How to minimise grassweed risk when drilling early

For many years, drilling wheat later into October and into early November has become a key tactic used by growers to help manage grassweeds, such as black-grass and Italian ryegrass.

But two difficult autumns in a row where wet weather has brought a premature halt to drilling or led to wheat being drilled into less-than-ideal seedbeds, is changing the mindset of growers heading into the autumn, with many considering bringing drilling dates forward to guarantee crops are successfully established.

The drilling date decision has always been about risk management, considering the competing factors of weed and disease risks, crop establishment success, and impact on yields, says John Cussans, principal weed science consultant for ADAS.

“The calculation for downside risk has changed for many growers,” he suggests. “And that’s not illogical, if you’ve gone through two of the most difficult seasons in terms of the window to establish crops in living memory back-to-back.”

Several tactics

For those who routinely want to drill winter cereals in September, the reality is that it will likely require more effective grassweed management through the rotation, including growing more spring crops and/or using cultivations to help facilitate that change.

However, many growers are unlikely to be in a position where changes across the rotation have already minimised grassweeds to an extent where earlier drilling can be confidently undertaken, John suggests.

“Advising growers this autumn that they needed to have undertaken grassweed management across the rotation to allow early drilling is unhelpful – it’s just telling growers not to start from here.”

But there are several tactics growers can use to help at least minimise grassweed pressure where it already exists to help facilitate earlier drilling, he suggests.

The first is to take every single opportunity to control as much freshly shed weed seed as possible. Post-harvest management should be adapted to the weather, he stresses.

If it is bone-dry dusty, which admittedly looked very unlikely this season as harvest approached, then don’t do anything post-harvest, he advises. “Leave the weed seed on the soil surface and, if possible, take as much straw away as possible or chop effectively to expose the weed seeds to those harsh conditions.

“And then don’t try to create a stale seedbed or cultivate until the weather breaks.

“On the other hand, if you’ve got soil moisture – when you combine, there’s already seeds germinating in the base, then try to create stale seedbed conditions and encourage more seeds to germinate.”

The idea with any post-harvest cultivation is to increase seed-to-soil contact to encourage germination. “Anything really shallow – a set of discs set up as shallow as possible, a straw rake, even a set of rolls will do the job.”

Treat glyphosate carefully

Optimising spraying-off emerged weeds before drilling with glyphosate will be crucial. “Treat glyphosate a bit like carrying a Ming vase across a slippery floor,” John stresses. “It needs to be treasured and looked after when used.”

He advises thinking about water volumes, rates, what nozzles to use, and whether you need an adjuvant or water conditioner.

Glyphosate is sensitive to water hardness and turbidity, for example. “In the long term, change the way you source and store water used for spraying, but in the short-term, one way you can mitigate that risk is to use lower water volumes, so you have more active per water volume. But that makes it harder to get a coarse spray, so it is worth looking at online resources to determine how best to trade off volumes, pressure and nozzle choice.”

Timing is also essential – don’t leave too big a gap between spraying off and drilling the crop. “If you leave too big a gap, you open up the window for the weeds to chit and you have more pressure in the crop you drill.”

Competitiveness

The next element is maximising the competitiveness of the drilled crop. While variety choice can make a difference, John suggests other factors will likely dictate that decision, but seed rates can be increased to improve competitiveness.

“Use a seed rate calculator for your drilling date and then add perhaps 50–100 seeds/m2 if you’ve got black-grass or Italian ryegrass.”

Higher seed rates, as well as earlier drilling, will increase risks from both lodging and diseases, so there is a limit to how far you can push it, he notes.

By drilling earlier, growers will have to accept that costs will increase for weed control. “To get the same outcome at the end of the season, you will need to apply more herbicide to an early drilled crop than a late drilled crop.”

But, it does open opportunities for more effective herbicide sequencing. With a later drilled crop in the autumn, there is probably only one opportunity to apply pre-emergence herbicides. In contrast, John suggests that for an earlier drilled crop, you could have effectively two bites of the cherry with a pre-emergence followed by peri-emergence treatment 10–14 days later.

That could allow more herbicide to be applied while potentially reducing the risk of crop damage. “You are hedging your bets with two applications – if there are bad conditions with one, the chances are the other will be better.

“Look at the conditions and adjust splits of herbicide to suit. Never skip the pre-em element, but if it is dry at pre-emergence, maybe change the balance to put more on at peri-emergence, whereas if conditions are good at pre-emergence, switch the balance the other way,” he concludes.

Use every tool available to control grassweeds

Frustration after several wet or difficult autumns and winters is likely to encourage more early wheat drilling this autumn, agrees Hank King, UK business manager of Gowan.

Excellent integrated weed management becomes even more critical with early drilling, he stresses. “Make sure you are prepared to put in the time, effort and money as required for stale seedbeds, maximising the potential to control weeds before drilling.

“Once you have considered all out of crop practices, then you need to turn to chemistry. If you drill early, there’s likely to be a longer period of grassweed germination that you will have to protect against, which will need every tool in the toolbox.”

That means using virtually every available active and timing. “You will need a robust programme from the moment you drill right through until it starts to get cold and weeds stop germinating,” Hank predicts.

Avadex (tri-allate), whether as a granule or liquid, will be a key part of that programme, he says. “Across all grassweeds, Avadex is incredibly consistent at adding to the control of whatever base treatment has been chosen.”

For example, Gowan data suggests an additional 12% control of black-grass and 20% extra control of both Italian ryegrass and sterile brome when using a base treatment of the latest residual herbicide technology.

“An autumn application of Avadex also gives helpful control of wild oats germinatng in the spring, which can be overlooked.”

While in lab tests, the extra tri-allate loading in the granule formulation of Avadex Excel 15G provides greater levels of control compared with the liquid Avadex Factor. Hank says choosing between them might be how confident you are with timing.

“If you can reliably apply Avadex Excel 15G granules within 48 hours of drilling, then by the probability of outcomes, you’re more likely to get more out of the granules.

“But if you can’t guarantee your applications within that period, then use Avadex Factor. A well-timed application of Avadex Factor will most likely be more effective than a poorly timed application of Avadex Excel 15G.”

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