Time to rethink black-grass control as new threats emerge
Black-grass emergence patterns are changing and growers must adapt their control strategies accordingly, leading agronomy firm Hutchinsons warns. ...
In response to significant shifts in the dynamics of black-grass observed in the field over recent years, the firm has just launched a new demonstration site at Cambourne, near Cambridge, to better understand these changes and how best to tackle them.
“Stale seedbeds and delayed drilling have been very effective measures for reducing September and early October-germinating black-grass, but the weed is adapting to what we do,” explains Hutchinsons technical manager, Dick Neale.
“We are now seeing a significant shift towards later-germinating black-grass populations in many situations, and evidence also suggests black-grass is becoming more vigorous, producing far more tillers than in the past.”
Showcasing changes
These shifts in population dynamics are clearly evident at the 40 ha Cambourne black-grass demonstration, kindly hosted by the Clear family at South Sea Farm, not far from the old Hutchinsons black-grass trials site at Brampton.
Like many farms, South Sea Farm has made great progress in depleting the September and early October-emerging black-grass population using techniques such as delayed drilling and stale seedbeds, however full control is still not being achieved.
That is primarily because black-grass emergence is becoming later and more protracted – something clearly highlighted in the 2023/24 season, when the wet autumn and winter prevented winter crop drilling on the heavy clay soils, resulting in many areas being left unsown until this spring.
Ironically, this provided a useful insight into how black-grass responds to management practices, Mr Neale says. “We tried various things before the weather broke last autumn, from different cultivations and cover crops, to no cover crops, or allowing volunteers to grow or not grow. Normally, these areas would have all gone under the drill by mid-October, but this season they didn’t; many areas had to be left undrilled, so continued through the winter.
“This showed the bulk of black-grass we are dealing with on this farm, is germinating from the 20 October onwards, not in September or early October, and is doing so in combination with additional cultivations.”
Farmacy agronomist Tom Smith, says individual black-grass plants are also noticeably more vigorous with greater tillering capacity. Indeed, some black-grass at Cambourne is producing up to 40-50 tillers per plant, around 50% more than the typical tiller number at Brampton a few years ago, although populations of individual plants were generally higher there.
At Cambourne, the average black-grass population across the 40 ha trial area is around 200-300 heads/m2.
“Growers and agronomists need to understand the dynamics of their own black-grass populations and tailor control strategies accordingly.”
So what can be done about it?
Delaying drilling even later into the autumn to catch black-grass emerging from late October onwards with stale seedbeds is simply not viable on heavy land, as autumn 2023 proved, so the team are looking at other ‘whole-farm’ solutions.
This covers everything from cultivations, rotation, soil health, and cover/catch cropping, to drilling dates, and herbicide options, with technology such as Omnia and TerraMap providing valuable data to inform decision making.
Focus on drainage
Improving drainage is a top priority at the Cambourne site, as the wet 2023/24 season, together with Terramap soil analysis, has clearly highlighted a number of issues that are exacerbating the black-grass threat.
“We’ve seen a number of springs pop up in some fields over the winter, and many areas have really struggled with drainage on the heavy clay soil, so will clearly benefit from mole draining this autumn,” notes 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 the high magnesium content and rebalance the calcium:magnesium ratio, opening the structure, and improving soil friability. It is an option the team will use again this autumn, as gypsum can deliver worthwhile improvements for a comparatively low cost.
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.
Drilling early
Another option that will be investigated this autumn is to see whether it is possible to sow winter wheat earlier (e.g. first week in October) in order to establish a competitive crop before the main flush of black-grass emerges.
Mr Neale says such an approach must be supported with a strong, correctly timed, residual herbicide programme, featuring 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.
“Drilling earlier may be possible and is something many growers might be keen to do this year, but you need to understand the dynamics of black-grass and observe what’s happening in each field on your farm to ascertain when most of the population is emerging,” he says. “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
Following the difficulties establishing winter crops last autumn, most land at Cambourne was sown with spring barley this season, which has helped reduce black-grass pressure, says Mr Smith.
This is partly because it allowed extra time for more black-grass to emerge over the winter, before being sprayed off with glyphosate prior to drilling. The team has also trialled a few different management approaches within the spring barley, including the use of placement fertiliser at drilling, variations in seed rates, and the use of a companion crop (spring beans).
Anecdotal evidence indicates the crops that received placement fertiliser were quicker to establish, making them better able to outcompete any black-grass 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 black-grass vigour slightly, Mr Smith notes.
The seed rate trial is also showing some 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 black-grass, he says.
For example, at the highest spring barley seed rate, an average black-grass population of 4 plants/m2 and 3 tillers per plant was recorded (ranging from 1 to 6 tillers). In contrast, at the lowest seed rate, there was an average of 20 plants/m2 and 16 tillers per plant (ranging from 3 to 20+).
“The majority of black-grass 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 black-grass control and crop yield potential.”
Interestingly, one area deliberately not treated with a well-timed glyphosate application before drilling, recorded an average of 44 black-grass plants/m2, producing 20 tillers per plant on average, although plants with more than 40 tillers were often found, he adds.
Look out for details of a results meeting later this year, where there will be an opportunity to discuss findings from the first year of trials at the Cambourne site.
Email Us