28 June 2002

Beware rising risk of herbicide resistance

Worried about herbicide resistance

in the weeds on your farm? Now is

the time to collect seed for testing. Our latest baseline advice article tells you what to do

GROWERS planning to collect weed seed samples for resistance testing this summer should consider including ryegrass if they have any concerns about the weeds control, advises herbicide resistance expert Stephen Moss of IACR-Rothamsted.

Herbicide-resistant ryegrass is an increasing threat in the UK, with 21 counties already affected and the number of cases rising.

"Ryegrass populations have developed enhanced metabolism and target-site resistance. The problem is more common in Italian ryegrass and most cases have come from all-arable farms, rather than from units where ryegrass is sown regularly."

The Rothamsted Rapid Resistance Test is currently being refined for ryegrass and will be fairly accurate, says Dr Moss.

Like blackgrass and wild oats, seed should be collected in July. "The second week in July will be right in most cases. The seed drops quickly, so once it starts to shed it will be over quickly."

Holding the top of the plant in a bag and shaking it is the best way to collect seed. "Dont strip the seeds. They should fall off by themselves."

Blackgrass

Testing specifically for target-site resistance is worthwhile in blackgrass, believes Dr Moss.

His advice is to target blackgrass plants in oilseed rape crops. "If the weed has survived graminicides in oilseed rape, theres a likelihood that target-site resistance has developed."

The best time for collecting is the second and third week of July, in oilseed rape and cereals.

Gently rubbing blackgrass heads over a polythene bag is the best way to release seed. "Dont collect them too early because the seeds wont be viable."

Wild oats

Place the panicle inside a bag and shake vigorously to collect wild oat seeds, says Dr Moss. "Plan to collect wild oat seed a bit later than the other two grass weeds. The second half of July or early August is right."

Aim to collect enough seed to allow for drying and cleaning losses. "We suggest that you get a mug full."

Also avoid collecting seed in very wet conditions and ensure seed is transferred from polythene bags to paper envelopes for transport and storage.

"Also ensure the seed sample is representative of the problem field. Get a few seeds from lots of heads and collect over an area of about 100m by 2-3 tramlines."

Broad-leaved weeds

Collecting seed from chickweed and poppies is much more difficult, but samples of both weeds have been confirmed as suffering from resistance to sulfonylureas in the past year, notes Dr Moss.

"There hasnt been a system set up yet to deal with seed samples from these weeds," he points out. "But a pot test can be done if necessary."

He advises growers with concerns to contact their agronomist or agrochemical supplier. "Its only chickweed and poppies which have been affected to date, so its not such a big issue as it is with grass weeds."

Test Results

Results from the Rothamsted Rapid Resistance Test come with an interpretation and should be back on the farm by September, helping growers with herbicide and cultural control choices, says Dr Moss.

"With absolute resistance, as happens with target-site resistance, the interpretation and advice is easy. But plants may have both types of resistance and can show complex patterns of cross-resistance." &#42

RESISTANCE TESTING

1 Resistant ryegrass Increasing threat in UK, Italian ryegrass worst.

2 Mechanisms Ryegrass has evolved target-site and enhanced metabolism resistance.

3 Seed collection Collect ryegrass in July, but be vigilant as seed sheds very quickly and different varieties flower at different times.

4 Blackgrass Collect seed in second and third week of July by gently rubbing heads.

5 Target-site resistance Test specifically for target-site resistance by getting seed from blackgrass plants in OSR.

6 Wild oats Collect seed later, towards the end of July/early August.

7 Broad-leaved weeds Seed collection much more tricky, use agronomist and/or agrochemical supplier.

8 Sample Collect mug of seed to allow for losses. Make sample representative by including a few seeds from lots of heads.

9 Seed storage Transfer seed to paper envelopes as soon as possible. Dry wet samples in trays.

10 Results Use interpretation to plan herbicide and cultural control.