Dirty dozen blackgrass counties

1 September 2000

Dirty dozen blackgrass counties

NOVARTIS has announced a change in the recommended timing for use of Hawk (clodinafop + trifluralin) on blackgrass in response to evidence that resistant blackgrass incidence is increasing rapidly across England.

In a phone survey of 500 growers commissioned by the firm earlier this year, nearly one in two wheat growing farms were found to have blackgrass and 19% of it is thought to be resistant by the growers.

Three years ago, more than 60% of that resistance was not known about and most of that rise is due to new resistant populations developing, believes the firms Mike Lickman.

Herts and Beds top the "dirty dozen" of counties, with 79% of combinable crops containing blackgrass (see table). But Lincolnshire has the highest proportion said to be resistant.

To stem the tide of resistance the firm is advocating a "total approach" to blackgrass management.

That starts with knowing the enemy, identifying hot spots of weed incidence and the type of resistance, if any, on the farm.

Reducing the seed burden by cultivation and straw management techniques should be the next step, followed by in-crop product selection, application, and timing, advises Mr Lickman.

"Growers should have the least reliance on chemical control as they possibly can. That way we can make maximum use of those herbicides when they are used," he says.

The survey reveals that stale seed-beds, pre-emergence herbicides and post-drilling rolling are widely recognised as important tools in control. But still only one in three growers are mapping weeds before harvest and only a half of those with resistant blackgrass have had it tested.

But few growers with blackgrass consider the weed "easy" to control. "Girlie blackgrass no longer exists," he says.

To cut the risk of herbicide resistance building up, the recommended timing for blackgrass control with contact and residual acting herbicide Hawk has been brought forward from two- to three-leaf stage of the weed to the one- to two-leaf stage.

While no change in efficacy is seen between the two growth stages, the move reduces the risk of delayed applications leading to doses on weeds larger than GS13, says herbicide product development manager, Jason Tatnell.

"Nowadays, we have to assume there will be some difficulty in control beyond the three-leaf stage," he says.

Any blackgrass smaller than one leaf at the GS11-12 timing should be taken care of by the residual trifluralin, to which there is no known resistance.

"We are much happier about the control of smaller weeds than we are about big weeds that might escape control in a resistance situation." &#42


&#8226 46% of English cereal land infested.

&#8226 19% of blackgrass resistant and rising.

&#8226 Make max use of all controls.

&#8226 Earlier timing for Hawk.

The dirty dozen

County % with % resistant


Herts 79 27

Beds 79 20

Northants 78 23

Essex 73 15

Suffolk 72 14

Oxon 64 24

Bucks 64 18

Cambs 62 13

W Sussex 61 7

Wilts 56 27

Glos 53 3

Lincs 53 34

*Combinable crops.

Based on survey of 500 growers across England, all with more than 100ha of cereals.

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