17 August 2001

Plan cultivations with autumn weed problems in mind

By Andrew Swallow

WHERE to plough, min-till and early drill must be planned with grassweed problems in mind this autumn, says a leading expert.

"Think strategically about what the correct cultivation will be," says ADAS arable technical manager James Clarke.

Widespread poor control last autumn means many growers will face high seed returns this year. Ploughing that down could store up a problem for future years.

Early indications that blackgrass seed has low dormancy should allow a good chit before spraying off and drilling a little later, he suggests.

Achieving that chit is crucial. If seed does not germinate in numbers until after the crop is drilled post-emergence products will come under high pressure, increasing the risk of poor control and resistance developing. Ploughing should be considered where the chit fails.

Delayed drilling is a key tool, and unless a good chit is achieved before early drilling, the practice should be limited to fields with low or no blackgrass pressure this autumn.

Where blackgrass is the main target, seed return and cultivation in previous years must also be considered. "Dont just go back to last year, you need to go back a little further," says Mr Clarke.

Ploughing is not always the best approach. High seed returns from previous years may be brought back to the surface and with an annual rate of decline of 80%, about 3% of buried seed can still be viable three years later. "That can be a lot of seed."

However, if seed return was low last time the field was ploughed and two or three years of minimal cultivations have followed, the seed bank is likely to be low.

On such a field, ploughing would be the best way to reduce weed pressure in an early-drilled crop, given the limited time to chit and destroy this seasons blackgrass seed.

With brome knowing which species you are dealing with is crucial to timing post harvest cultivations (see panel). Similarly, destroying volunteers depends on which crop you have just combined.

"You have to exploit our understanding of weed biology. I really want people to get to grips with control in the key window – between harvest and drilling." &#42

Watch timings

Cultivation as close behind the combine as possible is recommended to stimulate volunteer cereals and barren brome to chit, says Mr Clarke. Oilseed rape, meadow brome, and wild oats are best left untouched for at least four weeks. It is not known what is the best approach with blackgrass. Whatever the regrowth, it should always be sprayed off. "It is cheaper and more reliable than cultivation."