15 September 1995

Herbicide recipes help to control cover crops

FRENCH growers with cover crops on set-aside could soon be offered herbicide recipes to ensure they comply with the rules but avoid contaminating following crops with shed seed.

The work could help UK growers who decide to opt for a cover on rotational or flexible set-aside.

Unless obtaining a derogation – for sowing oilseed rape, lucerne or temporary grass – French farmers are required to keep set-aside covers in place until cultivation is permitted on Sept 1. (Under the derogations crops can be destroyed after July 15.)

Various reduced-rate options are being explored at ITCF to see which chemicals and doses are best suited to each set-aside species.

The aim is to keep green cover in place but prevent seeding, explains researcher Claude Rameau.

In the north much of the rotational set-aside is natural regeneration. But growers in other areas prefer to sow a cover to ensure better control of potential problem weeds. As much as 90% of set-aside in the Boigneville area, south of Paris, is sown, he estimates.

Phacelia, white clover, ryegrass and vetches are the most popular options.

But controlling their growth and avoiding seeding is not always easy. Mowing risks leaving uncut strips from the tractor wheelings across the field, explains Mr Rameau.

There is a list of nine approved chemicals for use on set-aside, of which glyphosate is one. For ryegrasses this is the most suitable when applied during stem extension, he suggests. But there are differences in the response according to variety and herbicide formulation, he notes. "Two litres/ha of Sting holds the crop for at least a month and then it starts to grow again." Half that dose risks allowing the grass to head and shed seed.

Less efficient

"But glyphosate is less efficient at getting rid of vetches." For that crop a reduced rate of dicamba (Banval in France) just as the first flowers appear seems a useful strategy. "It checks it for 35-40 days, after which it comes again. You can then make a second application or go in and cultivate from Sept 1 onwards."

On phacelia, mustard and some other covers half-rate sulfonylureas, such as Ally (metsulfuron-methyl), Cameo (tribenuron-methyl) and Scoop (thifensulfuron-methyl + metsulfuron-methyl), are the most efficient chemicals to use, he suggests. "They are all as good as one another to hold the crop at the first flower stage."

Keeping the cover but avoiding seeding. ITCFs Claude Rameau is homing in on reduced rate chemicals to keep phacelia and other set-aside crops like mustard and clover in suspended animation.