Herbicide resistance testing

Resistance testing methods

Grass Weeds

There are two methods to test for herbicide resistance - Petri-dish assay and Glasshouse pot testing using seeds. 

• With the Petri-dish assay, seeds are prepared and then germinated in herbicide solutions.  Seedling growth is recorded after 14 days.
• With Glasshouse pot testing using seeds, plants are grown in soil, treated with herbicide (at 2-3 leaf stage) and assessed 3-4 weeks later.  This method of testing is used for more effective assessment of ALS resistance.

Both test types are needed for black-grass and Italian ryegrass. The pot test only is needed for wild oats. 

Broad-leaved weeds

We also offers ALS-inhibitor (sulfonylurea) resistance testing for poppy, chickweed, and mayweed.

If you would like broad-leaved weeds tested for resistance to different actives, tests can be tailored to specific requirements, i.e. different herbicide choices and methods, different species. Please contact us to discuss this further and for quotations on different tests. 

Discounts are offered for bulk samples.

Order your herbicide resistance test

Sampling weed seeds for resistance testing

Resistance tests are only as good as the sample collected. Follow the guidelines below to ensure your seed sample is suitable.

• Do not collect from the edge of the field. Collect a representative sample from across the field; across 2-3 tramlines and across an area of 100m.
• Where patches occur in the field, sample the patches and mix together; the bigger the area the better.

Grass weeds

When taking samples of grass weed seed for resistance testing, make sure the seed is ripe. Black-grass, ryegrass and wild oat seeds are ready for sampling when seeds are brown and the seeds fall off the panicle on their own when gently brushed with your hand. You won’t need to force seed off the plant.

Black-grass growth stages - the ideal time to collect black-grass seed for herbicide resistance is when the plant is at 10-20% shedding

Best times to sample:

•   Black-grass – 2nd or 3rd week of July, when 10-20% of seed has shed
•   Italian ryegrass - early/mid July
•   Wild oats - late July/early August

How to sample:

1.   Collect a small mug of seed. Collect just the seeds; avoid collecting intact heads as they are unlikely to be ripe
2.   When you collect a mug of seed, allow the seeds to dry in a container for a few days.
3.   Transfer seed to a paper envelope.
4.   Samples are ready for sending.

Broad-leaved weeds

You will need:

• Paper bag/ envelope with the bottom taped up to prevent seed escape.
• Sharp knife or scissors

To collect poppy and mayweed:

• Cut off mature seed heads
• Place the heads in the paper bag or envelope.
• Leave the seeds in a well-ventilated area, making sure the bag remains open to the air to allow drying of the seed heads.
• Before posting, seal the bag/envelope with sticky tape to prevent seeds escaping.

 

To collect chickweed

Check chickweed plants for flowers and seed heads.  Seed heads will be always be green. 

• Pull large quantities of chickweed and place in a paper sack. Do not pack it in too tightly but allow enough room for air to circulate.
• Place the open sack in a well-ventilated area where the plants can dry out. As they dry seed will fall from the heads to the bottom of the sack.
• When dry remove the chickweed straw from the sack shaking any loose seeds into the sack.
• When the straw has been removed, tip the seeds into an envelope. Before posting seal the bag/envelope with sticky tape to prevent seeds escaping
• Alternatively post the sack containing the chickweed plants

Deciding which field(s) to have tested

Ideally fields should be tested every 3 years to coincide with a typical 3 year rotation. Understanding the resistance profile of the grass weed population in the field will help to optimise control measures.

If cost is a prohibitive factor in testing then the following points can be used as a guide when selecting sites:

1. Fields that have never been tested before
2. Fields tested 5+ years ago
3. Fields with continuous cropping (i.e. winter wheat only)
4. Fields where control of grass weed is poor
5. Fields where suspected resistant patches are still very small/discrete
6. Fields with the worst yields