Vigilance beats blackgrass
Herbicide-resistant grass weeds are on the wane after a 10-year battle on an Oxfordshire farm, thanks to a mix of cultural and agrochemical control methods. Robert Harris reports.
RESISTANT blackgrass and Italian ryegrass thick enough to produce a silage crop in some fields have plagued W Cumber and Sons Manor Farm, Marcham, Abing- don, for more than a decade.
A huge effort to control the weeds seems to be paying off. But constant vigilance is needed to keep on top of the problem, says Laurence Sim, a member of the Association of Independent Crop Consultants.
Both grasses are found on the alluvial soils that account for a third of the 400ha (1000 acres) of arable land.Blackgrass is also common on the heavy clay soils and on some lighter sands.
Chemical control started to slip in the early 1980s. Ash from burnt stubbles was blamed. "At that time, the thinking was that it locked up chemical," says William Cumber.
However a split field experiment to test the theory suggested something more sinister. Control was no better where ash was thoroughly incorporated compared with the farms minimal cultivation.
By the mid-1980s, resistance had been discovered in Essex. Mr Sim sent samples away for testing, and metabolic-type resistance, where plants break down chemical before it has time to work, was confirmed at Manor Farm. Such plants are partly resistant to a range of chemistry.
"In hindsight, we were the classic high risk case," says Mr Cumber. "We grew continuous cereals, and everything was minimum cultivated – we didnt own a functional plough. And we relied heavily on residual herbicides including chlorotoluron (as Dicurane) for grass weed control."
Isoproturon, which was less prone to lockup, replaced it and Commando (flamprop-M-isopropyl) was used to take out wild oats and suppress surviving blackgrass. But control continued to decline so Hoegrass (diclofop-methyl) was introduced.
"At first Hoegrass worked on both blackgrass and ryegrass," says Mr Cumber. "But euphoria only lasted a couple of seasons. "
Staff faced a losing battle, admits arable manager Gus Hartley Russell. "We relied heavily on a good burn to destroy seed and encourage chitting." Crops were sprayed with glyphosate (Roundup) pre-harvest to ensure the best burn. IPU was applied autumn and spring.
The straw burning ban accelerated the uptake of new control policies, says Mr Cumber. "We started to plough everything and introduced break crops."
Ploughing is now the cornerstone of a complex control programme. But the friable loam soils tend to break when ploughed. Mr Sim suspects seed scatters through the profile so it is difficult to achieve the theoretical 70% control if seed is buried for 12 months.
Winter rape is the main break crop, spring rape and peas are also grown on lighter land to allow a wider pre-drilling control window. Such broad-leaved crops allow the use of more potent chemistry.
Longmoor, one of the worst fields, was the first to be cropped with a break in 1992. The 22ha (55 acres) of winter rape emerged well. "So did the blackgrass. We sprayed it with Fusilade, but with little effect," says Mr Cumber.
Laser (cycloxydim) applied in spring worked well. "Evidence shows that Laser is probably the best of the fops and dims. A graminicide designed for use in broad-leaved crops can be stronger on grass weeds since there are no safety problems," says Mr Sim.
In winter and spring rape Laser often follows incorporated trifluralin. It is also used in the winter crop with pre- and post-emergence Butisan (metazachlor) where cleavers are a problem. Only in peas is Laser used alone.
In cereals, Cheetah Super (fenoxaprop-ethyl) and Topik (clodinafop-propargyl) have been tried individually after IPU, but with variable results. "We have now adopted the standard herbicide guideline treatments," he says.
Pre-emergence Avadex (tri-allate) granules applied after drilling are followed by an IPU/trifluralin mix at early post-emergence. That is more effective than IPU and either one alone. It costs about £50/ha (£20/acre), double most grass weed control budgets, but is cost-effective, says Mr Sim.
Although reluctant to use fops or dims in cereals, he is considering Hawk (trifluralin + clodinafop-propargyl) this autumn following encouraging reports last season. Grasp (tralkoxydim) also shows promise on ryegrass.
All chemicals are applied at full rates. "There is no scope to reduce them," says Mr Sim. Careful cultivations boost efficacy. "You have to be prepared to put in the extra effort to get fine enough seed-beds," says Mr Hartley Russell.
Seed is drilled 1.25-2.5cm (0.5-1in) deep to avoid spray delays. "Seed must be kept away from Avadex and if heavy rain follows the post-emergence spray a shallow-drilled crop will know about it," says Mr Sim. Timing is crucial, he adds. "Once blackgrass has tillered, youve lost the battle."
Heavily infested areas are spot treated with glyphosate after the crop has flowered to limit weed seed return.
All stubbles are ploughed straight after combining and left to green up before burning off with 1.5 litres/ha (1pt/acre) of Sting (glyphosate). If possible, badly affected fields are drilled last to ensure maximum regrowth. "The worst fields tend to be the heaviest," says Mr Hartley Russell.
Set-aside has been rotated around badly affected areas. Natural regeneration is sprayed when blackgrass finishes stem extension, but before seed set. It is then ploughed
The control programme is working. "Grass weed populations are definitely falling," says Mr Cumber.
"But we can still find plenty of grass weeds," adds Mr Sim. "Its vital to keep on top. In a ploughing regime you have to achieve 80-85% control with chemicals, which we are doing.
• Ploughing buries seed.
• Broad leaved crops compete with weeds and widen chemical choice.
• Stubble cleaning reduces burden, delayed drilling widens window.
• Spot treatment cuts seed return.
• Sequence and mix of different chemistry improves control.
• Fine seed-beds and correct timing boost efficacy.
Grass weed fight: William Cumber (standing) and Gus Hartley Russell.