Weed control in grassland

UK - March 24, 2011

Why control weeds in grassland?
Essentially where weeds grow, grass doesn’t. At any level of infestation, weeds such as docks, thistles and nettles pose a significant threat to grassland productivity as they compete directly for light, water and space.

Trials carried out by SAC showed that a 10% infestation of docks causes 10% yield loss of grass. In grazed pasture, spiky thistles reduce the area that livestock will comfortably graze – leading to even more wastage. It can also increase the spread of infectious diseases like orf.

Having docks in silage not only reduces yield but also brings down the feed value, as docks are less digestible than grass. This means farmers will have to buy more, costly supplementary feeds to supply the required nutrients to livestock during winter, than if the silage crop was weed-free.

An added problem when baling dock-infested silage is that the tough stalks can easily puncture the plastic wrap. This will let air in, which will adversely affect the fermentation, and create potentially dangerous moulds and unnecessary spoilage.

With the price of cereal-based feeds rising sharply and the high cost of fuel and fertiliser, farmers should be making the most of their grass. It is by far the cheapest feed on the farm, so encourage customers to grow and utilise as much as possible.

Which weeds need controlling?

Docks
Docks grow in soils rich in nitrogen and phosphorous, flowering from June onwards. They produce a large number of seeds which spread on the wind or in slurry and manure. They can exist in the soil’s seed bank for up to 80 years.

Poaching and ground disturbance encourages them to establish and they can withstand a fair amount of trampling. Cutting does not control docks as the parent plant simple sends up a new stem. The main tap root can reach down many metres into the soil.

Thistles
There are many different thistle species – the most common in grassland are Creeping and Spear thistles.

Creeping thistles spreads vegetatively and once established the root mass is greater than the plant above ground. Any fragmentation of the roots or cutting of the stems encourages aggressive re-growth from buds carried below ground. Thistles thrive on high levels of potassium in the soil.

Spear thistles are biennial and have sharp, spiny leaves. They only spread by seed which drops within a metre or two of the mother plant. Often unnoticed in the first year, in year two they can grow over one metre in diameter before flowering. It is vital to stop them seeding from July onwards.

Nettles
Nettles have creeping roots enabling them to spread easily and cover large patches of ground, shading any grass below. Constant cutting will eventually weaken them, but spraying with herbicide is more cost effective and offers longer term control.

It is important to re-seed the bare areas with grass after the nettles have gone to prevent re-infestation.

How can farmers stop weeds establishing in the first place?
The best defence against weeds is to stop them from gaining a foothold. Over or under-grazed, gappy leys with areas that have been badly poached in wet weather, will present bare patches of soil where weed seeds can easily germinate. A dense sward growing on well-structured, non-compacted soil will reduce the opportunities for weed ingress.

Farmers should also make sure the nutrient status of their soil is correct for growing grass. A cheap soil test will indicate whether phosphate and potassium levels are above or below the ideal index 2. An oversupply of either of these nutrients will encourage weeds like docks which love nutrient-rich soils, as will the over-application of manures.

Getting rid of weeds
Where weeds have established, there is a problem to fix. In grazing situations topping is really only a cosmetic solution that does not get down to the root of the problem. The weeds are still alive in the soil and retain the resources to re-emerge.

A longer term solution for controlling broad-leaved weeds in grassland is to use a modern translocated herbicide designed to tackle specific types of weed or situation. For example, Dow AgroScience’s Doxstar for docks, Thistlex for thistles, and Pastor for where a broad range of perennial weeds is present.

These herbicides move down into the roots to produce a more thorough kill than products based on older phenoxy chemistry such as MCPA or 2,4 D.  More importantly, fewer treatments are required to achieve the required result. Also, trials carried out by the Institute of Grassland and Environmental Research (IGER) has shown that these products do not hold back crop growth, as the sprays only affect the target weeds and not the grass.

Herbicides can be applied as a targeted spot treatment using a knapsack sprayer. This can be particularly useful for small patches of weeds, or where they are growing in gateways or around water troughs. However, broad-acre application will be more cost effective when tackling larger infestations (>5% of the area), using a weed wiper or a boom sprayer on the back of an ATV or tractor.

Anyone born before 1964, or who holds a  PA1 and/or PA6 certificate of competence in the safe use of herbicides (issued by the National Proficiency Test Council) can use professional use products. For others it is advisable to seek training (encourage farmers to visit www.nptc.org.uk for details of their nearest course), or to employ a contractor.

Whoever applies the spray should always read the product label and follow manufacturer’s recommendations for safe handling and application – including timing, dose rates, water volumes and stock withdrawal periods.

Spraying best practice
To make the most of an investment in spraying, farmers should follow a few key principles. Dow AgroSciences has developed the PRIDE guidelines to help livestock producers.

Advise your customers to follow these principles for best results.

Product – use the right product for the target weed. For long lasting control and where grass growth is critical, use triclopyr- and aminopyralid-based products which move right down into the roots of the weeds but do not inhibit grass growth.

Rate – Apply the correct rate of product in the appropriate amount of water. The rates have been carefully calculated and tested for optimum control. Reducing the dose rate reduces performance, particularly in the longer-term.

Skimping on water volumes is false economy – particularly when spraying in established grassland where weed plants can be overshadowed by the grass.

When using Dow herbicides at least 300 litres/ha of water should be used; up to 400 litres/ha in thick swards to get as much chemical as possible onto the weed leaves.

Information – Consider the cutting date and post-treatment use of the field. Allow at least three weeks, preferably four, between application and planned cutting date in silage crops. This allows enough time for the herbicide to be carried deep into the roots before the leaves, which have an important role to play in driving the transport of chemicals around the plant, are removed by mowing. Timing can be quite tight when treatments are applied between first and second cuts.

Where animals are grazing, adhere to the guidelines on the product label for stock withdrawal periods.

Density – Check the density of the infestation and the size of the weeds before spraying. For best results, translocated products should be applied when the weeds are actively growing and at the perfect stage for spraying – not too little and not too big.

Docks: Spray at rosette stage up to 25cm high or across
Thistles: Spray at rosette stage up to 20cm high or across
Nettles: Spray before flowering

Environment - Consider the state of the target plants before spraying. For best results, weeds must be healthy and free from disease and insect attack, and not suffering stress from drought, frost or water-logging.

Herbicides should always be used with due regard to the wider environment and countryside. Carry out a risk assessment if spraying next to water courses or where there is public access.

Weeds or clover?
Having 30% clover in a grass sward saves on fertiliser costs, increases animal intakes, provides late season feed and improves soil structure – so is a valuable resource.

Where infestations are small or concentrated in particular areas, applying herbicides as spot treatments or through a weed wiper may be the best way to kill the weeds, leaving the clover relatively unscathed.

Chemical options to control weeds in clover swards are very limited and not that effective on the target weed. Where weeds are, or could potentially restrict the productive capacity of the sward, there may not be enough of a saving in bagged nitrogen fertiliser to be worth saving the clover.

In these situations farmers need to consider using powerful translocated herbicides which will also take out the clover, and then re-establish the clover once the field is weed-free.

For further information visit www.grassbitesfarm.co.uk