How to get the most from pre-em herbicides

A failure to get the most out of pre-emergence herbicides this autumn will leave an unmanagable number of weeds for post-emergence applications, potentially putting next year’s wheat yields at risk.

This high reliance on getting an effective pre-emergence hit is because of the declining efficacy of post-emergence herbicides, limiting growers’ ability to take out blackgrass plants later in the growing season.

Therefore, with so much riding on the effectiveness of pre-ems, growers are being urged to take all the necessary steps to get the best from them.

These products are all soil-acting, residual herbicides applied pre or very early post-emergence of the crop and weed, so there are some actions which can be taken to help boost their performance, says Tudor Dawkins of ProCam.

See also: Innovative weed wiper offers hope in blackgrass battle

“As such, they are designed to work best in fine, consolidated seed-beds,” he emphasises. “So seed-bed preparation is one of the first things that growers can do to help their cause. It’s impossible to over-emphasise the benefits of a good seed-bed.”

Good consolidation means that the roots of germinating blackgrass stay in the herbicide-treated zone, where they will pick up the chemical, rather than grow unhindered from outside the treated area, he explains.

Soil temperature

Furthermore, the pre-emergence herbicides also perform well when soils are cool and moist, as this slows their degradation in the soil and keeps them active for longer, extending their activity through the main germination period of the weed.

“That proved to be a bit of a problem last year,” recalls Dr Dawkins.

“Soil temperatures remained stubbornly high well into the winter. This meant that the degradation of residual herbicides was greatly accelerated – or to put it another way, they ran out of steam.”

Soil moisture is important as it allows the herbicides to be picked up by the emerging weeds, he adds.

“These herbicides just don’t work as well when it’s dry.”

Drilling date

Sowing date also has a role, as later drilling means that soil conditions are more likely to be suited to getting the best performance from residual herbicides, he notes.

“Of course, a later start also gives you more time to remove a flush of blackgrass with glyphosate.

“As up to 80% of blackgrass seed germinates in September and October, weed numbers tend to reduce going through October and into November.”

Work done on behalf of AHDB by Stephen Moss and John Cussans of Niab Tag confirms that pre-emergence herbicide performance improves with later drilling dates.

“On average, these products gave 26% more control when drilling was delayed by three weeks, from mid/late September to early/mid October,” reports Dr Moss.

“Lower temperatures and higher soil moisture levels were probably responsible for this boost.”

That extra performance is worth having, he stresses.

“When you consider that adding another herbicide to the pre-emergence stack gives a 10-15% improvement, and increases the cost, it’s a sensible move.”

However, it is important the herbicides are applied at the actual pre-emergence stage, stresses Mr Cussans.

“If you have drilled early, don’t be tempted to wait until the post-emergence stage to think that you’ll get the efficacy benefits associated with later application.

“These herbicides must be applied relative to crop and weed emergence.”

Product choice

Pre-emergence chemistry is the most important when it comes to blackgrass control, as most of the strength in a programme comes from the first hit, says Dr Dawkins.

“You need to hit it hard early – smaller blackgrass plants are easier to control,” he stresses.

ProCam assessed 24 different pre-emergence products and combinations in a replicated trial last year, he reports, with no crop being sown to give the chemistry a tough test.

A consistent population of blackgrass and a uniform seed-bed were also used, providing constant conditions.

“We wanted to be able to look at all of the pieces of the jigsaw and tease out any differences between the treatments,” he explains.

The best results came from a co-formulation of flufenacet, diflufenican + flurtamone, with prosulfocarb also added to the mix.

“It was reassuring to see that a tried and tested combination, which is often used in our stacks, performed so well, giving over 70% control.

“Additional reductions came from adding other chemistry to the programme, such as an Avadex (tri-allate) application.

“However, it also illustrated the importance of a full programme, including peri- and post-emergence herbicides, so that you end up with at least 97% control.”

In most situations, the more chemistry that you can apply, from alternative modes of action, the better the outcome, he confirms.

“Of course, there comes a point when it isn’t cost-effective to do that, even though we need to squeeze every percentage increase in control out of a treatment.

“It isn’t possible to get these results from post-emergence herbicides anymore, so the majority of the herbicide budget should be spent in the very early stages,” he says.

Soil being ploughed

© Tim Scrivener

What about ploughing?

A well-set plough which fully inverts the soil will effectively bury seed for at least two to three years, reducing the seed burden, notes ProCam’s Tudor Dawkins.

“It’s only effective if the seed is not already uniformly distributed through the soil. Blackgrass can emerge from as deep as 10cm, so one plant coming through can undo the benefit of ploughing.”

Repeating the exercise too soon means that viable seed can be returned to the surface, perpetuating the problem, he warns.

“We saw a 60% reduction in the June head count from ploughing in last year’s trials, so it can pay dividends. But a robust herbicide programme will still be needed on ploughed land.”

What else can be done?

Blackgrass is favoured by wet soil conditions, so take a look at your drainage, says one expert.

“Are the drains functioning properly and can the water be carried away from the field?” asks Procam’s Tudor Dawkins.

It’s not uncommon to find conduits under field entrances completely blocked, with water being unable to move away, he reveals.

“Ditches need to be cleared, so that water can flow.”

Soil compaction also has to be addressed, as it can prevent water percolating down through the soil profile, he warns.

“Remove it by working with tines or discs just below the impeding layers. If the soil can maintain a mole drain, mole ploughing may help and should last for several years,” says Dr Dawkins.