Following a year of high grassweed pressure backed by limited success in pre-em applications last autumn, it seems as though a higher than average seed return is on the cards for many. CPM investigates ways to get back on track.
“It’s the underlying competition between the crop and the weed – what herbicides do is manipulate that in favour of the crop.”
By Janine Adamson
Not only did conditions last autumn cause havoc with drilling, but the relentless wet weather also hampered some growers’ ability to apply pre-emergence herbicides to regain control of proliferating grassweed populations.
And for those who did travel and apply chemistry, depending on drilling date, crops may have struggled, providing little competition in high blackgrass situations.
So in many instances, this makes for two back-to-back years of poor grassweed control, meaning there’ll be higher than average seed return entering the coming season, says weed expert, Will Smith.
Additionally, due to the adverse ground conditions last autumn, some remedial cultivations could be required to rectify wheelings and restructure soils following waterlogging. However, this could also prove an opportunity to get back on-top of grassweeds at the same time, he continues. “If you’re having to do that, could this be the year to undertake a deep cultivation and resurrect the plough? If so, it’s worth ensuring it’s done properly to deliver a true inversion.
“It’s also key to monitor for persistent species such as wild oats and broadleaf weeds which may emerge as a result, particularly if the plough hasn’t been used for a while.”
Where fields are in good condition, there should be an opportunity to utilise the stale seedbed technique, comments Will. “Growers will have to look for ways to manage this season’s seed return to elongate the opportunity for weed control, particularly for blackgrass, and this starts with post-harvest management.”
But, how to approach this depends on weather conditions at the time of the operation, he warns. “Where you have dry conditions, the first step will be to do nothing – leave seed on the surface in the case of blackgrass and ryegrass, to maximise predation as well as natural decay prior to any cultivation.
“Conversely, if soils are damp, then cultivating can help to encourage germination of weeds and volunteers but remember, these cultivations have to be at no more than 5cm depth to avoid bringing older seed to the surface. By reducing the impact of freshly shed seed, it’s hoped growers can enter next season at net seed-bank levels.”
Although Avadex (tri-allate) is a familiar product for many, this year could see it particularly prove its worth, adds Will. In his role as technical lead at Gowan, he says the company recognises it’s an additional product within programmes, but believe it can play a role in a ‘weed reset’.
“In high grassweed situations, we see an uplift of control across all species including brome, which is a benefit that other actives might not offer to the same degree.
“Particularly for those who did miss their pre-em applications, Avadex is an effective broad-spectrum herbicide which could assist in catching up on previous years of poor control,” suggests Will. “For those without an applicator or with wider constraints, the liquid formulation (Avadex Factor) is also effective.”
Association of Independent Crop Consultants (AICC) member, Jonny James of CCC Agronomy, agrees that Avadex provides an uplift across key grassweed species. “This is particularly the case where Luximo (cinmethylin) is the starting block for a programme,” he says.
“An added benefit of Avadex is that there are no known resistance issues, so it’s a very solid option. For weed such as wild oats, where control is becoming increasingly variable, the additional spend could indeed prove worth it this year.”
According to a joint research project undertaken by NIAB and Adama, achieving optimum blackgrass control is down to manipulating crop competition, irrespective of drilling date. In fact, upping seed rates can also help crops to bounce back from potential damage, as a result of high herbicide use.
The work was instigated after technical specialist Dr Bill Lankford noted that despite growers using more robust autumn herbicide stacks, blackgrass control wasn’t always as expected. Together with John Cussans, a winter wheat experiment was designed to understand the potential contributing factors using extremes of variables, in hope of pinpointing the sweet spot.
“The trial was based around variable seed rates and herbicide treatment regimes across two drilling dates (mid-September and late-October). The aim was to understand the relationship between these three variables and the resulting impact on grassweed control,” explains Bill.
Seed rates varied from very low (75 seeds/m2) to very high (600 seeds/m2) and herbicide programmes were designed to reflect low through to high input (see table). At high herbicide input, Bill says although the aim is to achieve ultimate weed control, when all applied together, there could be resulting crop safety concerns.
“Such a highly loaded tank mix isn’t something which manufacturers would endorse and best practice is to sequence, but we have to acknowledge that due to resistance issues in post-emergence herbicides, more active ingredients are being added into residual stacks.
“Equally, drilling later can reduce the opportunities to make repeated residual herbicide applications,” he adds.
| Different herbicide regimes (all applications made at pre-emergence) | |
| Untreated control | – |
| Low input | Diflufenican plus pendimethalin plus flufenacet |
| Medium input | Flufenacet+ diflufenican plus aclonifen plus prosulfocarb plus pendimethalin |
| High input | Tri-allate plus cinmethylin plus pendimethalin plus diflufenican plus prosulfocarb plus flufenacet |
With the results now in, John says at the heart of the findings is the importance of competition – both weed versus crop through seed rates, and, closing the gap on herbicide selectivity.
