Mature sterile brome showing its purple colouration at maturity pulling down a crop of winter wheat.

Recent Teagasc research has suggested that a number of populations of Irish grassweeds have developed resistance to post-emergence herbicides. The grassweeds currently being studied are wild oats, the brome family and blackgrass.

It is important to introduce this article by first explaining that all the populations of grassweeds that have been examined by Teagasc thus far come from fields where weed control had been an issue. With that in mind, the levels of resistance observed in these studies are not representative of the levels of resistance on farms throughout the country.

Wild oats

Wild oats are a logical species to begin with, as this grassweed is one of the most common throughout grain-growing regions, particularly in areas where spring cereals are heavily grown. Wild oats predominantly germinate in the spring and can inflict significant yield losses in crops due to their competitive nature.

In total, 42 populations of wild oats have been tested for resistance to the ACCase-inhibiting herbicides cycloxydim (Stratos Ultra), propaquizafop (Falcon) and pinoxaden (Axial), as well as the ALS-inhibiting herbicide mesosulfuron/iodosulfuron (Pacifica).

Fifteen of these 42 populations demonstrated outright resistance to one or more of the ACCase-inhibiting herbicides, with three populations showing partial resistance.

Furthermore, five populations were resistant to the ALS herbicide (Pacifica), with a further three populations partially resistant. All such populations were cross-resistant to the ACCase herbicides tested.

Wild oats are a problem in all cereals and a proportion of the population is now resistant to a number of herbicides.

Resistant strains came from a wide area throughout the grain-growing region, but a number of the problem populations seemed to be concentrated in the southeast, particularly Co Wexford.

It is important to remember that only 42 populations are represented in these tests. It may be just a quirk of statistics that threw up a number of samples from Co Wexford, but it is worth noting, considering the abundance of spring barley grown in this area.

The bromes

Sterile brome is another common grassweed and is probably now familiar to all readers. In contrast with wild oats, brome tends to germinate in the autumn. This partially explains its prevalence in winter cereal crops. Work done in Oak Park on 37 brome populations indicates that resistance to the sulfonylurea herbicide mesosulfuron/iodosulfuron is present in virtually all samples (33 of 37).

Again, caution is advised when interpreting these results. These samples originated from fields with heavy brome infestations, such as the one in the picture on P30. Within these populations, no resistance has been noted to cycloxydim (Stratos Ultra) or propaquizafop (Falcon), so break crops may still provide an option for control of resistant populations.

In order to illustrate the ability of some of these populations to withstand herbicide applications, increasing doses of mesosulfuron/iodosulfuron were used. Doses ranged from one quarter of the manufacturer’s recommended rate, right up to 16 times this rate. Analysis of these results allows us to estimate the amount of herbicide required to kill 90% of the population.

In some cases, it took up to 14 times the recommended field rate to kill 90% of the population – extremely high. Again, these results come with the caveat that these are just estimates, but nonetheless they indicate that herbicide resistance in brome populations can become a serious issue if left unchecked.

Blackgrass has a blunt and finely serrated ligule, which is the little piece of tissue that grows up from the top of the leaf sheath at the leaf axil.

Blackgrass has a blunt and finely serrated ligule, which is the little piece of tissue that grows up from the top of the leaf sheath at the leaf axil.

Blackgrass

Blackgrass is a weed that is well known to growers in the UK, but it is becoming increasingly common in Ireland as well.

Thus far, Teagasc has recorded 28 farms in the country with confirmed blackgrass infestations. In reality, the number of farms affected is likely to be much higher. Blackgrass has been recorded in Louth, Meath, Kildare, Carlow, Kilkenny, Wexford, Tipperary, Cork, Waterford and Limerick.

An issue with the sampling of blackgrass is that many are unable to accurately identify blackgrass, even after flowering. It is important that farmers and agronomists familiarise themselves with blackgrass to have a chance of stopping its spread.

During the vegetative immature stage of its development, blackgrass can be identified by its purple/red colouration at the base of its stem. The ligule is the translucent piece of plant material at the junction of the leaf sheath and the leaf blade. Blackgrass has a blunt and finely serrated ligule. The leaves themselves are generally long, slender and dark green.

As the plant matures, it grows in an upright manner. Blackgrass produces a number of seed heads that have the potential to shed hundreds of seeds per plant. The seed heads are long, cylindrical and narrow in shape, with colours ranging from green to deep purple (hence blackgrass).

At this stage, the seed heads are quite difficult to distinguish from other foxtail species, such as meadow foxtail, but the difference between these species can be told by looking at their growth habit, as blackgrass is much more upright.

As is the case in the UK, herbicide resistance is a significant issue with Irish blackgrass. Virtually all populations sampled in Ireland in 2016 and 2017 were found to be resistant to at least one herbicide active ingredient.

It must be stressed that it is probably best to assume that blackgrass is herbicide-resistant. Therefore, your control strategy must be something other than a post-emergence herbicide.

Control strategies

Beginning with blackgrass control, the most important thing is to avoid the spread of the weed throughout your farm and on to other neighbouring farms.

If the infestation is small (ie a small patch of plants under 10m2) hand rouging may be an option.

It is important to remain vigilant and observe this patch throughout the growing season for the spread of blackgrass. Desiccation with glyphosate may seem like an extreme option, but thinking long-term, this will help limit seed return.

If you’re operating on a mixed enterprise, you may decide to put the affected field back into grass for a number of years. Although estimates vary, most people believe that blackgrass will not survive burial in the soil for longer than three to seven years.

If this option is not available to you, consider switching to spring cropping in affected fields. Blackgrass is predominantly winter germinating, so spring cropping to allow the use of stale seed beds in the autumn is effective. However, it must be said that after a number of years, blackgrass populations will evolve to favour spring germination.

Grassweed control is a numbers game. Every plant that survives through to harvest gets the opportunity to develop herbicide resistance in subsequent seasons. In the case of wild oats, where dormancy in the soil can be very long, this year’s problem may still be an issue 10 to 15 years down the road.

Central to all grassweed control is integrated management. These strategies comprise of everything you do to limit grassweed populations, except for the application of post-emergence herbicides.

These strategies may include crop rotation, which allows you to rotate herbicide chemistries, spring cropping as mentioned previously, increasing seeding rates on headlands to provide increased competition for weeds, planting competitive crops such as spring barley and oats or drilling winter cereals as late as weather permits to allow for post-harvest weed germination.

Individually, these techniques may not provide a huge amount of weed control on their own, but when employed together, control is adequate.

Crucially, these strategies not only offer sustainable grass management, but also counter the evolution of herbicide resistance. This is very important if we are to protect what remaining active ingredients we have in future.

Key points

  • Resistance to different families of grassweed herbicides is becoming an increasing issue in Irish fields.
  • A Teagasc survey of fields that encountered control problems found about one-third of wild oat samples tested showed resistance to one or more of the ACCase-inhibiting herbicides. Five populations showed resistant to the ALS herbicide (Pacifica) and these showed cross-resistance to the ACCase herbicides.
  • The bulk of sterile brome samples tested from fields that had control issues were found to show resistance to the sulfonylurea herbicide mesosulfuron/iodosulfuron, but not the ACCase herbicides.
  • Virtually all blackgrass populations sampled in 2016 and 2017 were found to be resistant to at least one herbicide active ingredient.
  • Control of grassweeds must be helped by other husbandry decisions and actions.