Grab the chance for late season weed control

UK - September 15, 2009



Ideal grass growing conditions with continuing wet, warm weather offers farmers an unexpected opportunity to treat problem weeds such as thistles and docks this autumn.

“If producers didn’t manage to spray their grassland weeds in the spring they can do so now, and may achieve even better control,” says Dow AgroSciences’ weed specialist Duncan Connabeer.

“At this time of year, plants are drawing nutrients down into their roots in preparation for winter, rather than directing their efforts upwards to produce seed-heads. This allows the herbicide to work more effectively.”

Broadleaved weeds compete with grass in pasture for light, space, water and nutrients. In some cases, such as thistles, they also present a painful physical barrier to grazing – which is wasteful.

At a time when profit margins are being severely squeezed, maximising output from grass is crucial. Controlling deep-rooted and difficult weeds is an effective and relatively simple way of improving grassland production and utilisation.

Uniform weed growth
Herbicides such as the broad spectrum Pastor, Doxstar (for use against docks) and Thistlex (to treat thistles), work best when applied to weeds that are growing uniformly across the field. Spraying a few weeks after cutting for silage or hay or after topping is ideal.

Trials carried out by Dow compared topping and spraying simultaneously, with topping and spraying three weeks later. The results show that allowing time for regrowth gives better results and is kinder to the grass.

“Where late fourth cuts of silage are taken, weed regrowth will be perfect for spraying two to three weeks later,” says Mr Connabeer. “The plants will all be the same size and, as long as night time temperatures remain above 5ºc, they will still be actively growing.

Autumn treatment
Glastonbury dairy producer David Cotton runs a herd of 160 milking cows and related youngstock on a 245 hectare mixed farm. His grassland comprises a mixture of permanent pasture and short and medium term leys.

To control the docks and thistles, Mr Cotton prefers to spray in early autumn after topping the fields twice in mid to late summer.

“I find it hard to judge exactly the right time to treat in spring - the weeds and the grass are growing so fast it is easy to miss the ideal spray window,” says Mr Cotton. “I get much better kill from spraying in September.

“It is much easier to see the weeds at this time of year, which means I can target my applications more tightly, which saves on the amount of spray I use.”

Creeping thistle is the main problem weed on the farm, but treatment with Thistlex has kept even the worst effected fields weed-free for several years.

“One particular field was almost entirely covered with thistles – but has been pretty much clear since an autumn treatment back in 2004.”

Target healthy weeds
Independent grassland consultant Charlie Morgan agrees that late season weed control can be just as effective as spring applications, as long as the weather holds.

“In the south, autumn weed control in grass will work well certainly during September and maybe even into October,” says Mr Morgan. “But it is essential that the plants are at the same growth stage and actively growing– so a few weeks after cutting or topping is ideal.

“It is also important to make sure the weeds are healthy. It is no good spraying docks that have been shredded by beetle – because this will restrict the uptake of herbicide.

“If conditions are not quite perfect, splitting the application with a half rate now, followed by the second half in the spring, will do just as good a job.

 “Where there is clover in a sward – producers need to weigh up how much grass production they are losing from the weed infestation, against the contribution the clover is making to late season grazing. Where the weeds are very bad it may be worth treating and introducing clover back into the sward later.”