5 September 1997

The ideal match knocks unwanted weeds for six

Good control of grass weeds and volunteer cereals in oilseed rape boosts yields and reduces the burden in the following cereal crop. Robert Harris reviews the options for this autumn

OILSEED rape growers may not have to be so quick off the mark this season to control weeds. Matching the right product to the problem means fast growing crops can be kept clean relatively cheaply.

Seed drilled into moist seed-beds will produce good crop cover quickly. "The better the establishment, the fewer the weeds," says Peter Lutman of IACR Rothamsted.

Growers may be able to wait until November to spray without losing yield. That will ensure most grass weeds will have emerged, making it possible to achieve a complete kill with one application, he points out.

However, cereal volunteers can hit backward crops hard so early removal is essential. Fops or dims like Laser (cycloxydim), Fusilade (fluazifop-P-butyl) or Pilot (quizalofop-ethyl) are a good choice, he notes.

If the main target is blackgrass, growers should not use those products if similar products like Hawk (trifluralin + clodinafop), Topik (clodinafop), or Cheetah (fenoxaprop-ethyl) have been used widely in cereals – especially if a reduction in efficacy has occurred on the farm or nearby. "Resistance is increasing, and people should do something about it before they get it."

Products with different modes of action include Butisan (metazachlor), Kerb (propyzamide), and Carbetamex (carbetamide).

Exploit break crops

Berks consultant Lawrence Sim reckons fops and dims designed for use in oilseed rape give better control then those used in cereals.

For that reason he avoids them in cereals wherever possible. "I work in an area where we have difficulty controlling blackgrass. Resistance has been confirmed in many fields. So I use non-cereal break crops to the utmost for grass weed control."

Fops and dims slot into the sequential approach he favours. The programme starts with a quarter rate or less of a glyphosate-based product (eg Roundup) applied before sowing where enough weeds have emerged.

Trifluralin may be applied pre-drilling at 2.3 litres/ha to provide some control of blackgrass, meadow grass and wild oats, as well as chickweed, speedwell and poppies, for £7.40-10/ha (£3-4/acre).

Butisan which has some effect on volunteer cereals, blackgrass and meadowgrass is only used if there is a severe cleaver or mayweed problem, since it is costly, about £30/litre last season. It is usually applied in a split of 1.5 litres/ha pre-emergence, and another 1 litre/ha at early post emergence, to boost efficacy.

Surviving volunteers and grass weeds are controlled as late as possible in the autumn after they have all emerged, but before they compete with the crop or blackgrass becomes well tillered, says Mr Sim.

Laser is preferred on blackgrass with enhanced metabolism type resistance due to its better activity, says Mr Sim. "Where blackgrass is easy to control, a cheaper graminicide like Fusilade is adequate."

Application rates are usually kept at or near full rate to ensure maximum kill. Survivors are monitored carefully for the potential development of target site resistance, which renders fops and dims useless, he adds.

Kerb is used on the rare occasions that type of resistance is suspected. Otherwise it is avoided because at about £25/ha it is expensive and does not control weeds like mayweeds, poppies and cleavers.

Tailor to weeds

Peter Taylor of Herts-based Samco matches fops and dims according to the weeds present. "Pilot is very good on barley volunteers, used at 75-100mls/ha + oil at the two to three leaf stage. But it is not so good on wheat, and may not be the best product at those rates on blackgrass." Cost at the higher rate is about £17/ha, within a £ or two of the equivalent rates of other fops and dims.

Fusilade gives better control of that weed, provided it is applied at 0.3-0.5 litres/ha, the exact rate depending on weed size and number. It is also better at controlling wheat volunteers, he adds. Falcon (propaquizafop) is a suitable alternative, especially if meadowgrass is present.

Laser is his preferred blackgrass killer. "It gives very good control, and is possibly the best for metabolic resistant blackgrass. It is also very good on barley, though rates may need to be stepped up a bit for wheat volunteers."n

Rapidly emerging rape provides a good chance to hit grass weeds hard, agree experts (l to r) Peter Lutman, Laurence Sim and Peter Taylor.

GRASSWEEDCONTROL

&#8226 Moist seed-beds boost emergence and crop competitiveness.

&#8226 Later spraying possible but before canopy closes.

&#8226 Consider sequence to boost efficacy.

&#8226 Broad-leaved weeds may dictate product choice.