THEY say a good botanist can tell the day of the year by the type of plants that are flowering. In my job as an advisor, I am beginning to think I can tell the week of the year by the questions on weeds that cross my desk. For example, I rarely get questions on dandelions before St Patrick’s Day as that is when dandelions begin to flower.

In my opinion, the most effective way to control grassland weeds is to know a little bit about the weeds themselves. It is surprising how effectively and cheaply you can control certain weeds just by knowing a little about how they grow and reproduce. In this article I will concentrate on the main weeds that are common on good, productive swards and the key points which will help you control them and make the best use of the chemicals applied.

RAGWORT

Potentially deadly to livestock, ragwort is listed as a noxious weed. Ragwort is a danger to all stock and the main symptom from ragwort poisoning is cirrhosis of the liver. Although largely unpalatable, ragwort may be eaten when green, particularly when other grazing is sparse. It becomes more palatable to animals when cut or sprayed as it releases sugars. Any control strategy should be based on the fact that ragwort is a biennial (lives for two years) and also that just because you killed it with a spray does not mean it cannot harm livestock.

Small amounts of ragwort can be effectively pulled or dug up and safely removed. For larger numbers, sprays such as MCPA; 2, 4-D; Dicamba and Forefront provide good control but measures must be taken to avoid stock eating any dying or dead ragwort present. There is no absolute time between spraying and grazing/cutting – it’s when the ragwort is too low for animals to graze it or below the cutting bar of the mower.

THISTLES

There are two main types of thistle and knowing one from the other is critical to controlling them effectively. Creeping thistle is the most widespread and troublesome thistle. It mainly spreads by creeping roots which can be metres in length but also spreads by wind-blown feathery seeds (July and August).

To make matters worse, it can grow new plants from small (one-two inch length) fragments of its roots thus appearing to explode when reseeding is carried out. Yield losses of up to 15% have been recorded but they cause most damage by preventing animals grazing around them.

The extensive root system of creeping thistle is its key mechanism of survival in your fields. Frequent topping can reduce the root reserves but will seldom eradicate the problem as root fragments can lie viable and dormant for years. Thistles emerge in the spring at different times so topping is a useful tactic to even up the growth stages before spraying. Chemicals such as 2,4-D, MCPA and Dicamba reduce top growth but do not translocate down to the roots.

In comparison, the spear thistle only spreads by seed, albeit by a lot of seeds, which are designed to fly long distances by wind. Each plant lives for two years (like ragwort) producing a flatted rosette of leaves in year one and then the familiar tree-like structure in year two.

Once controlled in the re-seed, it rarely is a problem in grazed fields except after poaching or other sward damage. Topping is not effective to control the growth in year one but can be carried out on the second year growth before seed is set. However, the plant may re-grow the following year once again requiring topping. Chemical control options are the same as for creeping thistle.

DOCKS

Docks are the most common and troublesome weed in grassland. Docks have everything going for them in terms of surviving in your fields. They are perennial (live many years), have a large root system, produce many hardy seeds and are very opportunistic in terms of where they germinate.

For a dock seed to germinate, the amount of light reaching the soil surface is the main limiting factor as moisture and temperature are usually adequate. One key fact: when sunlight passes through green grass, some of it is absorbed by the leaf. What is not absorbed will not activate the dock seed to germinate so that is why docks find it so hard to germinate in grassland.

Chemical control of docks

In the established sward, best control of docks will be achieved in good growing conditions when docks are actively growing and nutrients are actively being transported to new foliage and roots. If seed stalks are seen on the plant or if the dock has diseased leaves or is under pest attack it is better to cut/top or graze and allow re-growth of the docks before applying chemical. Do not apply chemicals in a period of drought as the chemical will not be taken up by the plant leaves in sufficient quantities.

Use the highest water rates on the manufacturer’s label for best effects. CMPP/Dicamba or Fluroxypyr mixes offer short-term control of docks and are good at rapidly bending the dock so that it is not ensiled with grass silage.

For longer-term control, Doxstar Pro and Forefront T are the best options available as their chemicals are moved down to the roots. However, in all cases it is impossible to eradicate docks completely as the ‘hole’ left vacant by the older plant is quickly filled by new seedlings unless there is competition from the surrounding grassland. Where clover is of consequence, Eagle or Prospect offer year-long control and reduce the effects of the docks in future years.

One thing that is crystal clear from Teagasc experiments - prioritise a post-emergence spray after re-seeding. This has given five years dock-free grass when carried out correctly. It is as important as ordering the grass-seed!

What management practises can I apply to reduce weed effects?

Grassland management practices such as drainage, fertility, grazing, topping and mowing are very important when it comes to controlling weeds. All of these reduce seed return while encouraging the grassland to be competitive and dense which is the key to keeping weed numbers at a level that will not impact on your grassland productivity.

Perennial plants such as docks, creeping thistle etc. are genetically primed to survive via their underground root systems. These underground root systems make control very difficult and usually require repeated interventions. It may be the case that the weeds become so numerous that re-seeding is required. Now the farmer has the upper hand, as desiccating the old sward with glyphosate (Roundup etc.) offers the best chance to effectively kill these extensive root systems.

After re-seeding, the soil will be full of weed seeds but crucially the seeds of perennial weeds are very poor to grow and establish. It is now a very easy task to control these small seedlings and let the grass do the rest over the following years. To prove this point, see the photograph on the page.