Thistles come a close second to docks in the list of most troublesome perennial weeds in grassland. The most common thistle species in the UK are spear thistle and creeping thistle.
Spear thistles are biennial and have sharp, spiny eaves. They only spread by seed that can disperse as far as 30m from the mother plant. Often unnoticed in the first year, in the second they can grow as wide as a dinner plate before flowering. It is vital to stop them seeding in July.
Creeping thistles spread across the ground and once established, the root mass underground is greater than the amount of plant above ground, robbing the grass of moisture and nutrients.
Where thistles grow, grass cannot grow – so in silage crops they take the place of valuable feed. Research shows a 10% infestation of thistles leads to 10% less grass for cutting. Silage containing a high proportion of thistles will also be rejected at the feed trough due to the nasty prickles.
Thistles can also help spread infectious diseases such as Orf, a highly contagious viral disease of sheep, causing scabby lesions around the mouth and nostrils of lambs and the teats of lactating ewes. Topping is wasted effort Chopping down thistles with a mower or topper gives instant satisfaction but offers only short-term control. The roots remain largely unaffected and vigorous re-growth soon appears. It is a maintenance operation at very best.
Topping also leaves trash on the surface, which can smother any grass trying to grow underneath it.
Eighty per cent of livestock farmers asked in a survey about the level of weed control achieved 12 months after topping, said it had not delivered good enough results to warrant the time, effort and cost of doing it.
Spraying with an appropriate herbicide designed to tackle the specific weeds or situation offers a longerterm solution which, if done correctly will eradicate the problem.
There are two main options when choosing a grassland herbicide – contact or systemic/translocated products. Contact herbicides based on phenoxy carboxylic acid chemistry like MCPA have been around for years, but only work on the areas of the weed they touch. This has a quick effect on the plant above ground – but the active ingredient fails to penetrate inside sufficiently to finish it off.
In reality it is merely chemical topping. Some infestations may need spraying several times with this type of product to achieve the same effect as spraying just once with a more modern translocated one, with all the associated costs of making successive applications.
Modern translocated herbicides such as Thistlex from Dow AgroSciences are regarded as the most reliable solution for controlling perennial weeds in grassland.
The way they work ensures the active ingredients get right inside the plant, travelling around the internal liquid transport system, reaching deep into the roots as well as the stems and leaves.
Safe to grass
An added advantage of Thistlex over MCPA is that it is much more selective – only working on broad-leaved plants and not affecting the grass in any way. In some situations, older products can hold back grass growth. At a time when farmers are seeking to grow and feed as much grass as possible to cut input costs – this is an important consideration when selecting which herbicide to use.
“Thistles need controlling because they compete with grass for space, light, nutrients and water and are unpalatable to stock, so large infestations have to be treated,” explains Dow Agrosciences’ grassland specialist Andy Bailey.
“The combination of clopyralid and triclopyr in Thistlex maximises the activity on thistles, giving more than 85% control 12 months after application.
“Apply between 1 March and 31 October and use once per year at a rate of 1l/ha in a water volume of 200l/ha. Use higher water volumes up to 400l/ha, where thistle numbers are high or the grass sward is dense.
“Thistlex can be used on silage and grazing fields. Remove stock during spraying and for seven days after. Where applied to silage land, spray at least three weeks before cutting to allow time for the active ingredients to work fully.
“Spray when the weeds are at the rosette stage and up to 20cm across or high. If they are larger than this, it would be better to top them and spray the active regrowth two to three weeks later.”
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