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Weeds in Spring crops, April 2019

20 April 2019

Spring barley

If annual meadow-grass is a potential problem in establishing spring barley, then a residual herbicide is the best option for effective control.  This is particularly relevant to reduced tillage systems where annual meadow-grass can become a significant problem.  The downside to residual herbicides is that they don’t work well in dry soil – which may be an issue this year as some soils are currently quite dry.  Also there are gaps in the weed spectrum of these residuals, so a follow-up post-emergence herbicide may be needed.

Spring Oats

Spring oats are more competitive with weeds than spring barley or wheat and so less reliant of herbicides to suppress weeds.  There are no on-label recommendations for residual herbicides.  For broad-leaved weeds, various post-emergence herbicides are available. Always check the label because quite a few cereals herbicides can’t be used in oats because oats tend to be more sensitive to herbicides than wheat or barley.

Oilseed rape

A residual herbicide is not always needed in spring oilseed rape in good conditions where the crop grows rapidly away from weeds. However, if there are weeds that could contaminate the harvested sample or if there are problem weeds such as shepherd’s purse, chickweed and cleavers then it is prudent to apply a herbicide.

Forage and vegetable brassicas

Due to limited post-emergence herbicide options, pre-sowing and pre-emergence weed control are crucial, particularly in poorly competitive crops such as Swedes. A stale seedbed followed by pre-emergence treatment is a good option. The stale seedbed is useful to tidy up any perennials and flushes of spring germinated broad-leaved weeds using glyphosate pre-sowing. The stale seedbed is particularly useful if there are species such as charlock or runch in the field not controlled by the residual herbicides.  Kale is a competitive crop, however a residual herbicide will ensure it gets off to a clean start. Once established, it can be very effective at suppressing weeds.

Transplanted brassicas (sprouts, calabrese, cauliflower)

These can be relatively slow to come away, increasing the potential for weed competition if weed control programmes are inadequate. There are on-label recommendations in brussels sprouts, Calabrese and cauliflower for a number of products. The following species are NOT controlled: fumitory, hemp (day)-nettle, pennycress, field pansy, knotgrass, fat hen, small nettle, redshank and corn spurrey are poorly controlled.

Many early sown crops are grown under cover for at least a part of the season. This can also increase weed growth, and yet there are major limitations to the use of herbicides where covers are used.  Some residual herbicides all stipulate that they are not to be used on protected crops.  If a crop is covered temporarily some of these residual products can be used once the crop has hardened-off after removal of the cover to kill weeds not yet emerged. But care should be taken or crop damage may ensue.

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