Pressure is increasing for pre-emergence herbicides to deliver reliably high levels of control since the widespread decline in performance of post-emergence products – notably Atlantis.

Just how well pre-ems need to perform to achieve 97% control is illustrated in the recent HGCA Blackgrass Information sheet (see table). Yet reaching these high levels is a huge challenge given their intrinsic variability, says Rothamsted’s Stephen Moss.

This variability is down to a number of factors.

“Every pre-em herbicide is affected to some degree by enhanced metabolic resistance (EMR), which is partial and evidence suggests that it doesn’t build up very quickly, unlike target site resistance to fops, dims and ALS inhibitors.”

But in-field levels of EMR are difficult to quantify and the only way growers can see what works well is to undertake trials to see which products work best on their fields, he says.

“Stacking or sequencing is commonly used to counteract EMR, although it is not a sustainable long-term solution.”

In addition to resistance, soil moisture affects performance of residual chemistry while other factors such as seed-bed condition and organic matter, although not quantified, will also contribute to performance.

Realistic control

One of the biggest hurdles for the pre-ems is the sheer size of population that growers expect the chemistry to cope with, says Hutchinson’s Dick Neale. With 100 plants/sq m emerging at drilling recognised as the acceptable ceiling-level of blackgrass plants to present to the pre-emergent products, it is clear that many current blackgrass populations far exceed this, leading to disappointing results.

“We know that 80% of the blackgrass that wants to emerge in the autumn period will be during September and October.”

This means there is much work to be done by use of cultural controls, starting with cultivation, so as to ease the pressure on pre-em chemistry by reducing the number of blackgrass plants that will potentially emerge in the crop, explains Hutchinson’s Dick Neale.

Min-till cultivation techniques make poor-quality “stale” seed-beds, the blackgrass doesn’t germinate and the stale seed-bed opportunity is lost. The grower then waits 
for rain and drills into the highest risk possible seed-bed in terms of blackgrass.

Delaying drilling into October will allow more blackgrass to emerge and be sprayed off. And as the weather in October also tends to be more overcast, the soil remains wetter for longer and any residual chemistry will have more of a chance to maintain efficacy.

Delayed drilling

In fact, there is clear evidence from a joint project between Rothamsted and Niab Tag, plus an evaluation of 375 distributor trials, that demonstrates delaying drilling from mid-September to the first week of October results in appreciably better activity from flufenacet-based applications.

As with all trials, the range of control varied as there were situations when October remained dry, explains Dr Moss, but the average figure is 15-20% better control.

“That is equivalent to an extra one or two herbicides in a stack, or upping the rate of DFF.”

Flufenacet, the least affected by EMR, will form the cornerstone of any stack or sequence. Which products to add in will be farm specific, but what is recognised is that the greater the number of actives applied the better, says Dr Moss.

Mr Neale advocates a minimum of three different actives to make a stack worthwhile. “That’s quite easy to achieve because all flufenacet-based products are a stack in a can. Then growers need to decide what to apply next.

“Flufenacet has to be at 240g/ha and there is an increasing level of use at 360g/ha. The next best is Avadex, if you have the application technology. It delivers about 15% extra control over what you get from flufenacet or flufenacet stack and has a different mode of action.”

As to adding in other products, Mr Neale suggests mixing pendimethalin with Liberator and DFF with Crystal, then there is Defy and Lexus SX.

He also warns that using the same products season after season is not sensible, and recommends where perhaps Crystal has been the mainstay for years, including Avadex or Defy would be prudent.

Adding DFF in with Defy also has the benefit of some synergy on blackgrass.

As to whether to apply pre-, peri- or very early post-emergence, Mr Neale advocates always going pre-emergence where possible. But there is, he says, the option of applying the stack as a sequence every 10 days in order to catch moisture and new emergence going into October.

Good, fine consolidated seed-beds will also help residual performance, as will fine-tuning spray application techniques.

Covering all sides of the clods is useful by using forward- and backward-facing or twin-cap nozzles or the Defy or potato nozzle.

Paying attention to boom height is very important, ensuring 50cm above the ground for flat fans, down to 40cm with the Defy nozzle angled out and ensuring speeds do not exceed 12-14kph, he adds.