How to create an integrated weed management strategy for grassweeds in cereals

Grassweeds, such as blackgrass but also increasingly ryegrass and bromes, are probably the main agronomic threat to cereal production in the UK.

Without a good control strategy, over time they can increase to levels that threaten the economic viability of crop production.

For many, the main control strategy has been the use of herbicides. The most recent pesticide usage survey in 2016 suggested over 98% of wheat crops received a herbicide. That’s unlikely to have changed much in the intervening years.

But reliance just on herbicides to control weeds is fraught with danger. History has shown that grassweeds are incredibly adaptable and with active ingredients both becoming more difficult to register for use and being removed from the market, the remaining ones are even more at risk from grassweeds developing resistant to them.

That increasing difficulty to control grassweeds with chemicals has been the primary driver for growers to consider and use non-chemical methods of control.

In a lot of cases incorporating those alternatives has coincided with improved grassweed control – and is now commonly cited as the key to controlling blackgrass especially.

But it just highlights that integrated weed management is usually the key to long-term sustainable weed control programmes.

What is integrated weed management?

At its simplest integrated weed management is about using multiple methods of controlling weeds, including cultural, genetic, mechanical, biological and chemical controls, rather than just relying on one method alone.

In reality, for most that means reducing the reliance on herbicides by integrating a wide range of cultural control options including cultivations, drilling date, cropping choice, mechanical weed control and other physical controls.

What do you need to know to put an integrated weed management plan together?

Understanding a weed’s biology and life cycle – a weed’s seasonal pattern of growth and reproduction – is perhaps the most important starting point for an integrated weed management plant, after knowing what weeds you’re trying to control.

Within the life cycle there are generally five potential ways to control weeds:

1. By preventing seed return

This is crucial for grassweeds, which produce high levels of seed and can establish large viable seedbanks in one season.

Example control measures that can help prevent seed return include the use of glyphosate to aggressively target blackgrass patches in early June and harvest weed seed management such as cage mills retrofitted on combines to pulverise ryegrass seed.

2. By depleting the seedbank

The seedbank is the seeds in the soil resulting from seeding in previous years. Seed numbers can decrease in time as they germinate, decay or are eaten by wildlife, but some buried seed can survive for many years.

Understanding this dynamic is crucial – it’s both possible to deplete the seed bank by good management and make it worse.

For example, ploughing can be a good tactic to reduce grassweeds as it will bury seeds to a depth from where they are unable to germinate, but can also be a poor tactic if done too often and weed seeds that had been buried are brought back to the surface and are able to germinate.

Other examples of control tactics that will help deplete seedbanks include stale seedbeds and delayed drilling, which encourage weed seeds to germinate and then be destroyed either mechanically or with glyphosate before the crop is drilled.

3. By killing weed seedlings

Knowing when weed seeds will emerge can help determine the most effective control methods. For example, blackgrass typically germinates in the autumn and means that delaying planting a crop until the spring can help reduce the amount of germination in the crop.

As weeds grow, they will compete with the crop, but the damage this causes depends on the species, the density of weed, the competitive ability of the crop and the growth stage when the crop and weeds compete.

Some weeds might be highly competitive, while others pose little threat and can be left uncontrolled and may be valuable for wildlife. Most grassweeds fall into the highly competitive segment.

4. By stopping seed set

While by this stage weeds may have competed with the crop, as with preventing seed return, preventing seed set reduces weed seed production and in turn reduces the seedbank for future years.

This matters most with weeds that are difficult to control, such as grassweeds resistant to herbicides, and is easiest when weed populations are low. Hand rogueing, for example, can be a crucial tactic to prevent early-stage infestations from becoming a larger problem.

5. By applying good on-farm hygiene

Stopping weed seeds arriving on farm through good hygiene, for example on machinery, in seed, straw, compost or sewage sludge is a key step in managing weed spread.

There’s plenty of evidence that machinery has been a key factor in the spread of blackgrass, so for example insisting contractors blow down combines or balers before coming onto your farm is good practice.

The same applies to when moving machinery from a heavily infested field to prevent a weed problem spreading from field to field.

So why does this matter? Part one of building any good integrated weed management plan is considering your target weeds life cycle and how you can use as many of those opportunities to disrupt its ability to be successful and spread. If you can target weeds at more than one stage during the season, there’s a greater chance of a sustainable strategy.

What types of tactics are available to control weeds?

While herbicides are by far the most common form of weed control, and particularly for grassweeds, used proactively rather than reactively – e.g. pre-emergence rather than post-emergence, there are a surprisingly large number of alternative tactics that can be used.

