How stacking herbicides improves weed control

Higher blackgrass pressure in early drilled crops this autumn will require a strong stack and sequence of herbicides.

“The best practice and most robust advice when it comes to tackling difficult weeds such as blackgrass, ryegrass and brome is still to delay drilling until well into October,” says Bill Lankford (pictured), herbicide technical specialist at Adama.

“With that in mind, growers should consider that if they decide to drill early, they will also need to apply a strong stack and sequence of herbicide active ingredients to ensure satisfactory levels of weed control are achieved.”

Essential ingredients

New active ingredients such as aclonifen or cinmethylin will be essential at the pre-emergence timing to prevent blackgrass plants from stealing a march on early drilled crops, says Dr Lankford.

But even these herbicides will require help to maintain weed control further into the crop establishment and early development phases. Growers should consider upping seed rates to offset higher weed pressure.

Incorporating additional active ingredients with different modes of action can power-up herbicide programmes, says Dr Lankford. Protection can be boosted by using diflufenican, pendimethalin and chlorotoluron.

Post-emergence

A pre-emergence application of aclonifen, diflufenican and flufenacet will be enhanced by adding Anthem (400g/l pendimethalin) or a post-emergence treatment of Tower (250g/l chlorotoluron + 40g/l diflufenican + 300g/l pendimethalin).

Similarly, adding Hurricane (500g/l diflufenican) will power-up pre-emergence cinmethylin and pendimethalin.

This could then be followed by post-emergence Tower or Omaha 2 (40g/l diflufenican +  400g/l pendimethalin).

C

hanging emergence patterns are prompting advise for farmers to review and revise blackgrass control strategies.

Stale seedbeds and delayed drilling have helped reduced blackgrass emergence during September and early October. But the weed is adapting to the control strategy, says Dick Neale, technical manager for agronomy firm Hutchinsons.

“We are now seeing a significant shift towards later-germinating blackgrass populations in many situations, and evidence also suggests blackgrass is becoming more vigorous, producing far more tillers.”

A significant shift in blackgrass dynamics during recent years has seen Hutchinsons launch a new demonstration site at Cambourne, near Cambridge. The goal is to better understand and tackle the changes.

Showcasing changes

The shift in population dynamics is clearly evident at the 40 ha Cambourne blackgrass demonstration, hosted by the Clear family at South Sea Farm, not far from the old Hutchinsons blackgrass trials site at Brampton.

Like many farms, South Sea Farm has made great progress in reducing the amount of blackgrass emergence during September and early October.It is doing this by using techniques such as delayed drilling and stale seedbeds – but full control is still not being achieved.

That is primarily because blackgrass emergence is becoming later and more protracted – as seen this season, when the wet autumn prevented winter crop drilling on the heavy clay soils, resulting in many fields left unsown until spring.

This provided a useful insight into the way blackgrass responds to management practices, says Mr Neale, who tried various strategies before the weather broke last autumn – including different cultivations, cover crops and tackling volunteers.

“This showed the bulk of blackgrass we are dealing with on this farm is germinating from mid-October onwards – not in September or early October – and it’s doing so in combination with additional cultivations.”

Farmacy agronomist Tom Smith, says individual blackgrass plants are noticeably more vigorous with greater tillering capacity. Some blackgrass at Cambourne is producing up to 40-50 tillers per plant, more than half the typical amount.

The average blackgrass population across the 40ha Cambourne site is around 200-300 heads/m2. “Growers and agronomists need to understand the dynamics of their own blackgrass populations and tailor control strategies accordingly,” says Mr Smith.

With wetter autumns, delayed drilling on heavy land to catch blackgrass emerging from late October onwards with stale seedbeds is extremely risky, so the team is looking at other whole-farm strategies.

This covers everything from cultivations, rotation, soil health, and cover- or catch-cropping, to drilling dates, and herbicide options, with technology such as Omnia and TerraMap soil analysis providing valuable data to inform decisions.

Focus on drainage

Improving drainage is a top priority at the Cambourne site. The wet 2023/24 season and soil analysis have highlighted a number of issues  that continue to exacerbate the blackgrass threat.

Many areas have really struggled with drainage on the heavy clay soil, so will clearly benefit from mole draining this autumn, says Farmacy agronomist Poppy Clark. “The priority is to get the water moving through the soil and remove those wet patches.”

Terramap analysis also shows the silty clay soil is relatively high in magnesium, which is creating a ‘tighter’ structure that can be hard to work and is likely to impair root growth and water infiltration.

To address this, one field received 3 t/ha of gypsum (calcium sulphate) last autumn to counter and rebalance the high magnesium content while improving soil structure and friability. It is a low-cost option the team will employ again this autumn.

Other work is looking at the use of multi-species cover crops to improve soil structure, build organic matter, manage soil water, and break up any tight or compacted layers, thereby improving drainage.

Early drilling

Another option to be investigated this autumn is whether it is possible to sow winter wheat as early as the first week in October. If successful, this would help establish a competitive crop before the main flush of blackgrass emerges.

Such an approach must be supported with a strong, correctly timed, residual herbicide programme, says Mr Neale. It should feature limited applications at drilling, but with the most robust input of chemistry in mid-October.

A pre-Christmas top-up should also be considered, and possibly again in the spring, to catch later-emerging populations, he adds.

“Drilling earlier may be possible and many growers might be keen to do this year. But you need to understand the dynamics of blackgrass and observe what’s happening in each field to ascertain when most of the population is emerging.”

“If it is a late-emerging population, stale seedbeds and delayed drilling may not be working for you any longer, and there may well be a place for drilling earlier and being more focused with residual choices and timings.”

Spring cropping

Most land at Cambourne was sown with spring barley this season following a wet autumn.

This helped reduce blackgrass pressure – partly because it allowed moretime for blackgrass to emerge over the winter, before being sprayed off.

The team has also trialled different management approaches within the spring barley, placing fertiliser at drilling, varying seed rates, and using  spring beans as a companion crop.

Anecdotal evidence suggest crops that received placement fertiliser established quicker, making them better able to outcompete any blackgrass emerging with the barley.

Applying fertiliser with seed, rather than across the whole field, also reduced the likelihood of indirectly stimulating weed growth, which may have reduced the blackgrass vigour slightly, he adds.

The seed rate trial is also showing interesting results. Seed rates varied from 200 kg/ha (equivalent to 350 established plants/m2) up to 275 kg/ha (530 plants/m2), with higher rates offering much greater competition against blackgrass, says Mr Smith.

At the highest spring barley seed rate, an average blackgrass population of 4 plants/m2 and 3 tillers per plant was recorded, ranging from 1 to 6 tillers. The lowest seed rate saw an average of 20 plants/m2 and 16 tillers per plant, ranging from 3 to 20 tillers.

“The majority of blackgrass was smaller with fewer tillers at the higher seed rates, but we need to strike a balance,” Mr Smith says. “The spring barley was also more cramped in at higher rates, resulting in smaller plants and fewer tillers, so we need to find where the optimum is for both blackgrass control and crop yield potential.”

One area not treated with a well-timed glyphosate application before drilling, recorded an average of 44 blackgrass plants/m2. This produced some 20 tillers per plant, although plants with more than 40 tillers were often found too.