The key to achieving the best control of the hard-to-kill annual grass weeds – which are the main threats to most growers – is to take advantage of every pre-planting opportunity to get autumn cropping off to the cleanest possible start.
Eliminating large numbers of grassweed and volunteer seedlings at their youngest and most vulnerable stages ahead of sowing avoids inevitable in-crop herbicide compromises, while providing valuable insurance against less than ideal pre-emergence control conditions. It also reduces the risk and expense of relying too heavily on post-emergence treatments.
The best strategy involves a careful integration of chemical and cultural controls out of the growing crop, based on a thorough understanding of the enemy.
Starting well before harvesting and extending into early winter, this should involve: Knocking back weed populations with pre-harvest glyphosate as part of modern cereals and oilseed rape harvest management to get control off to an invaluable early start.
Correct combining to reduce cultivation requirements and substantially improve the quality of stale seed-beds for weed control. Effective cultivation and consolidation immediately after combining to ensure the best conditions for good stale seed-bed weed growth in even the driest of summers.
Timely glyphosate treatment of weed and volunteer seedlings to allow at least one more stale seed-bed cycle ahead of drilling. Well-planned pre-emergence and/or early post-emergence herbicide treatment in cereals and oilseed rape to optimise residual, as well as contact control of later-emerging weeds.
Quality stale seed-beds
With good tillage management, four weeks between harvesting and drilling is enough time to get on top of annual and perennial weeds and volunteers with at least two good pre-planting glyphosate treatments in most seasons. Combining should always be seen as the first cultivation. Leaving a short stubble, avoiding baling, chopping straw consistently to about 100mm (4in) and spreading straw and chaff evenly over the full width of the combine will minimise the trash problems that get in the way of stale seed-bed consolidation and maximise soil moisture retention.
Because soil moisture losses increase dramatically as soon as the crop canopy is removed, it is vital to cultivate within 48 hours of combining, if possible, to provide the best environment for weed seed germination. Cultivating only the top few inches will further limit moisture loss while giving enough stimulation for good germination.
The most important tool in the stale seed-bed armoury is the Cambridge ring roll, which should be used within a day of cultivation to ensure the high level of soil contact essential for the rapid germination of small weed seeds. Particular care should be taken to avoid excessive speeds which limit consolidation effectiveness.
Prompt, early spraying
Stale seed-beds should be inspected regularly so emerging weeds can be sprayed when the biggest has a single leaf. At this stage – between seven and 14 days after cultivation in most cases – the fields still look brown from a distance, so close inspection is essential.
Cultivating (if necessary) and consolidating the ground again as soon as product recommendations advise will allow a further flush of weed growth ahead of final seed-bed preparation and drilling.
Two stale seed-bed cycles are invariably better than one, because the first weed seedlings to germinate secrete auxins which build-up in the soil to inhibit further seed germination.
As well as cutting several days off the stale seed-bed cycle, prompt treatment of the early germinators limits this inhibition, enabling another flush of weed growth (which would otherwise come through in the true seed-bed) to be stimulated and sprayed off.
Timely drilling – within a maximum of five days after the last stale seed-bed cycle – can be ensured both by using a glyphosate formulation with a short cultivation interval and by employing a high capacity cultivator drill.
By ensuring even depth drilling at speed under a wide range of conditions, a cultivator drill rather than a traditional Suffolk coulter machine will also maximise the opportunity for effective pre-emergence herbicide treatment.
Good glyphosate management
If good pre-planting weed control is to be achieved without any delay to the main winter drilling schedule, glyphosate spraying must be managed as effectively as possible. With little more than a month between harvesting one crop and drilling the next on many units, poor or delayed weed control is simply not an option. Nor can hot and dry conditions, rainfall within a few hours of spraying, or hard water conditions be allowed to compromise efficacy.
Under these circumstances, it is important to use glyphosate formulations designed for challenging conditions rather than merely increasing rates.
These should be complemented with specialist adjuvants or water conditioners in hard water areas. Medium flat fan nozzles are recommended for optimum results on one-leaf grasses. With low water volumes, in particular, well-formulated adjuvants can be valuable in reducing drift to safeguard margins and hedgerows. Water volumes of 100 litres/ha are recommended under normal conditions, with very low volumes avoided if the weather is particularly hot and dry.
Spraying early in the morning is recommended to optimise uptake under dry conditions. Multiple applications must, of course, not exceed the specific product label’s statutory maximum dose of glyphosate. Steve Townsend is an independent tillage specialist, with over 24 years’ farm advisory experience. See ww.cropsystems.co.uk
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