How drilling date affects arable weed populations

The interval between harvesting one crop and drilling the next is important, as a non-selective herbicide can be used on emerged weeds. Learn about drilling tactics to help you establish integrated weed management strategies.

Before drilling:

• Where possible, wait for a weed flush before drilling
• Note that dry weather reduces weed emergence
• Aim to kill all weed seedlings
• Use a combination of non-selective herbicide and cultivations
• Follow best practice to minimise glyphosate resistance risks
• Account for dry soils, which reduce the effectiveness of pre-emergence herbicides
• Be aware that cultivations, especially in moist soils, will not kill all seedlings

At drilling:

• Drill fields with low weed populations first
• Leave fields with high grass weed burdens until last
• Maximise crop competition – a product of crop/variety, seed rate and drilling date

Lower seed rates leave more space for weeds to establish but early drilling means crops have longer for tillering and so are more competitive than those drilled later.

Crop establishment declines in cereals drilled after mid-October. Seed rates should be increased to maintain yield. Late-emerging weeds are less competitive and produce fewer seeds.

The window for drilling winter barley is narrower than for winter wheat. A low vernalisation requirement means barley is less suited to very early drilling, while yield declines rapidly when drilled after mid-November.

Delaying drilling increases the time available for weed control but it can reduce subsequent crop competitiveness, although increased seed rate can help compensate.

The effectiveness of delayed drilling will depend on the germination period of the weeds and will be most effective for weeds with low dormancy and a clear autumn flush.

Understanding dormancy and the effect of weather and soil conditions is important. Weather during ripening determines black-grass dormancy. Low dormancy occurs in warm, dry conditions and seeds will grow rapidly if moisture is not limiting. In some years, black-grass germinates as the crop ripens. Conversely, cold, wet weather leads to high dormancy and delayed black-grass emergence from seed shed in the current season.

Shallow-cultivate barren and great brome seeds to bury them as soon as possible after harvest, unless chopped straw provides good seed cover. Meadow, soft and rye brome seeds are usually under-ripe, and burial immediately after harvest enforces dormancy. Leave seeds to ripen for one month before cultivating. Brome emerges quickly in moist soil and dormancy has little effect on emergence. Wait until brome has emerged and spray off with glyphosate pre-drilling.

Delayed drilling allows a longer period for predation and seed germination.

Delayed drilling has little effect.

Seed dormancy is short-lived and most seed emerges by November. Delayed drilling reduces populations.