It’s not too late for weed control

Date published: 20 August 2024

Kieran McCartan, College of Agriculture, Food and Rural Enterprise (CAFRE) Adviser says that: “Weed growth has continued and it is important to control weed species in grassland swards as they compete for nutrients and space, reduce quantity and quality of the grass and hinder silage fermentation.”

Under The Noxious Weeds (Northern Ireland) Order 1977, ragwort, creeping thistle, spear thistle, broad leafed docks, curled leafed docks and wild oats are all classified as noxious weeds. The Department of Agriculture, Environment and Rural Affairs (DAERA) has powers under the Order to insist that these weeds are destroyed under notice. Failure to comply with such a requirement could result in prosecution. Two main weeds that we will look at in this article are docks and ragwort (benweed).

Dock control

Kieran says that: “When there is a 10% dock infestation in the sward, it reduces the grass yield by 10% and the docks will quickly spread to cover more of the sward.” Docks have a great survival mechanism from their deep roots, with one mature dock producing up to 60,000 viable seeds that can survive for 50 years. The best preventive management against perennial broad-leaved weeds is to prevent them establishing in the first place. This can be achieved by having a well-managed, dense sward, growing in well-structured, and non-compacted soil. Fields that have been poached with livestock during this wet summer and where slurry has been spread offer perfect conditions for weed seeds to germinate, as they have open areas to grow with a high availability of potassium and nutrients. Where dock-infested swards are ensiled, their high nitrogen content adversely hampers a good fermentation, leading to a high pH silage that spoils quickly when opened and depresses intake. Dock seeds can also survive in silage and pass through the animal, infesting a field when the slurry is spread.

Kieran continues that: “The long-term solution for controlling persistent dock weeds in grassland is to use modern systemic herbicides. To achieve maximum chemical uptake docks should be treated at the large green and leafy stage which is called the rosette stage.” This ensures maximum leaf surface area on the dock, so enough chemical gets onto it and moves from leaf to root. If seed stalks are seen on the plant or if the dock has diseased leaves or under pest attack, then it is better to cut/top or graze and allow re-growth before applying chemical. It is important to follow the pesticide label for the application rate as well as the minimum grazing and cutting dates.

Research has shown that longer term (up to five years) control of docks can be achieved by applying a suitable herbicide onto small docks shortly after reseeding. By applying the herbicide at this stage of the dock’s development, docks are almost completely eliminated. Where clover is present, it is advisable to use recommended clover-safe herbicides.

Ragwort control

Ragwort, also commonly known as benweed, in the green and preserved state has been responsible for many animal fatalities. Ragwort is poisonous and can also show symptoms such as tenesmus, hind limb weakness and cause severe animal pain. It is a biennial plant (lives for two years).

Normally animals don’t eat ragwort in pastures unless grazed grass availability is extremely restricted. An animal must consume up to 12% of the animal’s body weight of the weed to cause severe problems. Ragwort becomes more palatable to animals when cut or sprayed, as this releases sugars in the plant. Most fatalities occur where there is a mixture of ragwort finely chopped in hay or silage which the livestock eat in the housed period and the toxins build up over a period of time.

At smaller infestation levels, pulling of ragwort can be a successful control option. For larger infestation appropriate sprays should be used. The best time to spray ragwort is at the leafy (rosette) stage this is when it is actively growing and avoid spraying when it becomes stemmy. Ensure that the plant is fully decayed into the soil before grazing again.

An option is to spray in the autumn, if there is mild and dry weather, because at this stage fields can be closed off and the ragwort has time to die and decay over the winter.

Spraying regulations

Comprehensive advice on all aspects of using pesticides is contained in the DAERA Code of Practice for Plant Protection Products. The Code explains how to use plant protection products safely and so meet the legal conditions which cover their use.

Kieran advises that: “If you or a contractor are applying pesticides, you are responsible for ensuring the person applying pesticide on your land has the necessary certificate of competence, this is a legal requirement.”

All boom sprayers over 3m width must have a certificate of pesticide application, in the case of a new sprayer this will be before its fifth anniversary and then every three years thereafter. The National Sprayer Testing Scheme (NSTS) is currently the only officially designated provider for certification of pesticide application equipment in the UK.

Kieran concludes that : “It is also the responsibility of all farmers and contractors to understand the pesticide label and ensure they use pesticides at the correct rate and method. With the current difficult weather, ensure that you choose a dry day for spraying. Remember to keep proper records as you can be asked for these at a cross-compliance inspection.” You can discuss all aspects of weed control with a local CAFRE Adviser.