Don't let up on weed control this spring

UK - January 20, 2009

Broad-leaved weeds in grass are bad news.  They compete for light, water and nutrients.  Without them more grass would grow, more silage could be made, more animals could be grazed, and more milk and meat could be produced per hectare.

At a time when profit margins are severely squeezed - maximising output from a grass crop is crucial.  Controlling deep-rooted and difficult weeds - such as docks, thistles and nettles is an effective and relatively simple way to improve grassland production.

Eliminating weeds can have other benefits.  For example, thistle spikes can act like dirty hypodermic needles to lambs that get too close - breaking their skin and spreading diseases such as orf within the flock.

For the past three years, many farmers have used Forefront - a herbicide developed by Dow AgroSciences specifically to tackle the major grassland weeds.  Based on an active ingredient called aminopyralid, Forefront has proved extremely effective, and the significant uptake by farmers reflects how well the product works. 

However, some problems have occurred on farms and in gardens and allotments, where manure produced from animals that ate forage made from grass treated with Forefront, was applied to ground subsequently planted with susceptible crops, such as potatoes and legumes.  If you have FYM which you think could have traces of aminopyralid within it, then ensure you comply with the product label - applying to preferably grass or land destined for grass, cereals or maize.  Certainly in such circumstances do not sell on such FYM to gardeners or allotment holders.

Aminopyralid poses no health risk to any animal that eats treated grass, but enough active ingredient may be found in the manure to affect plants that are sensitive to it.

Last autumn, Dow AgroSciences sought a voluntary suspension of approval from the Pesticides Safety Directorate (PSD), for the sale and use of its aminopyralid containing products while it takes steps to minimise the risk of problems re-occurring.  It is unlikely that they will be available for use by farmers this spring.

Doxstar, Thistlex and Pastor are tried and tested alternatives and when applied at their full application rates, give the highest levels of control of the available products on the market.

For autumn sown leys - now is the time to zap weeds such as seedling docks and over-wintered chickweed.  If docks are the main problem use Doxstar.  Where the range of weeds is wider, Pastor delivers better control. Both are reliable when applied in variable, early season conditions.

As thoughts turn to silaging, treating dock-ridden leys four weeks before cutting with Doxstar will significantly increase the amount of grass that ends up in the clamp, and improve silage quality.  Docks have only 65% of the feed value of grass and pull down the dry matter content of the resulting forage.  If controlling docks  ahead of first cut, plan a Doxstar application in April.

Applications of 1.5 l/ha will give good seasonal control.  To achieve longer lasting results, a full 3 l/ha rate of Doxstar can be used.  As with all herbicides, for the best results weeds should be young and actively growing, with four or five healthy leaves.

By the end of May, nettles and thistles will be reaching maximum growth.  In fields where a lot of different weeds are growing, Pastor is the product of choice.

There is an old adage that relates to thistle control: "Cut them in June - they come back soon, cut them in July - they will soon die".  However, cutting thistles is dealing with the problem after the event.

An alternative to cutting has been an application of MCPA - both do a similar job - removing the biomass above ground quickly, but with little overall impact on root systems below ground.

A more lasting effect requires herbicide treatment when the thistles are young and actively growing with little stem.  Thistlex applied at 1 l/ha offers a more targeted, cost-effective solution.  Identify pastures where there is a thistle problem and consider applying Thistlex in April or May.  If Thistles get too large for an effective herbicide application, top them and then spray the re-growth six to eight weeks later.

There will be no aminopyralid products in the market this year.  Additionally, older products, such as CMPP, which has had its registration revoked for use on grass and MCPA, are both outdated solutions which lack the performance that's needed to deliver a more lasting effect. 

However, this is no excuse for skipping weed control this year, as all the good work done in previous years will be undone.  The temporary loss of Forefront is disappointing.  However, Doxstar, Pastor and Thistlex are excellent products and will do a very good job in the mean time.