Don’t let grass be overrun with weeds

A random traveller could be forgiven for noting that more and more of our national grassland area is increasingly occupied by plants that are not grass. Indeed, one might wonder if there is any grass present at times.

In some fields, grass swards can be cloaked in a sea of yellow from buttercup or, even earlier in the year, a sea of dandelion can multiply to infest the remainder of the nation. If we allow these sights to be the norm, you can expect legislation to constrain land use in future.

That is not to suggest that a few weeds do much or any real harm in grassland. It depends very much on what is there. This would not apply in the case of ragwort but how many intensive grass paddocks are free of docks? With so much talk of biodiversity, one must wonder if other species will be imposed upon grassland in future.

In the meantime, we still have the opportunity to minimise the problems with weed infestations in grassland. This should be considered where the level of infestation significantly reduces sward productivity or where the presence of certain weeds reduced either the grazable area or the utilisation of the sward – grazed or conserved.

Look at management

Many different weeds cause such problems and farmers can easily identify with docks, nettles, rushes, thistles, dandelions, buttercup and ragwort. But it is worth noting that, in many instances, it is mainly one species that causes the problem in individual fields.

Think about that for a second and you must realise that it is something that we do, or do not do, in individual fields that influences the weed problems present. Say that another way and you realise that husbandry can and should also be used to tackle these individual problems.

Tackling weeds culturally can mean a number of different things. Topping is the most common method used and this can be useful against rushes, creeping thistles and nettles by forcing regrowth from the root reserves to weaken the plant in time. Topping is essential to help prevent seed formation and spread and to remove old growth in rushes to have new growth to take in chemical when spraying. It can also be very useful when spraying for thistles as cutting back the first flush of emerged weeds can help to ensure that all the plants are emerged where spraying takes place.

Fertility is also a key tool. Everything you can do to promote grass growth makes it more difficult for the weeds to flourish. Lime and soil pH are the most critical here.

Having pH up at around 6.5 makes a huge difference to how grass will grow. I am not sure if pH has a major direct influence on most weeds, but things like sorrel significantly favour a low pH.

Potash levels can be important too, especially for docks. If you drive up soil K levels through the continued use of high amounts of cattle slurry, this high K level significantly favours the growth of docks. And where they prosper, they leave no room for grass to grow beneath them. And as those dock roots get even bigger, the challenge of controlling them gets bigger too.

So if you are having problems, any problems, look first at soil fertility levels and test areas in your field with bad weeds and no weeds separately.

Fertility is not necessarily uniform in any field. It is always important to be cognisant of the problems you have and why you might have them in those areas. Understanding these issues can be key to cost-effective control or your problem weeds.

Herbicides are essential

While cultural considerations can be key to preventing or minimising problems, the use of herbicides is still likely to be necessary to clean up pastures. Most emphasis should be put on grass reseeds initially. It you get these clean, it is easier to keep them clean. The use of glyphosate between reseeds can only eradicate annual and perennial weeds that are easy to kill. It will not eradicate perennials like docks and it can do nothing against any weed seeds which subsequently germinate and grow when the soil is cultivated.

The easiest weeds to kill are always the seedlings. Once they establish, they become more difficult to eradicate. For the same reason, it is important to try to avoid or minimise any form of poaching as this exposes seeds in the soil to light and this can trigger them to germinate. This is very obvious in the case of things like chickweed which has become troublesome in some systems.

Control tips

Most weeds can be controlled using herbicides – new and established. The main product actives available in the market are shown in Table 1.

Rushes

There is a high awareness of rushes due to its implication for land eligibility and cross compliance. Improved drainage is the ultimate help to get rid of this weed but a good grass growth environment is also useful where the grass can be grazed. Soft rush can be controlled using MCPA or 2,4-D applied in June or July when growth conditions are good.

Topping rushes about three to four weeks before spraying will improve control because the spray can more easily penetrate fresh foliage. The use of a wetting agent can help the spray to stick to and penetrate the waxy rush surface.

Dandelions

Dandelions are perennial, have a deep taproot and the plants survive from year to year. They are primarily spread by the airborne seeds and can eventually cover a huge proportion of a pasture if left uncontrolled.

