21 February 1997

Act soon to keep those grass species in check

Many crops will soon need a spring clean. This special herbicide focus examines winter pea and linseed treatments, and considers broad-leaved and grass weed control in cereals, including tips on avoiding blackgrass resistance. It also investigates novel weed control in sugar beet. Edited by Robert Harris

CEREAL growers need to act soon to achieve effective grass weed control. Chemical choice and rate depends largely on what has been applied so far.

By now, most growers will have put something on, says Jim Orson, ADAS head of cereals.

"Last autumn was a good average one for grass weed control," says Mr Orson. "About 70-80% of the intended area was sprayed. Residual chemicals worked reasonably well and the frost will have helped kill some late germinating blackgrass."

Trials last winter showed 50% of blackgrass was killed by the hard weather around Christmas. "But temperature at plant level is important – most areas which had the coldest weather also had a reasonable snow cover, which may reduce the effect."

Other growers who sprayed after this winters cold, dry spell should keep a close eye on crops. "Many stuck with straight IPU because crops and weeds were small. But there was a fair amount of dry weather afterwards. If IPU is not moved down towards the roots fairly soon after application, it can degrade on the soil surface so weeds will receive less than the expected dose."

If a follow-up spray is needed, growers have a limited choice. "If they have already used IPU at the full rate, then further applications are out," says Mr Orson. "However, if they have used lower rates, then they can apply more up to the maximum allowed – 2.5kg of active ingredient a hectare."

A foliar-acting chemical will also be needed, as insurance against a dry spell and to deal with bigger blackgrass. If weeds have survived a previous treatment with such compounds (for example Hawk (clodinafop-propargyl + trifluralin) or Cheetah (fenoxaprop-ethyl), growers should think hard.

Poor timing, too low a rate or bad application may be responsible, but growers should ensure that resistance is not to blame, he advises. "Each application increases selection pressure and raises the risk of target site resistance developing." If the infestation is heavy and control is a must, ADAS has found Topik (clodinafop-propargyl) often offers best control, although control may often fall short of desired levels.

Those who have not yet sprayed can use IPU in the spring up to the recommended rate, and add a contact material as needed. Exact ratios of residual and contact materials depends largely on the time of year, says Mr Orson.

Growers should aim to spray as soon as possible. Soil moisture is now likely to be plentiful, important for IPU to work well. Fops and dims also appear to work better in moist conditions, since weeds are growing well and more likely to take up chemical, he says.

Blackgrass is also smaller. Once it tillers, IPU control falls away dramatically, so growers will have to rely more on foliar chemistry.

"Provided blackgrass is small, there is plenty of soil moisture and the weather is fairly unsettled to encourage movement of residual herbicide to the root zone, then consider using IPU, if possible, and a little bit of foliar chemistry where needed," Mr Orson advises.

Timing wild oat sprays correctly is tricky, says Mr Orson points out. Some fields produce several flushes, often until late April. "Growers have to judge, based on experience of their own fields, whether the most recent flush is likely to be the last one, or they can end up spraying more than once." It may pay to wait. "Cold weather stimulates wild oat germination, so I would expect a bigger than usual flush this spring."

Resistance should also be considered when controlling wild oats, he adds. Some cases of resistance to fops and dims have been noted in the UK. "If fops and dims have been used elsewhere in the rotation, then an alternative like Avenge or Commando should be considered." In dry conditions they can match fop and dim performance, he maintains.

Growers should be aware of field history when choosing chemicals and rates, whatever the weed, says Mr Orson. "All weed stocks are different, not only in their behaviour, for example when they germinate, but in their susceptibility to herbicides too. It may mean decisions have to be taken field by field." &#42

Using contact herbicides alone encourages target site resistance in blackgrass to develop, says ADASs Jim Orson. To delay that, growers should spray earlier and mix them with a residual material where possible.

Wild oats:Timing can be tricky.


&#8226 Autumn sprays generally worked well.

&#8226 Early Jan IPU sprays reduced by dry soils?

&#8226 Spray soon while soil moist.

&#8226 Use IPU if possible, as part of early spring sprays.

&#8226 Pep up with contact material.

&#8226 Wild oat timing tricky – allow as big a flush as possible.

&#8226 Observe IPU stewardship guidelines.