A good herbicide strategy is vital – and must be flexible too, say experts
Growers are being urged to think carefully about how best to tackle changing grassweed threats with herbicides this autumn. Sequencing carefully chosen actives through autumn into winter will be far more effective in many cases than relying on big “stacks” of chemistry, says Hutchinsons technical manager Dick Neale.
There are several reasons for this – one being the changes in blackgrass emergence patterns seen recently at Hutchinsons’ blackgrass demonstration site at Cambourne in Cambridgeshire – and indeed elsewhere.
“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,” says Mr Neale. “We are finding blackgrass 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.”
Seedbeds
Stale seedbeds during September and early October remain valuable, particularly for controlling volunteers.
But in many areas, they are generating far fewer numbers of emerged blackgrass plants than in the past, adds Mr Neale. “Sequencing 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 crops were drilled early, thus widening the gap between pre-emergence spray applications and October grassweed emergence. Sequences are also suited to situations where warm, dry conditions have reduced the residual activity of pre-emergence applications, says Mr Neale.
Beyond blackgrass
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, he adds. 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 blackgrass focused, and increasingly, the control of other weeds should be factored into product decisions.”
Many 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.
Weed flush
Agronomist Richard Watkins says 80-90 mm of rain during September stimulated a flush of grassweeds. Where growers drilled early in dry conditions, follow-up herbicides will be central to tackling mounting pressure within the crop. Some farms still get a good early autumn flush of blackgrass, which this year came through in late September. Growers are being advised to wait to spray off this first flush with glyphosate before drilling, and in the case of ryegrass, maybe wait for two flushes.
But Mr Watkins explains: “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 increased in some situations, where wheat was drilled early in mid-September. But no pre-emergence spray was applied due to forecast heavy rain at the time.
Soil conditions
In other cases, where pre-emergence application was made 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 the right conditions helped.
Mr Watkins says this year is very different, so growers have to react appropriately to every situation. In some high pressure situations for ryegrass, 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.
For blackgrass and ryegrass, Mr Watkins favours a sequence of an aclonifen-based pre-emergence spray, then cinmethylin-based products at the peri- or post-emergence stage. This should possibly be followed by a prosulfocarb and chlorotoluron + diflufenican + pendimethalin mix later on if required – depending on weed pressure and crop growth stage.
Grassweed headache focuses on brome
Brome is 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,” says Hutchinsons agronomist Richard Watkins.
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. “Having a later applied herbicide top-up in the new year can be really useful. But we’re in a wet part of the country and getting on to apply late-season herbicides can be quite challenging. Some farms have resigned themselves to doing 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. This is followed by chlorotoluron + diflufenican + pendimethalin, or prosulfocarb later to extend coverage as long as possible, Mr Watkins suggests.