Resistance tests: only as good as the sample

Resistance testing black-grass seed may deliver misleading results and could make the problem worse, says Bayer CropScience, unless growers pay close attention to gathering seed correctly.

While testing seed can provide important management information, such as confirming which active substances are affected and whether resistance is target-site, enhanced metabolism (EM) or both, a false sample could do more harm than good.

“These are expensive tests to undertake, both in terms of time and money” says Ben Giles, commercial technical manager at Bayer CropScience, “and done well, they are a useful part of the strategy for managing grass-weeds.

“But to do them well, you must take an accurate sample,” he stresses.

According to Mr Giles, common failures include taking seed only from plants around the field edge or from only one spot in the field; forcing unripe seed from the head; and sending damp seed to the lab.

“If the tests don’t work, because you’ve sent non-viable seed, then you’ll have wasted your money.

“But if the sample isn’t representative of the field population, then there’s the possibility that you won’t know the complete resistance profile, and so unwittingly select for another generation of resistant plants by again using the active substances that are causing the problem,” he warns.

“To get a representative sample, walk a ‘w’ shape across the entire black-grass patch or field taking seed as you go,” Mr Giles advises. “Don’t just walk up and down the tramlines as you won’t get a representative sample. By sampling from only one spot, it wouldn’t be unusual to find out that the seeds collected had the same parent plant one or two seasons ago.”

Tapping the seed head in a clean plastic container (such as a plastic pint glass) is the best way to collect seed. “If they fall off on their own then they are ripe,” explains Mr Giles. “It’s ok to gently brush your fingers along the head but don’t dig in a fingernail to strip all the seeds off; these seeds won’t be ripe or viable and the test won’t work.”

Remember that because of the weather conditions we have had this spring, blackgrass seed may be delayed in maturing and the best time to sample could be one or two weeks later than in dry seasons.

Mr Giles says the tests work best on dry seed. “It is quite possible this season that seed may be damp when sampled; if so place in a shallow open container and allow the seeds to air-dry. Always use a paper envelope, rather than a plastic bag when sending the sample.”

Finally, Mr Giles says it’s important to give the labs as much information as you can. “Fill out all detail requested on the sample form as completely as possible – this will allow for better analysis of the sample and more tailored recommendations when interpreting the results.

“Just remember, however, that while resistance testing provides some really useful information that can be used to put together weed control programmes suited to your own circumstances, it only delivers you a snapshot of part of your weed seed bank at one point in time. These are, after all, seeds collected from plants that have survived a herbicide programme.”