Sequences not stacks for effective autumn weed control
With cereal drilling well underway, growers are urged to think carefully about how best to tackle changing grassweed threats with herbicides this autumn ...
Hutchinsons technical manager Dick Neale says that in most cases, sequencing of carefully chosen actives through autumn into winter will be far more effective than relying on big “stacks” of chemistry.
He highlights several reasons for this; one being the changes in black-grass emergence patterns seen at the Cambourne black-grass demonstration site and elsewhere in recent years.
“Where growers have focused on cultural controls and spring cropping over the past 8-10 years, it has done a terrific job in reducing overall populations, and we are finding black-grass is now emerging in smaller numbers, but it’s extending far longer into the autumn. In many situations the population is now below 50 plants/m2 but their tillering potential is significant.”
Stale seedbeds during September and early October are still valuable, particularly for controlling volunteers, however in many areas, they are generating far fewer numbers of emerged black-grass plants than in the past, he adds
“Sequencing of herbicides is therefore far more effective at covering this protracted emergence than relying on single big stacked applications.”
The case for sequencing treatments is further strengthened where growers have drilled early, thus widening the gap between pre-em applications and October grassweed emergence, and where warm, dry conditions have reduced the residual activity of pre-ems, he says.
Look beyond black-grass
Another reason for sequencing chemistry is to help tackle the mix of other grassweed species that are creeping into more fields, such as mixed populations of sterile and rye, soft or meadow brome, Mr Neale says.
This increases the risk of spring-emerging elements in the weed population, making it necessary to use later season residuals to extend control into the new year.
“The choice of active ingredient may not be exclusively black-grass focused, and increasingly, the control of other weeds should be factored into product decisions.”
Many leading herbicides offer good control of a range of grassweeds, and often bring additional activity against difficult broadleaved weeds too, such as groundsel, poppy, and bur chervil.
Tackling a flush of weeds
In the West Midlands, Hutchinsons agronomist Richard Watkins says 80-90 mm of rain during September has stimulated a flush of grassweeds, and where growers drilled early in dry conditions, follow-up herbicides will be central to tackling mounting pressure within the crop.
“In this area, many farms usually do still get a good early flush of black-grass in the autumn, which we are seeing coming through now (at the time of writing in late September). The aim is to always wait to spray off that first flush with glyphosate before drilling, and in the case of ryegrass, maybe wait for two flushes.
“However, where growers haven’t done that, and perhaps drilled earlier than they should on higher-risk fields, weeds are appearing within the crop.”
Pressure has been compounded in a few situations, where wheat was drilled early in mid-September, but no pre-em was applied due to forecast heavy rain at the time, he notes. In other cases, where pre-em’s were applied to early-sown crops, there is a risk warm, dry conditions will have quickly reduced their efficacy.
“It’s surprising how much moisture you need to get some pre-ems to work effectively. Big stacks of residuals did a great job last season, but they had the conditions to help. This year is very different, so growers have to react appropriately to every situation.”
In some of the highest ryegrass pressure cases, where people have drilled early, three sequential herbicide applications may be needed before Christmas, or early New Year to cover protracted germination within the crop, he suggests.
For black-grass and ryegrass, Mr Watkins favours a sequence of an aclonifen-based pre-em, then cinmethylin-based products at peri-/ post-emergence, possibly followed by a prosulfocarb and chlorotoluron + diflufenican + pendimethalin mix later on if required, depending on weed pressure and crop growth stage.
brome headache
Mr Watkins says brome is becoming an increasing issue on some farms, with cases of resistance to contact herbicides making control a real challenge. “We’re seeing meadow, soft, and sterile brome, but it’s the meadow brome that’s the main problem.”
Germination is often protracted, with an initial flush in early autumn, then further emergence in February. It is these later emerging populations that can pose a particular headache, he says.
“It’s where having a later applied herbicide top-up in the new year can be really useful, however we’re in a wet part of the country and getting on to apply late-season herbicides can be quite challenging, and some farms have resigned themselves to having to do some roguing.”
Where resistance to contact chemistry is confirmed, a change of thinking may be needed, with more focus on autumn pre- or peri-emergence timings, he notes.
Herbicide programmes for brome generally centre around a pre-em based on tri-allate, flufenacet, or ethofumesate, depending on the species present, followed by chlorotoluron + diflufenican + pendimethalin, or prosulfocarb later to extend coverage as long as possible, Mr Watkins suggests.
Getting the most from pre-ems
1. Weed emergence – grassweeds need to germinate within the herbicide active zone in the 10-14 days following a pre-em application. Also consider whether or not initial cultivations have moved grassweeds to a depth where germination into the herbicide zone will occur
2. Soil moisture – sufficient moisture is needed for weed germination, to facilitate uptake of residual herbicides, and to move herbicide into the soil to prevent rapid degradation at the surface. Soil moisture conditions are highly variable across the country, so react appropriately to individual situations
3. Application – use sufficient water for good efficacy (ideally 200 L/ha), keep forward speed and boom height low, and use the correct nozzle
4. Crop safety – this is driven by seedbed quality and drilling depth. Avoid cloddy seedbeds, which, by definition, are likely to be too dry for residual activity and will mean variable seed depth. Heavy rainfall can exacerbate the crop safety risk to shallow-sown seed for some chemistry.
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