9 July 1999

Good sample key to grassweed resistance check

NOW is the ideal time to collect blackgrass, wild oats and Italian ryegrass seed to check for herbicide resistance.

But simply grabbing seed by the handful is pointless, says IACR researcher Stephen Moss.

Taking a good sample is vital to ensure meaningful results, Dr Moss says. Only seeds easily rubbed from the heads and stored in paper envelopes are suitable for testing. Keeping the seed in polythene collection bags risks sweating and loss of germination.

The best time for collecting blackgrass seed is usually the second and third week of July when about 10-20% of seeds have already been shed. In spring sown crops, such as linseed, seed collection may need to wait until August.

For wild oats the second half of July or early August is ideal, while early to mid July is best for collecting Italian rye-grass for testing, says IACR Rothamsted.

Collect seed by gently rubbing seeds from the seed head over a small plastic bag. This will avoid stalks, chaff and other debris contaminating the sample. Do not strip heads by pulling off all seeds – many will not be viable if they are not easily detached.

Indeed, seed quality is more important than quantity, only a few hundred seeds being needed for the test. But aim to collect at least a mugful of seeds to allow for losses during drying and cleaning, says IACR Rothamsted.

Unless the problem is confined to a small, very distinct area aim to collect over an area of about 100m by two to three tramlines within the main problem area, avoiding obviously unsprayed areas. The sample needs to be representative of the problem field or area, so a few seeds from lots of heads should be collected.

Avoid collecting in wet conditions and do be sure to air dry seed immediately after collection for about a week before mailing to ADAS or Oxford Plant Sciences. Do not heat seed rapidly.

The new Rothamsted Rapid Resistance Test (Arable, Apr 9) not only confirms whether herbicide resistance is present, it also indicates which herbicides groups are involved to help subsequent grassweed management. Results will normally be available by mid to late September for seed samples collected in July.

Tests are offered as a charged service by ADAS and Oxford Plant Sciences and are being evaluated by several agrochemical distributors and consultants.

To date, only about 10% of farms with a blackgrass problem have checked for resistance. &#42

RESISTANCE TESTING

&#8226 Tests available for blackgrass, wild oats and Italian ryegrass.

&#8226 Collect seed in late July.

&#8226 Mugful required per test.

&#8226 Rub ripe seed off gently.

&#8226 Air dry before posting.

&#8226 £100-£125/test through ADAS or Oxford Plant Sciences.