Thistles creeping further into sugar beet

UK - May 19, 2005

Arable farms have seen a sharp increase in creeping thistle over the past ten years, with crops offering little competition, such as sugar beet, starting to suffer a yield penalty.

Professor Keith Jaggard, a research leader at Broom's Barn Research Station, believes the reason for the increase is natural regeneration set-aside fields with wind-dispersed seed, as well as the incomplete control of established populations.

"Creeping thistle robs yield in arable systems," says Professor Jaggar. "Deep-rooted rhizomes make it very difficult to kill completely and its thistledown seeds can be blown over many miles.

"Trials in past years have established that an average of just one creeping thistle stem per square metre causes a loss of 1 t/ha in beet yield. In practice, thistles occur in patches with numerous stems a square metre in each patch, often as many as several dozen," says Professor Jaggard.

"Glyphosate and in-crop cereal herbicide treatments seldom gives an adequate kill," says Dow AgroSciences' broad-leaved weeds specialist Colin Bowers. "Little wonder that creeping thistle is listed as a 'noxious weed' under The Weeds Act 1959. With fallow margins set to increase in area the weed population is likely to rise further."

A thistle patch that has just four stems per square metre will cut beet yield by the equivalent of 4 t/ha on that patch. At a mean ABC current quota value of £28.80/t, this equates to a loss of £115.20/ha.

Spot spraying is one solution. However, fields have to be walked thoroughly before offending areas can be accurately sprayed. Often patches with just one to two thistle stems a square metre are missed.

Spraying around identified patches will ensure light infestations, spread by cultivations, are not missed. Creeping thistles germinating from wind-dispersed seed are more difficult to identify, so an overall spray may be appropriate.

"Dow Shield can be applied twice for good control of established populations of creeping thistle," says Mr Bowers. "The first application at 0.5 l/ha should be made once the thistles are at rosette stage, with a follow-up application of 1 l/ha three to four weeks later."

Dow AgroSciences recommend that if Dow Shield is being used overall as part of a herbicide mix at 0.25 l/ha at the first application, then the follow-up treatment specifically for thistle control must be 1.0 l/ha once the thistles are clearly evident, probably some four to five weeks later.

"Ideally, the total rate applied should be 1.5 l/ha for reliable control. No single application of Dow Shield should exceed 1 l/ha with no more than a total of 1.5 l/ha over the season," says Mr Bowers.

Dow Shield will also control spear thistle, and provide moderate control of perennial sow-thistle at the above rates. Creeping thistle and smooth sow-thistle growing from seed will be controlled up to the four leaf stage with a single application of 0.5 l/ha of Dow Shield.