Italian ryegrass appears to mostly emerge in the autumn, contrary to popular opinion, new research suggests. The results cast doubt about leaving its control until spring and provide evidence to support the use of pre-emergence herbicides, especially where resistance is present.

A two-year research project co-funded by the HGCA and Syngenta, and carried out by PhD student Rocio Alcaron-Reverte, under the supervision of Rothamsted Research and the University of Reading has found the vast majority of Italian ryegrass seedlings emerge in the autumn, soon after drilling.

As a result, growers should aim to achieve effective chemical control in the autumn, Stephen Moss of Rothamsted Research says.

“Many growers delay ryegrass control until the spring because they believe it has a protracted emergence pattern and are waiting to spray until it has all emerged.”

But the latest research has found no evidence of a spring flush. “Indeed, most emergence takes place in October, at or soon after drilling. Far less occurs in November and December, with very little in the spring.”

Italian ryegrass, like blackgrass, is best controlled in the autumn, Stephen Moss of Rothamsted Research suggests

Results from five sites show that 81% of total emergence took place in October, 13% in November and December and only 6% in the period from January to April (see table).

“Emergence patterns were similar over two years, despite seasonal and weed density differences,” notes Dr Moss.

It means growers can treat ryegrass in the same way as blackgrass, which has a similar germination pattern.

“With both weeds, there will always be a small amount of spring germination. But in a well-established cereal crop, this doesn’t create a problem, as the resulting plants are less competitive and produce far fewer heads.”

The fact that resistance is common in ryegrass is another reason for making good use of pre-emergence control methods, he says. “You have a better chance of control when the weeds are small. In the project, we found 70% of ryegrass samples had resistance to at least one herbicide.”

While resistance levels in ryegrass are lower than in blackgrass, there’s a likelihood they will increase, he warns. “We don’t know enough about ryegrass resistance to the sulfonylureas. But we do know mixtures and sequences should be used and the emphasis should be on a strong pre-emergence treatment.”

The most effective pre-emergence herbicides for ryegrass control in wheat are Defy (prosulfocarb) and Crystal (flufenacet+pendimethalin), says Dr Moss. “Defy has a particular strength on ryegrass. There’s less need for an autumn follow-up spray where a good pre-emergence has been used.”

Jason Tatnell of Syngenta agrees. “If the pre-emergence treatment has done a good job, there’s no need to spray again. But we would never advocate starting a ryegrass control programme with a post-emergence spray.”

The other two active ingredients commonly used for ryegrass control are pendimethalin and chorotoluron,. “It’s important that growers make use of everything that’s available, rather than over relying on just one active ingredient.”

His advice is to use a Defy/pendimethalin mix pre-emergence, followed by an Axial (pinoxaden)/CTU mix post-emergence.

“If you’re not intending to do an autumn follow-up, then it’s fine to do a spring application of Axial. The only exception to this is where you have known ACCase resistance, in which case you should switch to a sulfonylurea.”

Most ryegrasss emerges in the autumn, new research has found

Ryegrass seed dormancy

Environmentally imposed dormancy is less significant in ryegrass than it is in blackgrass, reports Dr Moss.

“It’s not a major consideration with ryegrass,” he says. “So it’s another nail in the coffin for the delayed spray.”

Even this year’s later harvest shouldn’t make a difference for ryegrass germination patterns, he believes.

“Seeds were shedding a bit later this year, which means they could be slightly more dormant and less opportunity to get rid of them pre-drilling,” he says.

That will result in more coming up with the crop and an increase in weed pressure, says Dr Moss. “It adds weight to the argument for a strong residual pre-emergence treatment.”

Delayed autumn sowing can greatly reduce ryegrass infestations, he adds. “But only if all the emerged plants are effectively destroyed with glyphosate pre-drilling.”