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Thistles come a close second to docks in the list of most troublesome perennial weeds in grassland. The most common thistle species in Ireland 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 longer-term solution which, if done correctly will eradicate the problem.

Herbicide choice
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 William Corrigan.

“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.”

The best time to tackle dock infestations in silage crops is before first cut according to William Corrigan, National Business Manager for Dow AgroSciences.

“There are three good reasons for spraying docks in grass in early to mid April,” says Mr. Corrigan.

“Firstly killing them now allows the grass time to spread into the gaps they leave which will maximise silage yields at first and subsequent cuts.

“Secondly it improves the quality of the silage. Docks only have 60% the feed value of grass so the fewer of these that make it into the silage pit the better.

“Thirdly, when using modern translocated herbicides like DoxstarPro, leaving three to four weeks between treatment and harvest gives the active ingredients plenty of time to reach right down into the roots for thorough and long-lasting control.”

Mr. Corrigan suggests farmers plan a potential spray date now by working back at least three weeks from the expected cutting date, and noting this in the diary or booking the contractor if spraying is out-sourced.

“Weed growth has been slow so far this spring due to the cold nights and dull days. But recent warm warmer may spur them into action, especially in fields that have had fertiliser as they respond very well to nitrogen.”

DoxstarPro controls mature and seedling docks – both broad-leaved and curled. For best effect, weeds should be sprayed when they are actively growing and at rosette stage, 15 to 20cm across or high. The dose rate for one spring application is 2l/ha, applied in a water volume of 300l/ha; or 400l/ha if dock numbers are particularly high or the grass sward is dense.

The mild winter and kind spring weather means many soils have now reached 5-6°C at 10cm depth. This has triggered both grass and broad-leaved weed growth and the competition for light, space and nutrients is under way.

“As day length increases, every day soil temperatures reach 5oC or more, root and shoot growth starts,” says Andy Bailey, grassland specialist for Dow AgroSciences. “Soil processes also start to kick in as bacteria start to break down organic matter producing nitrogen in plant-available form, which also helps stimulate new growth.

“However, weeds do not start all growing at the same time. Buttercups and dandelions are the first to get going. Bad infestations may need spraying with a translocated herbicide in the next two to three weeks, when rosettes of actively growing, fresh green leaves can be seen,” Mr. Bailey advises.

Waiting until the field is a carpet of yellow in April is too late and spraying then will be less effective. If docks are also a problem, farmers should not be tempted to spray for both at the same time – when the docks are at the ideal stage for treating, buttercups and dandelions will be too far advanced.

“Broad-spectrum herbicides such as Pastor on silage ground and Forefront T on land grazed by cattle and sheep, are good options for early applications to catch both buttercups and dandelions.”

Volatile milk prices, high animal feed costs and uncertainty over the effect quota removal may have, are driving many dairy farmers to reassess the role grass plays within their business.

Well-managed grazed grass costs £97/tonne of dry matter (t DM) to grow, but has a feed value of £197/t DM. These figures come from a study* of 12 dairy farms monitored between 2011 and 2013, which set out to identify how farmers can increase milk output while lowering overall costs.

The farmers that grew and utilised the most grass per hectare and produced more milk from forage, made the most money.

And with land prices reaching record levels, expansion plans based on maximising grassland productivity on existing fields, rather than buying more, will generate more immediate returns.

More grass from what you’ve got!

The presence of broad-leaved weeds will always hijack grass yield and quality. The main problems on dairy farms are docks, chickweed and nettles, which all relish nutrient-rich areas where cattle regularly manure and trample.

Research at SAC showed that a 10% infestation of docks leads to a 10% reduction in grass yield – essentially where weeds grow, grass cannot grow.

Docks have only 65% of the feed value of grass and are less digestible. Where silage is baled, their tough stalks can puncture the plastic wrap letting in air, leading to spoilage and wastage. Dock seeds can also survive in silage and pass through the cow, infesting a field where slurry has been spread.

Tackling infestations

The best defence against perennial broad-leaved weeds is to stop them establishing in the first place. This can be achieved by having a well-managed, dense sward, growing in well-structured non-compacted soil. Over or under-grazed leys that have been poached, offer perfect conditions for weed seeds to germinate.

Topping or mowing weed plants provides short-term visual satisfaction but stimulates active regrowth – so the problem gets worse not better.

Spot treatment with herbicide through a knapsack sprayer can be cost effective when weed levels are less than 5%.  Where infestations cover a greater area, spraying the whole field is advised.

“The long-term solution for controlling persistent weeds in grass is to use modern systemic herbicides, for example DoxstarPro where docks are the main culprit, or Pastor where there is a broad spectrum of perennial weeds present,” says William Corrigan, grassland agronomy specialist for Dow AgroSciences.

“A well-timed treatment with an appropriate herbicide can transform a weedy pasture into a productive ley, without the need for costly reseeding.

“Farmers may have to be more pro-active this year, as weeds have generally survived the mild winter and kind spring, so weed numbers are likely to be higher than usual this spring.”

Mr. Corrigan continues: “It is important to plan weed control to make full advantage of translocated products like DoxstarPro. The reason they do such a good job is the way they work – travelling around the plant’s internal transport system, killing it from within. To do this they need time to reach all parts, including deep into the roots. Spraying a silage crop three to four weeks before first cut will give good control and really benefit silage yield and quality.”

DoxstarPro gives excellent control of mature and seedling docks – both broad-leaved and curled. Spray the weeds when they are actively growing and at rosette stage, 15 to 20cm across or high. Use 2l/ha once per year, or split the application into two at 1l/ha each, one in spring and one after the last cut. Apply in a water volume of 300l/ha; increasing this to 400l/ha if dock numbers are particularly high or the grass sward is dense.

This product can be applied to grassland used for grazing or cutting. Stock should be taken out during spraying and return no sooner than seven days later. If spraying DoxstarPro after a silage or hay cut, allow two to three weeks for sufficient weed growth to occur.

DoxstarPro has been designed to be effective against broad-leaved weeds but safe to grass. Trials have shown that even at double the label recommended rate there is no reduction in grass yield.

DoxstarPro is not approved for use by handheld equipment such as a knapsack or lance.  Choose GrazonPro in these instances.

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