10 September 1999


After last years difficult

season grassweed pressure

is set to be high this autumn.

Growers need to consider all

options to optimise control,

as Charles Abel reports

MAKE no mistake – grassweed problems will be worse this autumn and herbicide resistance has almost definitely increased too.

To avoid big yield losses next year and extra weed control costs in future a concerted onslaught is essential this autumn.

"It is not the farmers fault; poor conditions last year mean seed return is likely to be greater this summer," says national weeds specialist James Clarke of ADAS Boxworth. Extra selection pressure for herbicide resistance in blackgrass adds to the need for careful planning now.

"There is a lot more blackgrass about, a significant increase in brome and a lot of wild oats too." With up to 10 times more seed being returned to the soil the need for rigorous control is clear, he says.

"Just 20 blackgrass plants a sq m can mean a 5% yield loss, which is equivalent to the cost of good weed control. Minimising weed numbers must be a top priority this autumn."

Most of this years crops suffered extra weed pressure, Mr Clarke notes. Early sowings were inherently prone to more grassweeds, and later sowings, which should suffer less in theory, succumbed due to inadequate control of early weed chit before sowing and poor seed-beds. Thin, uncompetitive crops were particularly vulnerable to weed growth, especially brome grasses.

Poor spraying conditions also meant most crops were left completely untreated until after Christmas. Many fop sprays were finally applied after late January. "The ideal time for grassweed control is two to three leaves, yet some blackgrass was two to three tillers. No wonder herbicides struggled," notes Mr Clarke.

"Whatever the herbicide resistance status was on a farm before, it is more than likely to have suffered further selection pressure and moved further towards resistance." That applies to blackgrass particularly, but also to ryegrass and wild oats if herbicides were applied late.

Recent news that promising blackgrass herbicide JV485 is to be delayed shows growers cannot rely on new chemistry to bail them out of the resistance problems, he adds.

Herbicide resistance test results are unlikely to be ready ahead of cultivations, so all growers must make their initial plans based on the risk of resistance. But by using a quicker test this year, results should be available in time to influence herbicide strategies.

Judging the resistance risk is relatively straightforward, using guidelines from the Weed Resistance Action Group, he says. Fields can then be prioritised for a more or less rigorous attack on grassweeds.

"It is quite likely that a lot of fields with poor grassweed control this year will fall into the high risk category, because of the treatments they have received," Mr Clarke says.

His advice is to try to do as many of the low risk factors as possible.

Ploughing helps a lot, especially for brome. Although more costly than minimal cultivations, the expense is likely to be less than the cost of managing resistant weeds in future, he stresses.

If minimum cultivations are used emerged weeds must be controlled properly, which generally means a non-selective spray before drilling, he says.

Also prioritise drilling according to weed pressure where possible, leaving weedy fields until last. "Do not dismiss delayed drilling just because it did not work last year. There were a lot of other factors at play too," Mr Clarke notes.

"In all cases ensure you have a good seed-bed to ensure a good competitive crop to compete with weeds and help herbicides."

When it comes to herbicides, timing is the key. If resistance is known or suspected use a sequence starting with Avadex or trifluralin. The main autumn spray should then be applied at the two- to three-leaf stage of the weed. Any later and control slips rapidly, says Mr Clarke.

Main spray choice then depends on conditions and weed profile. On good, moist seed-beds isoproturon can still be the mainstay, with or without pendimethalin. "If you understand how to use ipu you can still get good results."

The alternative is a fop herbicide. "But be aware that the more effective the product the higher the selection pressure. It is a balancing act between good control this year and further selection pressure for resistance in the longer term."

In all cases be sure to read manufacturer advice for application and ensure good sprayer operation. "Poor application can cause poor control," Mr Clarke says.

Provided a planned strategy is adopted, last years grassweed legacy need not have a lasting impact on cereal farms. &#42

Grassweed control strategy

JULY-AUGUSTMinimise movement of weed seeds between fields during harvest and cultivations. Review records of herbicide use and control. Plan herbicide strategy for next crop. Consider ploughing, delayed drilling, spring cropping and set-aside.

SEPTEMBER-OCTOBERDestroy weeds before sowing. Ensure you can apply pre-emergence herbicides at the correct time.

OCTOBER-JANUARYApply post-emergence herbicides for blackgrass and Italian ryegrass control when plants are small (2-3 leaves).

FEBRUARY-MAYApply spring herbicides to wild-oats when they are all emerged and still small. Consider need for over-spray if autumn grassweed control poor.

MAY-JUNEAssess herbicide performance.

JUNE-JULYMap weed patches. Rogue wild oats. Collect seed samples for testing.

Based on: WRAG guidelines


&#8226 Ploughing reduces the risk from grassweeds.

&#8226 Good rotations – broad-leaved crops can reduce grassweed populations and allow use of different herbicides.

&#8226 Spring sown crops cut the impact of most annual grassweeds.

&#8226 Set-aside/fallowing provides a chance to prevent seed return and reduce the soil seed-bank.

&#8226 Stubble hygiene – kill emerged weeds before sowing, preferably with a non-selective herbicide.

&#8226 Delayed drilling allows more time for seedling weeds to emerge and be killed before sowing the crop.

&#8226 Competitive crops are better able to suppress weeds.

&#8226 Prevent seed return by hand roguing, cutting or spraying off patches of weeds.

&#8226 Avoid introducing or spreading resistant seeds by sowing clean crop seed and minimising spread by combines, cultivation equipment, straw or manure.

Source: WRAG

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