Tackling grassweeds this autumn: Expert tips for growers – Farmers Weekly

Dry seed-beds also restricted the performance of residual herbicides. Residuals need sufficient moisture to keep the active dissolved in solution and get it taken up by emerging weeds.

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

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

The effect will be particularly noticeable where germination is protracted due to the weather or natural characteristics (for example Italian ryegrass).

All three factors were compounded in 2022, when stronger wheat prices encouraged a higher proportion of wheat plantings in the autumn.

This meant more growers started drilling earlier, when conditions were still dry, with insufficient opportunity for stale seed-beds.

Brome and Italian ryegrass are also increasing concerns in many areas, says Hutchinsons technical support manager Cam Murray.

Great brome has been particularly noticeable in southern England, while the rise in meadow and soft brome may be an unintended consequence of cultural blackgrass control measures, he suggests.

“Meadow and soft brome need UV light for three to four weeks to become viable, so where growers have used stale seed-beds for blackgrass control, doing so may have unintentionally buried brome seed, enforcing dormancy.

“The timing of cultivation influences how certain bromes survive, rather than the specific cultivation itself.”

But, for Cam, Italian ryegrass is one of the biggest threats given its “monstrous potential to rob yield”.

“With the ratio of 1 plant/sq m relating to a 1% yield loss, it places itself as one of the premier grassweeds in relation to yield loss.”

Warrington-based Hutchinsons agronomist Andrew Roberts says ryegrass is the biggest grassweed challenge for many growers in the North West.

Levels this year have been “higher than ever” as wet, mild conditions during winter and spring favoured protracted germination and growth.

Italian ryegrass germinates almost year-round (apart from July), and while it does slow in midwinter, it can continue germination throughout mild winter months.

It also has a significant spring germination period from March to June, rendering spring barley less effective to fight ryegrass than it is for blackgrass control.

Andrew says early-sown winter cereals have come 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 being complicated further by the rise of herbicide-resistant ryegrass populations, particularly to post-emergence chemistry, he adds.

“Our focus is on pre-emergence residuals and cultural controls, including stale seed-beds and delayed drilling.

“But, this isn’t always possible in this region, given how quickly the weather and field conditions can deteriorate once you get into October.

“Later-sown crops also need higher seed rates to avoid establishing thin plant sands that are more likely to allow grassweeds in.”

Managing weeds is a “numbers game”, so even just a few surviving plants can soon have a dramatic effect on seed return and weed pressure in subsequent years, Dick says.

“If a blackgrass head population is even noticeable in the field, it will generally be at about 100 heads/sq m, so has the ability to deliver 10,000 seeds/sq m back to the soil.

“That amounts to 100m 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.

“Single plants carrying 25 tillers are now becoming commonplace, compared with nine to 10 tillers per plant 20 years ago. The goalposts 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, says Cam.

As mentioned previously, placing soft brome into darkness at shallow depth immediately after combining will effectively lengthen seed dormancy and prevent any growth for control before autumn crop establishment.

In contrast, barren brome will respond positively, being ready to go straight after the combine, so will grow for spraying off before autumn drilling, he explains.

Soft brome also has an in-built delay in establishment of a proportion of the population, so a spring germination period must be built into any control strategy to successfully control this weed.

Ploughing buries seed beyond germination depth, so can be an effective “reset button” for many grassweeds.

However, Cam says it must be done well, and growers should recognise that what is ploughed down now will be ploughed back up in subsequent years, so ploughing forms part of the overall strategy, not the only strategy.

“Grassweeds are very good at adapting to what we do, so there must be a balanced approach to cultivations, which may include ploughing.

“When issues start developing, it’s no use continuing to do the same thing every year and expecting a different result.”

For ryegrass, Cam says a useful cultural control is to leave fields for a good couple of weeks after harvest, if possible, before any tillage, as birds feed on seed left on the ground, so it can help reduce overall numbers before looking to obtain a chit.

Another ryegrass-specific must-do is seed resistance testing. “Italian ryegrass does develop true resistance to residual herbicides, so it is vital you understand which herbicides are effective on your population.”

Resistance status varies from field to field, so multiple tests may be needed. “If you do not know what you are fighting, then how can you possibly apply the correct chemistry?” Cam asks.

Sequencing of residual herbicides is important for countering protracted germination of ryegrass, so consider how cultivation strategy may affect field access in late autumn and early spring, he says.