28 August 1998

Cut cost of weed control

By Andrew Swallow

ON a no-blackgrass farm, adequate autumn weed control should cost no more than £13/ha (£5.25/acre), and can be as low as £5/ha, say independent agronomists.

"Last year we acheived good control for £5-5.50/acre in non-blackgrass situations," says Tony Howell, amember of the Association of Independent Crop Consultants from near Market Drayton, Salop. "Full-rates are seldom justified."

He selects a broadleaved weed-killer according to the weed spectrum and soil type, usually with an isoproturon top-up. "For over 75% of growers in the west this means dff (diflufenican) plus ipu," he says.

Application rate needs tailoring to weed size and soil conditions on the day of spraying, he stresses. "In later drilled crops, weeds are smaller, so less chemical is needed. Similarly finer seed-beds mean lower rates. For example, a late-October drilled crop of wheat on a level and moist seed-bed may only need 25g/ha of dff," he suggests.

Isoproturon is topped up to 1000g/ha, to provide consistent meadowgrass control.

Where cleavers are a particular problem, pendimethalin replaces dff. Similarly, with severe ryegrass or autumn germinating wild oats, switching from ipu to chlortoluron may be an advantage.

Growers disappointed with the activity of ipu activity last autumn should look back at their spray records for reasons.

In very dry situations weeds stop chitting, so residual applications, especially ipu, should be delayed until sufficient moisture is available for them to work. "Current thinking is that ipu should be used as a post-emergence product to reduce leaching risk," he says.

In such a dry spell, if early emerged weeds are competing with the crop, a contact product such as CMPP or HBN applied with the BYDV spray would be justified.

Trifluralin can prove useful where there are specific farm preferences for the chemical. "It is very cheap, so where it has worked in the past, growers like to use it," he says.

Scottish Agronomys Andrew Gillchrist confirms this use of trifluralin. "We have seen good results with it used straight on later sown cereals in Scotland. At 2 litres/ha, that is about £5/ha."

Mr Howell anticipates using more pendimethalin than dff this autumn. "It is cheap, effective in moist conditions, and has a broadspectrum, including cleavers. I suspect ipu/dff mixtures are going to be in tight supply." &#42

KEY POINTS

&#8226 Select broadleaved product first.

&#8226 Top-up to 1000g/ha ipu, or equivalent.

&#8226 Keep cost under £13/ha.

&#8226 Use ipu post-em of weeds.

&#8226 Chlortoluron not ipu for wild oats and ryegrass.

&#8226 Diflufenican (dff) for most broadleaved weeds.

&#8226 Pendimethalin stronger on cleavers.