Metazachlor is a key herbicide for tackling weeds in oilseed rape crops and a new Farmers Weekly academy outlines how growers can reduce the risk of it being found in water by changing their autumn cultivations.

The residual herbicide is considered to be the foundation of most early-season oilseed rape herbicide programmes.

But it is one of the autumn-applied active ingredients that has been found in untreated drinking water supplies.

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There are two ways that metazachlor can end up in our water supplies, says Steve Townsend of the Soil First Trading Company.

First, there are farmyard sources, caused when the sprayer is filled and cleaned.

Second, field sources, when herbicides get into water via field drains or as surface run-off.

Looking at cultivations, he says: “The conditions that give rise to optimum oilseed rape establishment also work well for metazachlor stewardship.”

The fine, firm, moist seed-bed required for the crop to get off to a good start also helps to keep metazachlor where it belongs.

Moisture is needed to activate residual chemistry, just as it is required for the crops to get away.

To achieve a fine seed-bed, he explains it is important to maximise natural crumb and soil structure.

The best way to maintain this natural soil structure is to do nothing to it.

Seed-bed moisture will also be maintained by not cultivating the soil, while keeping any previous crop residue on the soil surface also helps to prevent moisture loss.

Heavy soils will shrink and crack as they dry out, which is a good sign that the soil is structuring itself.

Any large cracks are a potential route for herbicide, which may then end up in surface water.

“The long-term solution to cracking is to improve organic matter content. This helps to reduce the amount of shrinkag,” says Mr Townsend.

He advises that oilseed rape should be drilled by the last week of August and planted at least 2cm deep.

“This places the seed into an area of consistent moisture,” he adds.

Any metazachlor can then be applied before the end of September, in good conditions for it to work well, and while there is little risk of it getting into surface water.