other weeds are common problems. Residual herbicides can be affected by soil organic matter, surface trash or rainfall. You might think that more weeds would survive residuals because of the way they work in the soil, but control is generally good.

Several factors may be operating at once to reduce control. Also, it seems obvious to say but any weed needs to be susceptible to the herbicide at the growth stage it is applied.

 

2. Are survivors more likely to carry genes that confer resistance?

It doesn’t automatically follow that survivors are resistant but they could be, you need to consider other factors. Speedwell for example has no known incidence of resistance so if it survived you would assume it is for some other reason. Weeds with known resistance problems like ryegrassblack-grass or poppies need to be investigated in more detail. If it happens repeatedly and only one susceptible species survives while others are controlled effectively, resistance is a strong possibility

 

 3. Are weeds that survive an application of one type of herbicide likely to have cross resistance to others?

Cross resistance is not something we fully understand and sometimes the patterns can be odd. For example, some years ago we found cross resistance to pendimethalin and chlortoluron which are different modes of action but not pendimethalin and trifluralin which are the same mode of action.

It seems to be the case that if a weed has enhanced metabolism resistance (EMR) to one herbicide it is likely to have EMR to some other herbicides, although the impact on herbicide efficacy varies considerably. Also, there appear to be many mechanisms for EMR, and there is still a lot to learn about these. For example, over the years we have found black-grass populations with unusual cross-resistance patterns but the reasons for these are still poorly understood.

Worldwide, there are more mechanisms of resistance to glyphosate than for any other herbicide.  But this is probably because it is the most widely used and studied herbicide. There is no reason to think other herbicides aren’t overcome by many diverse mechanisms yet to be discovered.

I also think it is worth noting that, in the field, herbicide resistant populations don’t seem to suffer a notable fitness penalty. Resistance does not rapidly disappear if you stop using herbicides. From a practical perspective, if it is growing in dense patches in your field then there is no significant fitness penalty.

 

4. How much of a role do environmental factors, particularly ‘winter-kill’ have in determining how many weeds remain after the autumn pre-em programme?

Winter kill does have a role to play in killing grass weeds. In my experience with grass-weeds, this is more to do with the effect of frost heave exposing the roots rather than the direct effect of cold. There doesn’t tend to be the deep sustained cold needed to kill weeds like black-grass and ryegrass although there are many weeds (e.g. charlock) that are sensitive to the frosts we have in the UK. Milder winters mean that more weeds of all type survive and even grow during winter making spring control more of a challenge.

 

5. How can farmers effectively control surviving weeds in spring?

Spring emergence is often talked about for ryegrass, black grass and brome, but I think in some cases these are likely to be autumn germinating plants which are very small, so they are not noticed until spring. It is important that all autumn germinating weeds are killed effectively before sowing spring crops as otherwise some may simply re-emerge with the crop and I am sure these are sometimes wrongly considered as spring emerging weeds.

There is lots of scope for resistance testing surviving weeds, I think resistance testing concentrates the mind and reduces the amount of guesswork.

Controlling patches of weeds with glyphosate is definitely worthwhile as patches seem to persist year-to-year. I would not expect cost to be a major factor in patch management because, unless we are talking about spraying off whole fields, a few patches don’t add up to a huge amount of additional cost in glyphosate

In spring crops, it is important to spray off properly ahead of sowing. There is a temptation to lower rates of glyphosate or apply too quickly but it’s a mistake. Also, it could be the job given to the inexperienced operator. Applied properly, glyphosate can give near 100% control, so it’s worth taking the time to achieve this.


Discover more in our Insights