But unlike herbicides where if a weed is sensitive, and for grassweeds that is obviously a big ‘if’, control can be close to 100%, most other weed control approaches need to be integrated with a good knowledge of weed biology to be successful.

The 2019 AHDB ‘Research Review: Weed control options and future opportunities for UK crops’ (PDF) breaks down weed control tactics into seven distinct types: cultural, non-chemical, chemical, novel and emerging technologies, digital tools, genetic tools and preventative weed control.

In total the report describes over 50 different potential tactics that could be used, ranging from the common such as existing chemistry, rotation, drilling date and cultivations to emerging ideas, such as remote sensing and CRISPR technology.

Building a good integrated weed management plan will use as many of these as required to diversify weed management and reduce reliance on herbicides. Where possible IWM will also promote the use of site-specific weed management and target applications to reduce herbicide impacts.

It’s also important over time to diversify the control programme so you’re not relying on the same strategy over and over again, as that’s likely to also favour particular weeds, or lead to a weed adapting its biology to prosper.

A diverse rotation, including different times of drilling will help minimise the ability of a weed to adapt, but repeated spring cropping, for example, over time could allow a particular weed to flourish.

In the short- and medium-term it is very likely that weed control strategies for conventional growers will continue to use herbicides in some form, however. They are convenient, easy-to-use and generally cost effective.

How to maintain the effectiveness of herbicides?

The easiest way to maintain the effectiveness of a herbicide is not to use it. While that’s not always realistic, it does point to a key tenet of protecting them – minimising their use. But once a decision has been made to use them, ideally on as low a weed population as possible, it is also crucial to maximise herbicide performance.

For grassweeds, timeliness is key – most herbicides are more effective when targeting small weeds.

Application technique is important – herbicide performance can be significantly affected by poor application. So consideration should be given to water volumes, droplet size, forward speed and boom height to maximise the amount of active substance hitting the target, while also controlling spray drift.

For grassweeds in cereal crops, mixing and using an appropriate dose of several products together and / or sequencing products in close succession gives more effective control than individual products.

The use of a variety of mode of actions – the way the herbicide controls susceptible plants – is also the foundation of resistance management.

Resistance is the other main way herbicide effectiveness diminishes, and unlike application technique is not reversible once it is present in the population.

It occurs when a weed survives a rate of herbicide that would normally kill it. That’s passed down genetically to its offspring, and through repeated selection resistant populations increase. Spotting it early gives more options for potential mitigation.

Resistance is widespread in blackgrass and becoming increasingly problematic in other grasses, including Italian ryegrass and some brome species.

Blackgrass has developed significant resistance against the primarily contact acting with single site modes of action ACCase (‘fops’ and ‘dims’) and ALS (sulfonylureas) type products, but worryingly there are also signs of resistance developing against flufenacet, which is the key residual pre- and early post-emergence product for grassweed control.

Regular monitoring will help provide that key early warning of resistance developing, but remember not all poor performance of herbicides is due to resistance, so good investigation of the reasons behind an unexpected result is important.

Resistance testing can help with this. Mapping of infested areas – digital tools are helping to make this easier – is also good practice to help monitor weed spread.

Follow these tips from AHDB to help manage herbicide-resistance risks in arable weeds:

• Manage resistance threats – even before herbicide efficacy declines
• Reduce reliance on herbicides – a change of herbicide policy alone is unlikely to provide adequate lowering of resistance risk
• Adopt non-chemical control methods
• Maximise the benefit from pre-emergence herbicides, which are associated with partial and relatively slow resistance development
• Place less reliance on post-emergence herbicides – regular use of ACCase-inhibiting and ALS-inhibiting herbicides is associated with a high risk of herbicide resistance
• Where possible, use lower resistance risk post-emergence herbicides in the rotation in oilseed rape and beans
• Note that residual herbicides require moisture and an even seedbed for good efficacy, and most do not perform well in high organic matter (over 5%) soils
• Where soil does not cover crop seed do not apply herbicides until the crop has established.
• Use mixtures and sequences of herbicides with different modes of action to delay resistance
• Be aware that although the use of higher resistance risk herbicides with lower-risk modes of action will help reduce weed populations, this will not prevent further selection for resistance
• Follow the manufacturer’s recommendations on the label (e.g. for optimum dose and spray timing/application)
• Take note of label restrictions – introduced to reduce risk of herbicide resistance – on the sequential use of both ACCase-inhibiting and ALS-inhibiting herbicides
• Avoid treating if heavy rain is forecast, in waterlogged or frosty conditions, or if crop is suffering nutrient stress
• Note that resistance can vary considerably between and, to a lesser extent, within different fields
• Closely monitor herbicide performance to guide management
• Conduct regular – at least once every two to three years – resistance tests on seeds or plants