22 March 2002

Growers save time with post-em

By Andrew Swallow

THE wide range of post-emergence herbicides now available in sugar beet gives growers much more management flexibility and the opportunity to reduce post-emergence passes, especially on organic soils or where timesaving is a priority, says Morley Agricultural Consultants.

"If you have a high spring workload, post-emergence sprays can be very broad spectrum now," says Peter Riley. "Programmes can be made very flexible for those who want to reduce passes."

One of the most robust he recommends, usually on medium to heavy mineral soils, is phenmedipham, ethofumesate, Venzar (lenacil) and Debut (trisulfuron-methyl), plus oil.

"Two doses of that will retrieve most situations if the first pass is delayed, or you can deliberately delay the first pass if you have other operations you must be doing. It comes down to a balance of the chemical cost and the cost of spraying."

But keeping the interval between the two passes tight is still important, he stresses.

"The two sprays need to be relatively close together, especially if you are trying to retrieve large or difficult weeds – seven to 10 days maximum."

The four-way mix is approved by Debut manufacturer Du Pont from the first true leaf of the sugar beet at 2.5cm (1in). Where cleavers and/or brassica weeds are present Mr Riley says the Debut rate needs to be at or close to the full 30g/ha but elsewhere it may be reduced. Similarly the Venzar rate may be reduced if weeds are small.

"But I tend not to alter the rest," he comments.

Often two passes with such a mix will allow the gate to be shut on weed control. However, a watch should be kept for late germinating fat-hen. Phenmedipham, possibly plus a residual such as lenacil and oil should be applied if any is found.

For those on organic soils Debut post-emergence mixes are particularly useful because of the inactivity of residuals on such soils, says colleague Phil Strachan.

"They can cut the number of passes required by at least one, if not two," he says.

Generally, for key fen weeds such as redshank, fools parsley, pale persicaria, annual nettle and annual mercury, his preferred start to a programme is phenmedipham, ethofumesate and Debut, plus or minus oil.

But if bindweed is a problem Venzar replaces the ethofumesate. "In my experience it has given better results," he comments.

A similar mix is appropriate for the second pass, with oil. If volunteer potatoes are present Debuts rate should be raised to the full 30g/ha and 0.35litres/ha of Shield (clopyralid) considered.

"Id use the full rate too if weeds such as fools parsley are getting big but generally about 20g/ha is enough."

After two mixes containing Debut Mr Strachan suggests growers move away from it unless fools parsley or cleavers are persisting. Phenmedipham, ethofumesate and Shield is a possible third and fourth pass option where volunteer potatoes are present, perhaps plus Goltix (metamitron) at 1kg/ha to aid fat-hen control.

"Or you could go back to the old-fashioned Betanal plus Nortron if you are purely aiming for fat-hen," he comments.

One situation where Debut should not be used to strengthen or stretch the programme is where sugar beet has been sown into a barley cover crop, he stresses. "After a treatment of Debut the barley may not be growing actively enough for the graminicides to work," he explains. &#42

STRETCH POST-EMS?

&#8226 Consider where time limiting.

&#8226 Debut key component.

&#8226 Reduces passes on organic soils.

&#8226 Beware resistance risk.

Drilling progress

British Sugar estimates 30% of the intended sugar beet crop in East Anglia was sown by Monday this week, 25% in the Newark factory area, and 15% in the York catchment. In the West Midlands less than 5% had been planted.

Resistance risk

As Debut is a sulfonylurea and an ALS-inhibiting herbicide, growers should beware the risk of resistance developing to such products where used repeatedly throughout the rotation, says Mr Strachan. "Ensure these herbicides are tank-mixed wherever possible, especially on those weeds where resistance could be exacerbated and there are alternative controls, such as in cereals."