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Weeds

Controlling broad leaved and grass weeds is essential in winter crops to ensure profitable yields and trouble free harvesting. Our wet climate places increases challenges on the persistence of autumn applied herbicides whilst also favouring the growth of grass-weeds especially annual meadow grass (Poa annua). The main aim of autumn applied herbicides is to reduce the germination and competition effect of autumn weeds until crop growth takes off in the spring. Occasionally a follow-on herbicide may be required in the spring where autumn control was not satisfactory or a particular weed (e.g. Sterile Brome) is problematic. Timing of autumn herbicides will depend on the weed challenge and spraying opportunities in the autumn. Reduced (lower than recommended by the manufacturer) rates have been successful in Teagasc trials over a number of years but depended on the weed species and growth stage of the weeds at the time of spraying. Reduced rates give more consistent results in barley than wheat due to crop competition effects.

Integrated Pest Management (IPM) techniques should be used where possible. For more information on IPM visit: Integrated Pest Management

When deciding which product to use, assess your priorities. Key factors in your selection process will be:

  1. Weeds present in the crop.
  2. Safety of the crop, operator and the environment.
  3. Compatibility with other products.
  4. Cost of the treatment compared with the benefits.

1. Pre-Emergence Weed Control – This is recommended for fields sown early, or where there is a history of grass weed problems, or where there may be difficulty getting back onto the land later in the autumn due to wet weather. Adequate soil moisture is essential for the success of these products, and the seed should be adequately covered with about 3cm of soil.

2. Post-Emergence in Autumn/Winter – This allows assessment of the weed problem to be made. The chemical costs are generally lower, and an aphicide for Barley Yellow Dwarf Virus control can be added, if necessary. The disadvantage is that ground conditions may not be suitable for spraying.

3. Post–Emergence in Spring – This is recommended for late sown crops, where autumn control wasn’t possible.

Some weeds germinate mainly in the autumn, some mainly in the spring, and others will germinate throughout both times of the year. The following lists contain the germinating times of the more commonly found weeds:

Autumn mainlySpring mainly
  • Cleavers
  • Hempnettle
  • Field Buttercup
  • Parsley Piert
  • Polygonums
  • Chenopods
  • Red Deadnettle
  • Fools Parsley
  • Black Nightshade
  • Pennycress
Autumn and Spring
  • Fumitory
  • Field Speedwell
  • Common Chickweed
  • Shepherds Purse
  • Runch
  • Corn Marigold
  • Sowthistle
  • Mayweeds
  • Groundsel
  • Poppy
  • Charlock
  • Spurry
  • Ivyleaf Speedwell
  • Forget-Me-Not

 

Some weeds, for example Corn Marigold, have an extended germination period and for this reason it can be difficult to control it. If you go too early with a contact herbicide, you will miss the later germinating plants. If you go too late, you will have difficulty killing the more advanced plants. The answer is to catch it with a mixture which has contact and residual activity.

Spring spraying can start when crops are healthy, vigorous and at a suitable growth stage.

The problem with leaving everything until the Spring is that, generally there are only a few weeks available before the crops advance beyond the stage that is safe for the application of the cheaper, hormone based sprays. During this short period, there may be actually only a few suitable spraying days. A wet Spring can allow a weed problem to become very difficult to control.

It does not pay to spray under adverse conditions when the crop might be suffering from stress, e.g. after a cold or wet spell. This could cause damage to the crop or unsatisfactory weed control. Good growing conditions usually produce the best results.