Grassland Weed Control

Weeds can seriously reduce your available grass and grass silage yield and quality. For every 1% ground cover by weeds, you can assume that your pastures are losing 1% yield. While various weeds can cause problems in grassland, the three most common are docks, thistles and nettles. These weeds if left uncontrolled can quickly start dominating grasslands.  Seeds from these weed plants are very resilient, can remain dormant in the soil for years and then germinate anywhere the soil surface is exposed and conditions are favourable. 

Control Strategies
Good grazing management practices help to minimise grassland weed problems i.e.:
• Drainage
• Maintaining soil fertility
• Avoiding poaching

Where weeds manage to establish, weed control is highly dependent on herbicides. However, it is important to remember that as good as any specific herbicide is, only about 50% of the resultant control is down to the product used.

Product Selection
In established pastures the key to successful control is the correct use of a chemical that targets the specific weeds present.

Creeping thistle-Short term control is achieved using MCPA type products e.g. Mastercrop MCPA or M50 in May-June, before flowering. Long-term control can be obtained with Thistlex which gives a slower but more effective kill.  Optimum timing for Thistlex is in spring when the thistles are 15-20cm (6-8 inches) high or after silage when thistles are actively growing.

Perennial nettle-Tends to grow in clumps in pasture and can prevent grazing.  Its growth pattern makes it an ideal target for spot treatment e.g. with Grazon 90, Nettle Ban etc.

 

Docks-There is a vast range of chemical formulations and products available for dock control in grassland; a summary of a selection of these are shown in table 1..

Products can be split into three main groups.

  1. Short to medium term control

Straight CMPP formulations e.g. Duplosan which have traditionally been used for dock control are no longer permitted for use in grassland under cross compliance. The good news is that more effective products for dock control are available for roughly the same cost per acre.  These include products like Hurler, Reaper and Hi Load Mircam.

  1. Long term control

Products like Forefront/Forefront T and Doxstar can be applied when the docks are in full rosette. 

  1. Clover Safe

Use Eagle or Prospect.

Weed control in new leys
Grassland reseeding is an ideal opportunity to control problem weeds in a field as there is an opportunity to ‘burn off’ weeds with glyphosate prior to sowing and then hit seedling weed plants a few weeks after sowing, when they are weak. 

When using glyphosate products, it is very important to use the correct rate i.e. 5L/ha (3.5 pts/ac) minimum and 6L/ha (4.25 pts/ac) where established perennial weeds like docks are present for 360g/L products. Dairygold recommends Roundup Flex (480g/L product-therefore lower use rates) as it is very effective at controlling difficult perennial weeds, is rain-fast in one hour and facilitates a rapid turnaround time (only requires 4 days after spraying before seedbed preparation, compared to 7+ for other products). 

To complete the job and ensure a weed free start for new pastures, a post emergence herbicide should be used before the weeds reach 10cm (4 inches) in height.  Spraying at this stage ensures that weeds don’t get the opportunity to build up root reserves which makes them much harder to eliminate.  Where there is no clover sown products like Hurler or Reaper represent the best value for money. 

Key point: Failure to apply a post emergence herbicide early before weeds develop strong roots makes them much harder to kill.

Where clover is included in grass seed mixtures, it is important to wait until the clover has reached the characteristic trifoliate leaf stage and specialist herbicides e.g. Mastercrop Undersown or Legumex DB are used to protect the clover. Add Triad for clover safe control of chickweed.

Talk to your Dairygold advisor and select the most suitable chemicals for grassland weed control on your farm.

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