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Grassweeds are again a big talking point for arable growers – with some fields under significant pressure before autumn drilling. Growers choose plough to ‘reset’ grassweeds

• Bromes and ryegrass increasing problem

• Weather is among key influences

• Mix of control strategies useful

Grassweeds are again a big talking point for arable growers – with some fields under significant pressure before autumn drilling.

Blackgrass remains the dominant issue in many eastern counties – with Italian ryegrass and brome increasing nationally. This has prompted more growers to reach for the ‘reset’ button and plough-in stubble this autumn.

Some growers blame the shift to direct drilling and minimal tillage systems for rising grassweed pressure. And it is true that ploughing has long been an effective tool in the armoury alongside herbicides.

But experts at agronomy company Hutchinsons say the reasons are often far more complex. They include a combination of factors – from inappropriate cultivation choice and timing to poor herbicide strategy and unpredictable weather.

There are three important reasons why some growers have seen disappointing grassweed control this year, says Hutchinsons technical director Dick Neale (see panel). Understanding these reasons is key to moving forward, he adds.

Increasing concerns

Bromes and Italian ryegrass are also increasing concerns in many areas. Great brome has been particularly noticeable in some areas, with a rise in meadow and soft brome perhaps an unintended consequence of cultural blackgrass control measures

Meadow and soft brome need ultraviolet light for up to a month to become viable, so growers who have used stale seedbeds for blackgrass control may have unintentionally buried brome seed, enforcing dormancy.

But Hutchinsons technical support manager Cam Murray says cultivation timing – rather than the specific type of cultivation itself – influences how certain bromes survive.

Combating other grassweed shouldn’t be overlooked, with Italian ryegrass is one of the biggest threats given its “monstrous potential” to rob yield.

“With just one plant per square metre relating to a 1% yield loss, it places itself as one of the premier grassweed in relation to yield loss.”

Italian ryegrass germinates all-year-round apart from July. Ryegrass too will be a challenge for some growers this year because the wet and mild winter and spring favoured protracted germination and growth.

Cultural control

Early-sown winter cereals are under particularly high ryegrass pressure given the narrow weather window for cultural controls between harvest and drilling – and few opportunities for applying herbicides beyond the pre-emergence residual.

Control strategies are further complicated further by the rise of herbicide-resistant ryegrass to post-emergence chemistry. The focus should be on pre-emergence residuals and cultural control, including stale seedbeds and delayed drilling.

This isn’t always possible in areas where field conditions can deteriorate as autumn progresses. Later-sown crops also need higher seed rates to avoid establishing thin plant stands that are more likely to favour grassweeds.

Numbers game

Managing weeds is a numbers game, says Mr Neale. That’s because even just a few surviving plants can soon have a dramatic impact on seed return and weed pressure in subsequent years, he explains.

“If a blackgrass head population is even noticeable in the field, it will generally be at around 100 heads/m2, so has the ability to deliver 10,000 seeds/m2 back to the soil. That amounts to one hundred million seeds/ha – the numbers are phenomenal.”

Trials clearly show that 90% control of plants in the autumn only equates to 57% control of heads in the following May, because surviving plants are genetically stronger, with the ability to resist herbicides, and produce more tillers.

Seed return

“Single plants carrying 25 tillers are now becoming commonplace, compared to 9-10 tillers per plant 20 years ago. The goal posts are continually moving.”

Preventing seed return is the ultimate aim, but this requires an understanding of the types and species of weeds present in individual fields, their ecology, and resistance status, in order to determine the most effective control strategies.

This is particularly evident for brome, as the maturity profile or innate dormancy of barren brome is different to rye or soft brome, and shallow tillage timing will have totally different outcomes for either species, explains Mr Neale.

Placing soft brome into darkness at shallow depth immediately post-combining will lengthen seed dormancy and prevent any growth for control before autumn establishment.

In contrast, barren brome will be “ready to go” straight after the combine. It will grow for spraying off prior to autumn drilling. Soft brome also has an in-built delay, so a spring germination period must be built into control strategies.

Effective ploughing

Ploughing buries seed beyond germination depth, so can be an effective “reset button” for many grassweeds. But it must be done well, and growers should recognise that what is ploughed down now, will be ploughed back up in subsequent years.

That means ploughing is only part of the answer. Sequencing residual herbicides is important to counter protracted germination of ryegrass, so growers should consider how cultivations may affect field access in late autumn and early spring.

Factors affecting grassweed control

1. Poor stale seedbed chitting

In 2021 and 2022, dry conditions in summer and early autumn reduced the germination of volunteer grassweeds before drilling. This resulted in less effective stale seedbed control two years running.

In 2023, soil moisture is generally much higher, so stale seedbeds should be more effective this autumn. With sufficient moisture, rolling stubbles twice can encourage weed seed germination – and avoids the risk of creating an over-fine seedbed.

2. Poor residual herbicide performance

Dry seedbeds restricted the performance of residual herbicides in the last two seasons. Residuals need sufficient moisture to keep the active dissolved in solution and taken up by emerging weeds.

Again, more soil moisture this year should allow for better performance, but pay attention to seedbed quality, moisture/post-application rainfall, pre-emergence timing – within 24-48 hours of drilling – and application coverage to maximise efficacy.

3. Lack of application sequencing

Where the focus has been on stacking pre-emergence chemistry, this has generally meant fewer spring residuals to catch spring-germinating grassweeds.

The impact of this has particularly noticeable where germination is protracted, due to the weather, or natural characteristics, such as Italian ryegrass.

All three factors were compounded in 2022, when stronger wheat prices encouraged a higher proportion of wheat plantings last autumn. This meant more growers started drilling earlier, when conditions were still dry, with insufficient opportunity for stale seedbeds.