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Managing metazachlor in OSR

Course: Oilseed rape | Last Updates: 9th October 2015

Steve Townsend
The Soil First Trading Company
Biography >>


Getting into water

There are two ways that metazachlor can end up in our water supplies.

The first – farmyard sources – occurs when the sprayer is being filled and cleaned. Much has already been done to eliminate this point-source pollution, but it is essential that good operator practice continues to be applied at all times.

Good agronomic practice and sol management can help cut the risk of metazachlor reaching field drains.

The second – field sources – occurs when herbicides get into water via field drains or as surface run-off.
While these represent more of a challenge, especially when we have a wet autumn, they can be minimised through good agronomic practice and soil management.

Water legislation

Metazachlor is just one of the pesticides that water companies find in some surface water catchments.

When these reach high concentrations in raw water, they challenge water treatment processes.
Therefore, there is a risk that the water industry will have to take tougher action to reduce levels and prevent any breaches. 

Drinking water protected areas have been established within the Water Framework Directive, with those being considered at risk of deterioration also having upstream safeguard zones where specific action can be targeted.

To find out if your farm is in a designated safeguard zone, go to the Environment Agency’s website What’s In Your Backyard?

Seed-bed preparation

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, it’s important to maximise natural crumb and soil structure.
Oilseed rape is usually established after cereals, when there is a natural tilth on the soil surface and the previous crop has left the soil in good condition.
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.

For a crop being established after cereals, the combine harvester should be considered as the first cultivation pass, especially when it comes to residue management.

Chopped straw will help to build soil structure in the long term, so it’s best to start chopping now.
Pre-harvest glyphosate will help to take out any weeds in the bottom of the crop and allow a better straw spread, particularly at the beginning and end of the day’s harvesting.
Set the combine to leave the stubble as high as possible in relation to the drill type that you will be using.

Heavy soils will shrink and crack as they dry out, which is a good sign that the soil is structuring itself. However, 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 shrinkage – the soil will still crack, but not so severely. Organic matter can be built by:

  • Chopping crop residues
  • Growing cover crops
  • Reducing tillage to zero

After five years of reduced tillage (less than 10cm deep), there will be a noticeable difference.
The soil’s soak ability will have improved, as will its resilience to wet and dry weather extremes.
The aim to end up with damp and moist soil, so that crops can get away earlier and be more resistant to pest attack.

Drilling date

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", where it is most likely to germinate quickly and evenly.

This also gives more time for the crop to get well established and put its roots down into the soil.
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.

Early drilling will also help the crop withstand other pressures, such as flea beetle and slugs.

Application rate and timing

There are two rate restrictions which apply to metazachlor. 
A maximum of 750g of active/ha can be applied, with no more than 1,000g being used over a three year period in the same field.

On drained fields, avoid applying metazachlor after 1 October.

The best timing for metazachlor applications is before the weeds emerge, which means it needs to go on early.
However, if soils are wet, or wet weather is forecast, spraying should be delayed, as this is when the peaks in water are seen.

On undrained land, there are no timing restrictions. But on drained fields, applications should be avoided after 1 October, unless conditions are favourable and drains are not flowing, in which case it can be applied up until 15 October.
Applications to drained land in Drinking Water Safeguard Zones should not be made after 1 October.

Similarly, don’t spray if soils are very dry and cracked, as this allows the herbicides to go straight through.

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