5 April 1996

Low-dose split could give tricky weeds a hard time

Effective but cheap control of weeds in set-aside is in the offing as chemical manufacturers strive to refine product recommendations. Robert Harris reports

SEQUENTIAL low doses of glyphosate appear to offer good control of couch grass in set-aside, especially in more northern areas.

Other problem weeds like volunteer potatoes and oilseed rape could also be more easily removed using the technique.

Spraying when couch leaves and rhizomes are actively growing is crucial if best control is to be achieved with reduced rates, says Trevor Culley, technical services manager for Zeneca Crop Protections range of herbicides.

"Generally we would recommend a 4 litre/ha rate to control couch. But when the weed is growing well a three-quarter rate can be used with good results."

In the south, that usually applies to May or June applications. But further north the shorter growing season and colder weather means it is more difficult to time sprays effectively. And, given the narrower window, any opportunity is more likely to clash with other work, he maintains.

But trials carried out by the SAC in Scotland have shown two sprays of Touchdown (glyphosate-trimesium), each of 1.5 litres/ha (1pt/ acre), gives at least as good control as one correctly timed spray.

Applying the first split in late May and repeating it in mid-June gave complete control of couch in the trial. That compared with a 40% kill achieved with one 3 litres/ha (2.1pt/acre) application at the first timing, and 90% when the same rate was applied at the second. "We need to see if these results are repeatable before recommending them. But it is an interesting trend."

Another benefit is that the first spray removes much of the annual grass weed burden. This allows a more effective kill of problem weeds, like potatoes, which would otherwise have been shaded from the second spray, explains Mr Culley.

Trials have shown a 4 litres/ha (2.8pt/acre) rate of Touchdown sprayed in mid-July killed all foliage and reduced viable tuber numbers to 15-20% of that achieved with earlier sprays. Conversely, volunteer oilseed rape plants are difficult to control after flowering even with high rates of glyphosate, says Mr Culley.

"In trials last year, we found oilseed rape was at quite an advanced growth stage by the middle of April. Applications made after that time were less effective."

But early sprays also removed most other cover, reducing the value of set-aside as habitat. "Oilseed rape is a potential target for sequential application to see if we can achieve good control with a lower initial rate to preserve cover," says Mr Culley.


&#8226 Split doses more flexible -useful in north.

&#8226 As good or better control than one well timed spray.

&#8226 Reduced shading means better weed control.

&#8226 Mixing other herbicides effective on tricky weeds and cheap.

Low-dose sequences of Touchdown can be used to ease couch control, says Zenecas Trevor Culley. Adding broad-leaved weed herbicides may help growers to cheapen treatment on difficult-to-control targets.