“Fundamental to keeping on top of problem weeds like black-grass, bromes and Italian ryegrass in the most sustainable way is knowing exactly what infestations you have and where they are,” stresses Roundup technical manager, Barrie Hunt who co-ordinates the national Grassweed Action management resource. “And June and July are the only months when you can really tell this. Especially in cereals where the distinctive weed heads stand out above the crop for all to see !
“It certainly isn’t comfortable having your agronomic failures on such public display. But, unlike OSR which can hide a multitude of sins beneath its canopy, it provides the ideal opportunity to plan ahead and target your management for the best and most cost-effective control.
“After all, we know that different weed species need different approaches. As do different levels of infestation – whether on a whole field or individual area basis.”
So what exactly does Barrie recommend ? Well, essentially it’s a matter of getting out into your cereal crops and doing two things – identifying the precise weed species present and mapping where they are, either manually or using one of the GPS-based apps increasingly available for mobile phones and tablets these days.
While the mapping the infestations is best done from the tractor cab because it gives you a far better field view, he insists it’s important to collect any weeds you may wish to confirm the identity of and, where required, take seed samples for herbicide resistance testing.
“Most of us can distinguish black-grass, brome and Italian ryegrass easily from the cab,” he notes. “However, the five brome species can be difficult to tell apart even once they’ve headed in the summer. As the two main groups of brome need quite different autumn cultivation and pre-planting control strategies and the different species have different susceptibilities to post-em herbicides, taking the time to carefully identify what you have is essential.
“This is particularly important where you have mixed populations of weeds. With sterile or great brome on the headlands and black-grass across the rest of the field, for instance, it will pay to shallow till the entire area immediately behind the combine to establish your stale seedbed. If you have meadow, soft or rye brome on the headlands, though, it’s best to leave these areas for around a month before cultivating so you don’t induce dormancy.
“With herbicide resistance problematic in both black-grass and Italian ryegrass and suspected in brome, resistance testing every three to five years can also save you a lot of unnecessary chemical expense. In which case, you will need to take a sample of ripe seed from across the field.”
Alongside good identification, Barrie Hunt believes accurate mapping of weed infestations between and within fields each July should be a priority for everyone. As well as seeing this as vital to targeting the most cost-effective balance of controls, he points out that routine weed mapping is the only real way of monitoring how successful – or otherwise – your controls are proving for continual improvement.
“Not all your fields will require the same intensity of grassweed management,” he reasons. “So traffic light coding each field every season on the basis of its weed risk means you can concentrate the most rigorous controls on the worst-affected ground where they are most needed. Red-coded fields, for instance, are almost certainly best ear-marked for rotational ploughing, delayed winter cereal sowing or spring cropping, together with the most robust pre-planting, pre-em and post-em herbicide programmes.
“On the other hand, fields with less pressing problems may be drilled earlier in the autumn and with less intensive pre and post-em herbicide programmes for valuable savings in both cost and hassle. Beware, though, an apparent absence of grassweeds in a single season should not automatically result in a green coding. Instead, it’s important to consider the history of each field in your planning.”
The accurate mapping of weed problems across individual fields has, of course, become infinitely more practical and valuable with the digital tools increasingly available to growers. This allows fields to be zoned and sprayed for their specific weed burdens – either using a precision agronomy service or manually – to even more precisely concentrate the chemistry where it’s most needed for the greatest financial and environmental sustainability.
“Satellite, drone or ground-based imaging can be useful here in some cases,” agrees Barrie. “But there are still a lot of ‘ifs’ and ‘buts’ with these technologies. So there really is nothing to beat getting out into your crops in July and recording exactly what’s going on. In my view, it remains the Number One essential to keeping your grassweed problems firmly under control.”