Planning autumn herbicide programmes to control grass weeds and broad-leaved weeds

Written by Hugh Guinan, Field Technical Manager

It is tempting to think that once drilling is complete and a pre-emergence herbicide has been applied, you can shut the gate and not think about weeds again until the spring.

Unfortunately, this is not the case for cereal growers who face the annual challenge of grass weeds and broad-leaved weeds.

At the time of writing, farmers are at various stages in planting this year’s crop – some have barely started while others may have even parked their drills up for the year having done all they planned to do.

But regardless of the state of play on your farm, the importance of controlling weeds before they steal yield, cause problems at harvest, or hamper your future crop remains the same.

Challenges occur of course; over the past two seasons we have seen significant issues with both drilling crops and getting sprayers to travel, and it is no surprise to see plenty of grass weeds above the crop throughout the spring when this occurs.

Setting out with a plan, using cultural and non-chemical controls, and balancing crop growth stage with the appropriate herbicide applications can really help stamp out weed populations in your cereal crops.

A simple guide to planning an autumn herbicide programme in cereal crops

What are your target weeds? Identify the species you are trying to control.
Why does it need controlling? The most important weeds are those which are going to affect your yield, cause problems later in the season, or hamper crop quality.
When do the weeds need controlling? Some weeds are best controlled early in the autumn before they get the chance to establish and compete, while others can wait.
What chemical options do you have? Assess the strengths and weaknesses of each product, know when they are most effective and what partners can go in the tank.
Do you know your timings? Work out whether your best approach is to apply pre-emergence, peri-emergence or post emergence.
Can you use alternative modes of action? Mixing actives increases efficacy, limits selection pressure and supports resistance management.
Is a sequence possible? Combining a pre-emergence spray with further top up residual sprays can boost efficacy across the autumn and spring.

Why integrated weed management in cereal crops starts in the summer

Chemical control of weeds is only part of the solution.

Adopting an integrated weed management (IWM) approach helps prevent weed establishment in its tracks.

This is a blend of cultural control, non-chemical control, and chemical control used together and has the benefit of being environmentally sustainable and financially efficient while achieving high levels of long-term weed control.

This strategy starts during and immediately after harvest via prevention of seed spread, rotation diversity, stubble management, targeted cultivations, delayed drilling, considered seed rates, stale seed beds and much more.

Options for growers once the crop has established becomes more limited and this is where chemical control plays its part in IWM.

What should I consider for my autumn cereal herbicide programme?

Most growers and their agronomists will be familiar with the weeds that tend to present a problem annually. Any good cereal herbicide programme starts with the target weeds in mind.

Italian ryegrass and bromes are two of the major burdens Corteva’s field support team is asked about every autumn and spring. They are also becoming harder to control on many farms.

As a typical starting point, a robust pre-emergence herbicide will be followed up with one or two contact sprays to maintain efficacy throughout the autumn and early spring. This approach ‘tops up’ the residual levels rather than relying on a single hit at the start. This is especially important in early-drilled crops.

What’s the best way to control Italian ryegrass in cereal crops?

Italian ryegrass is a key target for cereal growers because it’s highly competitive, extremely adaptable and resilient as well as being at a considerable risk for additional herbicide resistance development.

Plant for plant, it is much more competitive than blackgrass so can carry a hefty yield penalty, and also produces up to four times’ more seed.

Herbicide programmes targeting Italian ryegrass should be watertight.

This starts with an autumn residual and Corteva trials have found cinmethylin-based products to be the most effective active to base the program around. Other key actives include aclonifen, flufenacet (specific to population sensitivity), pendimethalin and prosulfocarb.

The best control was achieved from a sequenced approach, associated with ryegrass’s prolonged emergence patterns, based on a pre-emergence application with one or two top up applications throughout the autumn or spring.

The best of these included a well-timed autumn application of a pyroxsulam-containing product such as Broadway® Star. Timed at the one- to three-leaf stage of the Italian ryegrass gave optimum efficacy, governed by ensuring active growth and good spray conditions around application.

Pyroxsulam can also be spring applied to small, actively growing plants and can be tank mixed with residuals such as pendimethalin for dual control.

Timing is key, and a balance must be struck between autumn and spring control. AHDB studies suggest autumn emerging plants produce 23 times as much seed as those emerging in the spring.

A well-timed autumn application would achieve better results than a poorly applied application in the spring.

Removing weed competition early and boosting crop competitiveness and biomass can also reduce light-stimulated germination within the canopy in the spring – further reducing numbers when compared to a poor spring timing where efficacy may be reduced by way of weather setbacks, large weeds, variable temperatures and shading of the weeds at application.

How do I control brome in my cereal crops?

Bromes are problematic, highly competitive grassweeds with good tillering ability, wide emergence patterns (depending on species) and good resilience to abiotic stressors.

Much like ryegrass, bromes are an increasing problem in the UK and Ireland, driven by increased use of min-till and no-till systems, close crop rotations and limited herbicide options.

There are five main species – more information on these can be found here.

Yield loss from brome results from direct competition but, due to the lack of control options in other crops such as barley and oats, they can be a challenge across the rotation.

One reason herbicide efficacy can be reduced is due to the plant’s physiology, the most common species have leaf hairs, varying levels of leaf waxiness and challenging leaf architecture in some cases, all making getting the herbicide through the cell wall to the target site more difficult.

Corteva trials have shown that the best control comes from a two-to-three-spray programme across the autumn, targeting emerged brome when small, optimally when the weed has between one and three leaves.

Key residual actives for control of bromes include flufenacet, triallate (pre-only) and pendimethalin. For emerged bromes, pyroxsulam should be partnered with pendimethalin and applied as soon as application conditions are conducive to active weed growth.

Insights and updates will be shared throughout the season.
For further information, please contact our technical hotline on 0800 689 8899, e-mail: ukhotline@corteva.com or send us an enquiry using this online form.