Published on 22nd August 2023
Actives for Autumn: Choosing Effective Herbicides for Cereal Weed Control
There are many herbicides for farmers to choose from when planning autumn weed control in cereals. The best option will vary from farm-to-farm but the principles behind the choice don’t change. Here are some important things to consider when choosing actives for autumn.
The recent Bayer Mode of Action Survey 2023 showed that farmers typically use 3–4 modes of action for weed control in wheat across the pre-em and post-em programme (See Chart 1). But it’s important to remember that some of most important actives are in the same Herbicide Resistance Action Committee (HRAC) groups.
Chart 1: Number of Modes of Action Used in Typical Wheat Weed Control Programme (Excluding Glyphosate Pre-Drilling)
Group 15 includes flufenacet, tri-allate, prosulfocarb and ethofumesate; Group 12 has picolinafen and diflufenican; and most post-em chemistry for grass-weeds and broad-leaved weeds is Group 1 or 2. Another question in the survey asked which actives farmers used over the last three seasons. The most used actives were glyphosate, flufenacet, pendimethalin and diflufenican (see Chart 2).
“The breakdown of which actives farmers are using doesn’t contain any great surprises, but it does highlight some risks and opportunities,” says Bayer’s Tom Chillcott. “Most obvious is Group 15, all those four actives still have a lot to offer and from an efficacy standpoint can complement each other well. But using them in the same programme isn’t sound resistance management. Make sure there are other modes of action alongside Group 15 actives in the program.”
“Metribuzin is an opportunity it’s the only HRAC Group 5 available in cereals. It’s sold as Alternator Met and Octavian Met in a co-form with flufenacet + diflufenican. But has been a niche product since launch mainly because of the label. That’s changed this year and the full 1 litre rate can now be used throughout autumn.”
“The new chemistry, aclonifen and cinmethylin, is both an opportunity and a risk. Our trials show that both can give a step up to the herbicide programme. But a new active isn’t an excuse to ease up cultural controls or rotating actives, our aim has to be to keep all this chemistry effective for as long as possible.”
Chart 2: Actives Used Over the Last 3 Seasons (Number of Users)
Target Weeds and Tailored Strategies
“The first thing to consider is what are you trying to control, very rarely are you trying to control just one weed, there’s usually a mixture of them. Then look at the actives you could use in your programme for those weeds,” says Bayer’s Jamie Oakley.
Black-grass is the biggest single problem, but Italian ryegrass, brome and wild oats cause difficulties for many. Advice for chemical and cultural controls is typically based on black-grass control but that doesn’t always translate to other weeds.
“Delayed drilling works very well against black-grass because the main flush is concentrated in early/mid-October. Ryegrass germination is more spread out, but the peak is around the same time as black-grass, so we still expect decent results from delayed drilling.
“Brome is more complicated; sterile brome and soft brome tend to germinate in September into October so delayed drilling once again works. Rye, meadow and great brome germinate later, peaking in November so unless you drill very late, after sugar beet for example, herbicides are the main control tool in wheat, so a top-up spray with good longevity is needed.”
Mr Oakley points out that performance of actives varies by weed. Prosulfocarb seems particularly suited to ryegrass control and tri-allate is a good option where wild oats are a problem. Flufenacet is still a very good active for black-grass control as long as there is moisture in the seedbed. Aclonifen still performs in drier conditions so is a good partner for Liberator to cope with all conditions.
Balancing Efficacy and Cost
The overall efficacy of the programme versus the cost is the most important factor for farmers. With so many situations and combinations of products there is no single correct choice. Added to that, it is best to vary programmes between seasons for resistance management.
“Liberator plus Proclus is a solid base; it has good efficacy on all grass-weeds and many broad-leaved weeds as well. It immediately gives you three different modes of action in the programme, so it ticks a lot of boxes.”
“Adding more actives on top whether it is in the metribuzin co-forms, prosulfocarb, tri-allate or pendimethalin can all improve efficacy but also cost. It’s all about finding the right balance.”
Mr Oakley points out that cinmethylin is another strong contender, but its cost does force you to put a lot of faith in one active rather than a diverse programme. But it is a different mode of action so will help with long-term resistance management.
Full details of product performance can be found here.
Importance of Timely Application
Correct timing makes a huge difference to pre-em performance. The standard guidance is that applying at the true pre-em timing within 48 hours of drilling gives more consistent results.
In the field, things aren’t always so simple, especially when delaying drilling into October. Farmers often prioritise drilling, so the pre-em gets delayed. “It’s a challenge to get everything done: Drill, probably roll, then spray all within 48 hours when there are many other fields waiting for the same.”
“Have you got the tractors or the drivers to do it all at short notice? And farms with outlying land often have to compromise somewhere. But a one-week gap between drilling and pre-em is too long really, anything less than that is probably ok, but it is best to aim for 48 hours.”
If you know delays are likely, product choice can help, in trials coforms of metribuzin + flufenacet + diflufenican has much less drop off in control compared to straight Liberator. Tri-allate also needs careful consideration if you are relying on a contractor with an applicator to apply it. Is the potential for a delay in application worth it compared with an alternative you know you can apply at the correct time?
Ensuring Physical Compatibility and Crop Safety
For pre-em herbicides, physical compatibility between different products is not usually a concern but crop safety is. The more chemistry applied, the more chance of crop effects. Depending on the actives in the mix, 3–4 actives is a sensible guideline for wheat tank-mixes; 2–3 in barley. However, conditions make a big difference to what is safe.
“First of all, it’s soil type, you are less likely to get issues on loamy or heavy soil. If you’ve got light stony ground that’s a red flag for me. If you can apply them closer drilling, you’re less likely to get issues. Leaving it just before it’s about to pop through the ground is high risk.”
Good firm seedbeds with consistent drilling depth between 3cm–3.5cm is ideal and puts the germinating crop out of the range of soil mobile herbicides. Make sure there is no heavy rainfall forecast soon after spraying as this can move chemistry through the soil to the crop.
“Even where we see some crop effects, they are usually transitory and don’t have any noticeable effect come harvest, but we understand no one wants to take chances. If you are in doubt, sequence rather stack. Put the main part of the programme on at the true pre-em timing and follow up with different chemistry 2–4 weeks later. Sequencing also helps to extend the duration of protection so there aren’t any real downsides.”