Controlling ryegrass with post-em chemistry

Many ryegrass populations still susceptible to post-ems

For some, a surprising finding of the survey is the number of populations still susceptible to post-ems. 45.6% of populations were sensitive to ALS-chemistry and, coincidentally, 45.6% susceptible to Axial (pinoxaden). And over 50% were either fully susceptible or exhibited only single R?. This is remarkably close to ryegrass samples submitted to Bayer over the last two years for resistance testing. Over 55% of these were also fully susceptible to ALS chemistry.

Some samples were cross sensitive to both actives, but others were sensitive to one and resistant to the other. The overall picture is as follows:

“If you surveyed people and asked how many think ALS doesn’t work on Italian ryegrass, I expect it would be a much higher number than what we found in the survey,” says John Cussans. “It’s not because those people aren’t having problems controlling Italian ryegrass, but the source of the problem is perhaps not what they imagine it is.”

Patterns of resistance

Previous work published in 2019 looked at 50 ‘difficult’ Italian ryegrass populations. It showed a higher level of resistance to Atlantis (mesosulfuron + iodosulfuron and pinoxaden compared with results from the more random 2021 survey. Which in itself is likely to be an overestimate as it attracts farmers worried about ryegrass, many of whom gathered samples after a post-em application which tends to select the resistant plants in the population.

Different regions also showed variation in sensitivity to the two types of post-em chemistry but we don’t know the reasons for this variation.

Pinoxaden tends to be applied late

Data from the survey shows that farmers using pinoxaden tend to apply it later than ideal in April, May and even June. This implies it is used as a ‘fire brigade’ treatment rather than as a planned final step in the programme. ALS-based products such as Pacifica Plus (mesosulfuron + iodosulfuron + amidosulfuron) tended to be applied earlier in spring when control is usually better.

For all post-ems, timing, conditions and application technique have a big effect on efficacy. If you have a sensitive population, make sure that herbicides are getting the best possible control to help prevent resistance developing.

Resistance test to confirm status

Weed control is too important to base it on guesswork. With such variable patterns of resistance in Italian ryegrass, a resistance test can help identify the right chemistry for the job. Once you know which actives are still effective, plan programmes that rotate different modes of action and integrate several cultural controls.