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The need for accurate weed ID for grass-weed management

As we enter June, we begin to be able to visibly judge the final performance of weed management efforts, and perhaps start to think about what the next steps might be. The first considerations should be about the species of weed that we are seeing – are they what was expected based on the history of the field, or are there are a few new faces popping up? Black-grass or wild oats might be the most familiar faces, but species such as Italian rye-grass or one of the several bromes are becoming more frequent, particularly as the frequency and intensity of cultivation is reduced. The accurate identification of these could be vital in guiding the control measures that could be adopted, or adapted, in the following season.

 

Italian rye-grass or perennial rye-grass?

Rye-grass has been used through agriculture for centuries and so it is little surprise that is has developed into a weed. We have perennial rye-grass (Lolium perenne), and Italian rye-grass (Lolium multiflorum) – which can either be a domesticated variety or a “weedy” population. The two species may appear similar, but are very biologically distinct, so can be treated in different ways. One key defining feature is how the leaf blades appear from the stem – Italian rye-grass leaves are rolled whereas perennial rye-grass have folded leaf blades. Additionally, Italian rye-grass has awned spikelets, whereas perennial rye-grass doesn’t. These are often difficult distinctions to make but critical. Perennial rye-grass is typically less of a concern with the source of the weed often straightforward to understand and apply remedial action. Italian rye-grass is of far greater concern, and depending on the scale of infestation will require careful consideration of all available control techniques. Italian rye-grass is a predominantly autumn germinating annual weed, however the key cultural control approaches that we associate for black-grass control e.g. delayed autumn sowing and spring cropping, are typically less effective in Italian rye-grass as a result of a lower, or even absent, vernalisation period. This means that even a very late emerging, or spring emerging, individual can produce as much seed as an early-autumn germinator, so even where reduction of individuals is high, the effective reduction in seeds is much lower. Herbicide resistance is common, and can be very acute, with actives acting through ALS (HRAC Group 2) or ACCase (HRAC Group 1) both significantly affected. Resistance to flufenacet has now been detected in some populations, which requires significant alterations to herbicide programmes. If this is appearing as a new weed then it is worth sending a seed sample for resistance testing to understand the issue and design a suitable herbicide programme. Worryingly, the risk of glyphosate resistance developing in this weed is high, with resistance already detected in other European countries. Taking appropriate steps to reduce the risk of resistance is vital to ensure the long-term protection of this vital active ingredient. In summary, Italian rye-grass is a seriously challenging weed to control in any individual crop and requires a truly Integrated Weed Management approach to reduce populations, and to protect against infestations.

 

The family of Brome

In the UK, we have five main brome species that appear in the arable landscape, which can be divided into two sub-groups on account of their physical appearance. Bromus sterilis (Sterile or Barren Brome) and Bromus diandrus (Great Brome) are clearly defined by their large, drooping seed heads and long awns on the seeds. Both of these species are solely autumn germinating, so adding spring crops into the rotation can be very effective, whilst the best control from herbicides is achieved by applying them in the autumn, as part of pre-emergence and early post-emergence applications.

The sub-group contains Bromus secalinus (Rye Brome), Bromus commutatus (Meadow Brome) and Bromus hordeaceus (Soft Brome), although the latter is mainly a field-edge species and rarely makes it into crops. Rye brome and meadow brome have more tightly packed panicles, with groups of seeds resembling an egg shape. The key feature between these two species is the cross-section of the mature seed – meadow brome has a saucer shape, whilst rye brome has a V-shape. The other key difference from either sterile brome or great brome is the germination periods. Conventional knowledge considers them to be autumn germinating, however new evidence indicates this is not the case and significant emergence from both can occur in the spring. A trial by ADAS using pots demonstrates this very clearly, and further investigation in field populations of great brome and meadow brome re-inforce this. Identification down to species level is important as there appears to be differences in sensitivity, with Great brome and Rye brome being slightly less sensitive in general, so may require a more robust herbicide programme to ensure successful control. Resistance to ALS herbicides has been detected at low frequencies, and whilst it should be monitored, it isn’t yet a widescale concern. 

 

Wild oat – spring or winter?

The wild oat is an all too common weed, although there tend to be particular years that see greater populations making its way through to June – this is a combination of germination timing and how successful control measures have been applied. Firstly, take a small seed sample and have a look. Avena fatua, the spring emerging species, has seeds that naturally separate, whereas the seeds of winter wild oat remain together as a unit and need significant force to break them apart. Wild oats will have two larger seeds, with a third, or sometimes fourth, tucked in between these. These secondary seeds will be awned in the spring wild oat, and awn-less in the winter wild oat. The primary seeds in both species are strongly awned. As the names indicate, they have reasonably distinct germination periods, which can be useful in setting out a management plan using crop rotation to avoid them, or to target with suitable herbicides. Tri-allate, the AI in Avadex Excel 15G and Avadex Factor, is very effective against both species of wild oat, and tame oats, when applied pre-emergence. It is an active with a longer half-life, which helps to provide long-term control against a notoriously unpredictable weed species.

 

Summary

Take the opportunity now to observe the species of weeds in a field, and to some degree map the location if patchy, and record the density, which is useful for benchmarking future control. If new weeds are appearing then reflect on where the source into the field is, and how your practices may need to change in the future to reduce population growth. Careful consideration of herbicides is needed, and using herbicides that have action across multiple species is a robust way of reducing the risk of other species entering the field. Products such as Avadex Excel 15G or Avadex Factor are effective across the grass-weed complex earning you flexibility later in the season for other field work. 

 

Useful Links

https://croprotect.com/

https://ahdb.org.uk/knowledge-library/the-weed-resistance-action-group-wrag