“It’s the underlying competition between the crop and the weed – what herbicides do is manipulate that by shifting it in the favour of the crop. Effective weed control is in fact an indirect result of reducing the weed size early doors,” he explains.
“But applying more and more herbicide doesn’t always mean greater weed control and it can also have an impact on the crop, particularly sensitive species such as spring barley and oats. Where herbicide begins to have a greater impact on crop suppressiveness than it’s efficacy – reducing weed seedling numbers – the eventual effect of weed control is reduced. So it’s asking whether you can overcome that knockback through increasing the seed rate,” continues John.
Furthermore, Bill says the trial results indeed reflect this hypotheses. “What we see, using aerial RGB ‘human eye’ imagery, is the impact on crop vigour from the high input herbicide treatment is compensated for when increasing the seed rate. In fact, at 600 seeds/m2, there’s little to no damage from any of the herbicide treatments.”
Subsequently, John says his advice will now be for growers and agronomists to be bolder with seed rates. “It’s well acknowledged that upping seed rates is a tactic against grassweeds, but what hasn’t perhaps been understood is how it’s compensating in two synergistic ways.
“You could be looking to add up to 150 seeds/m2 more in a high blackgrass situation,” he suggests. “Equally, at low seed rates, all herbicide regimes had an impact on crop vigour.”
As for drilling date, the trial also measured blackgrass seedling density at a high pressure site at Hinxton; counts were undertaken in December. Perhaps unsurprisingly, blackgrass populations were much higher at the earlier sowing date across all seed rates and herbicide treatments compared with 10 blackgrass seeds/m2 or less in the later drilled plots (including untreated).
However, although blackgrass populations were lower at the later drilling date, the higher the herbicide input, the greater the reduction in crop density. Bill says this means despite drilling later being better from a starting point of weed pressure, if herbicide programmes can’t be sequenced adequately due to a lack of opportunity, there’ll be a considerable risk to crop safety.
“The trial indicates a noticeable thinning impact as a result of all herbicide use at the later sowing date, reaching 40% at the highest input level,” he says. “Therefore, drilling early and having a greater chance of sequencing and multiple applications, allows improved crop safety.
“From a practical perspective, this means that growers dealing with herbicide insensitive blackgrass could drill earlier than the end of October and still achieve good blackgrass control by focusing on reducing the seed-bank present at drilling and using different herbicide active ingredients in sequence, plus manipulating the seed rates.”
The same can’t be said for Italian ryegrass, stresses John, which was assessed at a site at Faversham. Due to inclement conditions during the trial period, while the weed seedling data from both drilling dates could be reviewed, the later drilling date trial wasn’t taken to maturity because of uneven establishment. Despite this, the relationship between drilling date and ryegrass control remains clear, adds John.
Bill agrees that with an increase in herbicide-resistant ryegrass populations, active ingredients continue to struggle to perform. “Drilling date is pivotal in combating this tough weed.
“With blackgrass, there’s the flexibility to adjust and compensate which is a positive message, but herbicide insensitive ryegrass is just too difficult to control at an early drilling date. Growers won’t see the return on investment from the inputs required.”
To conclude, John adds there has to be an acknowledgement of how the weed seed-bank impacts the crop. “Historically, there’s been a school of thought that it doesn’t matter what the seed-bank level was, you’d achieve the required control from herbicides and all would be well.
“However, because this trial has illustrated the importance of manipulating crop competition, it’s evident that the seed-bank population does have an impact. The real solution is to reduce that population in the first place,” he says.
Reflecting on these findings from an agronomic perspective, Jonny says he’s still reluctant to advocate early drilling when such a wide array of other factors are at play. “You have to balance the benefits of early drilling and the potential weed control gains against broader management challenges, whether that be lodging or disease pressure. As for upping seed rates, this can have an impact on spec weights.
“There’s also the issue of herbicide persistency and half-life – products don’t have the same longevity when applied in dry conditions. This is especially pronounced in an active such as prosulfocarb (as in Defy) which is most effective in cooler, moister conditions,” he adds.
And whereas grassweeds are usually the primary concern for most growers, Jonny highlights the increasing prevalence of groundsel, poppies and other broadleaf weeds. “Populations of groundsel are certainly increasing but control using SU type herbicides is poor. This means there’s an increasing pressure to maximise the potential of residual products.
“As a result, options such as Tower (pendimethalin+ diflufenican+ chlorotoluron) and Alternator Met (metribuzin+ flufenacet+ diflufenican) seem to be helping with the control of these weeds,” he concludes.
This article was taken from the latest issue of CPM. Read the article in full here.
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