They can be knocked back by small amounts of MCPA or 2,4-D but long-term control requires the use of actives like aminopyralid, fluroxypyr, dicamba or CMPP-P in spray mixes. This weed is best targeted in summer or early autumn.

Chickweed

Common chickweed is an annual weed (lives for one year) and only spreads by seed. However, it can germinate and set seed throughout the year making it seem like a perennial weed.

It is fast-growing and ground-hugging. It is mainly prevalent after reseeding or where the soil surface is broken. Active grass growth is key to smothering it out but it can often appear in holes left after killing other weeds. Actives such as fluroxypyr and CMPP-P can be very effective for control.

Ragwort

Ragwort is potentially deadly to livestock and should be well-known to all farmers. The fact that it is more palatable to animals when dead makes it more awkward to control safely.

Ragwort is a biennial plant (lives for two years) and the dead sprayed plant can still be harmful to livestock. Where numbers of the weed are small, they can be pulled up or dug out effectively and removed off the field to somewhere safe.

Where numbers are large, many herbicides containing either MCPA, 2,4-D or dicamba can provide good control. Products such as Forefront T will also provide good control. Where pastures are sprayed, measures must be taken to prevent stock from eating any dying or dead ragwort plants.

Thistles

The creeping thistle is the most widespread and troublesome. It spreads within a field by its creeping roots, which can be metres in length. It can also spread by its wind-blown feathery seeds, mainly during July and August. Roots broken during cultivation can produce new plants, enabling a proliferation of this weed following reseeding.

Frequent topping can reduce the root reserves but will seldom eradicate the problem. Thistles emerge at different times in spring so topping is advised to even up growth before spraying.

Chemicals such as 2,4-D, MCPA and dicamba reduce top growth but do not translocate down to the roots. More persistent control is provided by Thistlex, or Forefront T, but follow-up sprays will be needed. Spray thistles at the rosette stage and before the seed stalk becomes obvious.

Spear thistle is a biennial plant, which produces flat rosette leaf growth in year one and the long stalk in year two. It spreads by the very many seeds it produces which move long distances by wind. Chemical control options are the same as for creeping thistle. Herbicides are effective but care is needed to prevent other seeds from germinating as a result of poaching or other sward damage.

Docks

Docks are perennial plants which can live for many years. They have a large root system, produce many hardy seeds and are very opportunistic. Light hitting the seeds is a major factor in enabling a dock seed to germinate in the ground, as moisture and temperature are usually adequate. So germination is most likely in open swards or after cutting. High soil K levels then favour the growth of the dock.

Dock seeds do not survive being ensiled in pit silage (due to low pH) but they can survive the passage through an animal’s intestine.

You need healthy docks to get good chemical control. Diseased, grazed or pest-attacked leaves will not give good chemical penetration down into the root.

Stress conditions must be avoided also. You need to spray when nutrients are being transported down into the root, which means before seed stalks are visible. When spraying, use high water rates to ensure a good wetting with the chemical. Dock seeds germinate and establish easily where there is little competition in establishing leys. They are less likely to establish successfully in established leys due to sward competition.

For the aforementioned reason, a post-emergence spray following reseeding can give effective control of docks that will persist for a number of years. This should be an essential part of reseeding.

Where docks are established in mature grassland, Forefront T offers the best control option. Some regeneration from rootstocks can occur and repeat applications may be necessary. Where clover is important, the clover-safe options for post-emergence application following reseeding offer greater and longer lasting dock control than the clover-safe options applied to established swards.

When targeting specific weeds the information in Table 2 should help with product choice and rate.

SUD obligations

Those spraying grassland are equally bound by the obligations of the Sustainable Use Directive (SUD). While anyone can purchase pesticides, like herbicides, only those registered as professional users with the Department of Agriculture are permitted to actually apply pesticides. And all boom sprayers to be used must have passed a recent certification test also.

One must be a professional user even to apply pesticide using a knapsack sprayer. Those who purchase pesticides must keep a record of where purchased products were applied and when. Relatively few products are cleared for use through a knapsack sprayer.

Farmers who spray herbicides on grassland must also show how they are using other measures (integrated pest management – IPM) to help combat these weed problems. They must also make every effort to keep pesticides away from water courses.

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