Herbicide resistance is forcing cereal growers to spend more of their grassweed control budget before weeds emerge, increasing the importance of planning, application timing and choosing cost-effective chemistry.
The loss of reliable post-emergence options has transformed weed control over the past 20 years. Growers with difficult blackgrass, Italian ryegrass and brome populations must now make many of their key decisions during the first month of the crop.
“The big picture is herbicide resistance has taken away options in the crop,” says Adas weed scientist John Cussans. “Grass weed management was a lot simpler when you could wait, look at the weed pressure and then put the herbicide on.”
Early decisions
Residual herbicides now dominate programmes, with flufenacet and cinmethylin forming the backbone for many growers tackling difficult grassweeds. Mr Cussans says the default strategy should be to use the strongest chemistry first, at true pre-emergence, where crop safety and conditions allow.
“I think if you have the conditions where you have a good scenario for crop safety, a good approach would be cinmethylin first with picolinafen or pendimethalin.”
Other partners could include ethofumesate, prosulfocarb or tri-allate, depending on the target weeds. Italian ryegrass particularly benefits from sequencing autumn treatments, although the same principle can improve blackgrass control. Growers should adapt that strategy where crop safety is more marginal. Rather than abandoning the pre-emergence treatment, Mr Cussans suggests starting with safer chemistry before applying cinmethylin at early post-emergence.
Dry conditions
That approach can also help when dry seedbeds threaten residual herbicide performance, says UPL technical sales manager Tom Wheelhouse.
UPL and ProCam trials last autumn tested a holding treatment using chemistry other than cinmethylin or flufenacet, followed by stronger products at early post-emergence.
“In those conditions, weeds were germinating for longer, so coming back with the strongest chemistry helped knock out the later germinating weeds.”
Trials comparing residual tank-mix partners showed relatively little difference between Xerton, containing ethofumesate, Avadex, containing tri-allate, prosulfocarb and pendimethalin, Mr Wheelhouse says. That puts greater emphasis on cost. Cinmethylin is a premium product, so growers may be able to contain programme expenditure by combining it with lower-cost partners such as Xerton rather than more expensive bixlozone or tri-allate products.
Target weeds
Price cannot be considered in isolation, however, because products need to match the weed spectrum in individual fields.
Xerton does not control Italian ryegrass, while prosulfocarb and bixlozone have useful activity against it. Ethofumesate and tri-allate, meanwhile, provide stronger options where bromes are the priority. Trials last season found that adding Xerton to a base programme of flufenacet plus DFF and Avadex Factor increased sterile brome control by eight percentage points.
The challenge is therefore to build programmes around both efficacy and value while maintaining diversity in chemistry and cultural controls.
“We are doing a lot of weed control in the first month, with very few options after that, and having to risk manage application by application, field by field,” says Mr Cussans.
That front-loaded approach leaves less room to correct mistakes later, he adds. Getting the first applications right has become increasingly important – both for weed control and the herbicide budget.