Trials underline value of perennial grass in AB15 Stewardship mixes

AB15 is popular for August sowing. But it must be done properly.

Balancing AB15 Stewardship mixes with perennial grasses is vital to optimise their effectiveness in managing problem grassweeds, confirm the latest trials.

Farmed environment specialist Marek Nowakowski conducted a range of field-scale trials on ground with historic grassweed problems. They took place at Agrii’s Newton Purcell iFarm and involved three legume fallow mixes established in autumn 2020.

Retained for two years, the trials replicated four times in 12mx 200m plots across the field included a legume-only mix, a mix of legumes with perennial ryegrass and a special Agrii multi-mix containing several alternative grasses with 15 legumes and other herbs.

All plots were managed to stewardship guidelines with three cuts in the first year to deal with emerging grassweeds. They had good ground cover and flowers in their second season – despite being sown later than ideal in strips with a cross-slot drill.

Six random 3m² quadrat counts per plot this June showed substantial differences in both blackgrass and brome populations between the treatments. An average of 19 heads/m² and 18 heads/m² respectively were recorded in the no-grass legume mix.

Annual grassweeds

In contrast, less than one head/m² and no brome whatsoever were found in the multi-mix; and the legume and ryegrass mix had less than one head/m2 of both blackgrass and brome (see graph).

“Control of annual grassweeds in the first year and a good competitive perennial ground cover into the second season meant both our grass-containing mixes did a very effective job at preventing more blackgrass and brome emergence,” said Mr Nowakowski.

There was almost 20 times the grassweed population in the no-grass legume mix. Mr Nowakowski said this underlined that blackgrass control, in particular, is much more effective if perennial grasses are included.

This didn’t mean grasses were always needed in an AB15 mix. Rather, it showed that including them invariably meant there was more chance of keeping on top of annual grassweed problems. 

“Herbs on their own tend to grow too slowly to develop competitive canopies going into the winter,” said Mr Nowakowski. “This gives far too much space for annual weeds to come through; especially on ground carrying significant weed seed burdens.

“The name of the game with annual grassweeds has to be to prevent them setting seed by repeated mowing in the first year while building-up a competitive sward that shuts out further emergence and establishment from then on.”

The latest trials confirm how effective perennial ryegrasses can be at doing this.  But for those nervous about growing ryegrass, Mr Nowakowski said there are plenty of other grass species which would do a great job.

These included the crested dog’s tail, slender creeping fescue, Chewings fescue and smooth meadow grass we included in our even more effective trial multi-mix.

“It’s important to appreciate that grasses like these are at least as valuable as many flowers as food sources for a wide range of insects and birds. So, having them in the mix is also an environmental positive.”

For wildlife, grassweed control and soil structure, Mr Nowakowski says birdsfoot trefoil is better than the official guideline which suggests including 50% common vetch – not least for its much more valuable and competitive canopy and its deep rooting.

“Of course, you don’t have to stick to the guidelines these days. The danger, however is if something goes wrong. You’re likely to be in the clear here if you’ve followed the guidelines. If you haven’t, though, you risk losing your AB15 payment.”

Mr Nowakowski recommends sowing in first half of August for the best establishment, and broadcasting rather than drilling for even groundcover. Alongside these he has several other long-standing science and experience-based tips for AB15 success.

First and foremost, he suggests those that need to put the maximum pressure on problem grassweeds think seriously about good inclusions of perennial grasses in their mixes.

“Even with the relatively open structure cross-slot drilling gave at Newton Purcell, we achieved much better blackgrass and brome control with mixes containing grasses,” he stresses.

“With the new rules specifying a maximum of three years in the same place against the minimum of three years we originally designed AB15 for, ensuring the most competitive cover from the word go is especially important.

“When and how often you cut in the first year is also key. As blackgrass re-heads more rapidly after each cut and seeds take a relatively long time to mature and become viable after heading, I never advise mowing before the end of flowering – generally in late May or early June. 

“Holding your nerve like this means two or three mowings should be quite sufficient.  Starting too early – unless you need to help the mix establish in the face of a very dense initial weed burden – could mean you have to cut more than three times to prevent any weed seed return.”

Wheat crop

The final, crucial part of the equation in Mr Nowakowski’s experience is getting the following wheat crop into the ground without waking-up any remaining grassweed seed. 

This may not have been so critical with the five-year duration AB15
was originally designed for, given the extent of the loss of viability of the weed seed reservoir in the ground over this time.

After just two or three years, however, there is likely to be more than enough viable seed remaining. So, the emphasis has to be on spraying-off the mix effectively with glyphosate and direct drilling the wheat with the least possible spoil movement.

“AB15 has proved very popular with growers for its combination of agronomic and environmental benefits,” says Nowakowski. “But getting it right is far from as simple as sowing a guideline mix and mowing it repeatedly in year one.

“It hasn’t been made any easier by the way the goalposts have been shifted either. All the trial work I’ve been involved over many years shows that having a legume-based fallow down for two years is better than one, three years is better than two and four years is better than three.

“A maximum of three years makes choosing the right mix and the managing it correctly  through to the establishment of the following wheat crop more essential than ever.”