Importance of Spring Weed Control – James Boswell – South East Farmer

Spring herbicides are likely to be more important than usual for keeping winter cereals weed free this season, given wide variations in autumn drilling and residual programmes, according to James Boswell, agronomist with Hutchinsons.

While many growers managed to drill early into favourable conditions for crop establishment and pre-emergence sprays, some were hampered by record breaking rainfall during October, resulting in crops going in late into sub-optimal conditions, potentially without any residual herbicide.

It is these later-drilled patchy crops that are at greatest risk from weed competition come spring, as underlying weed pressure is likely to be greater and they lack the competitiveness of better established, earlier-sown crops.

Wheat drilled early into high-risk grassweed situations could also come under more pressure in spring, especially if there was not sufficient time between harvest and drilling for the usual cultural controls such as stale seedbeds.

Attention often focuses on black grass, but while this remains a big issue in many areas, ryegrass is also a significant threat. It can be a beast to control if it’s allowed to spread and establish, so if you see any ryegrass, you have to take a zero-tolerance approach.

Even if residual chemistry was applied last autumn, there could be question marks over its efficacy given heavy rain in October, which may have washed actives deeper into the soil.

Where fluefenacet-based residuals were applied for grass weeds, top up levels with additional flufenacet as soon as conditions are suitable for travel in early spring.

By maintaining residual levels, we can hopefully tackle the problem at the root before weeds emerge.
Remember, applying a flufenacet top-up of 120g a.i/ ha is only effective if there is some residual level left in the soil after autumn treatment; that amount won’t do much on its own.

If young black grass is already present, contact- acting products based on iodosulfuron-methyl- sodium and mesosulfuron-methyl are generally most effective. It may also be worth including a residual product in the mix to pick up any later emerging weeds.

Mr Boswell says crops should be monitored closely over the coming weeks, allowing growers to respond with the appropriate herbicide programme.

For all grass weeds, the best results come from applying herbicides early to small weeds, before tillering, and by making sure they are actively growing.

Good application technique is also vital to ensure contact-acting chemistry hits the target, so slow down and get boom height right to maximise spray coverage on weed leaves.

Broadleaved weeds tend to be less of an issue in cereals where effective residual programmes have been applied, and many species will be controlled by sulfonylurea chemistry applied for grass weeds.

Their later emergence also means crops should be better established and more able to outcompete emerging broadleaf weeds, although there can be issues in gappy and thin crops, or where there is a field history of a particular weed problem.

Where targeted broadleaf weed control is required, there is a wide range of chemistry based on ALS and hormone actives, so it is all about matching the herbicide to the weed spectrum.