Credit: Gary Naylor
Effective control pre-drilling can make the difference between success and failure in the battle against problem grass weeds. It's one of the main building blocks of integrated weed management across the rotation so give it the time and thought it deserves.
The cumulative effect of glyphosate rate, timing, application technique and product choice can lead to big differences in the efficacy of the pre-planting programme. Do everything possible to deliver the herbicide where it needs to be – the growing points in the roots and shoots of the plant.
"Roundup® is highly effective and relatively easy to use, but these strengths are also the biggest dangers as you can overlook the basics," says Bayer Agronomist Roger Bradbury. "Sub-optimal performance means you aren't getting the level of weed control you've paid for and expect. It is also a risk for the development of resistance."
Clear benefit from effective Roundup use
A Bayer sponsored demonstration at the NIAB black grass management site in Lincolnshire shows the value of a good pre-drilling programme. It compared poor glyphosate practice of a low dose rate at 2+ weeks pre-drilling, versus good practice, applying at a suitable rate within one week of drilling.
On top of that, it looked at the interaction with good and poor weed management strategies in successive years to highlight the compounding effect of different management choices on seed numbers over time.
"The starting point in this demo was a black grass population of around 5 plants/m2, the graph shows how successive poor management choices can quickly result in rapidly escalating weed numbers. It also underlines the importance of effective use of Roundup pre-drilling; you can see from the graph the benefit of this."
Rate selection
Check the weed spectrum and size to pick the ideal rate. For pre-drilling applications, the target is typically small grass weeds. 540g/ha a.i (equivalent to 1.5L/ha of a 360g/L product) should be the minimum use rate and reserved for black grass seedlings only. For Italian rye-grass and brome 720g/ha should be the minimum starting point. If weeds are larger and have started to tiller – a possibility this season with the early harvest – then increase the rates. In direct drilled situations be aware of carryover of weeds from last season which will most likely need a higher rate
"Don't attempt to offset a lower rate adding adjuvants or other mix partners," says Roger. "In trials, it hasn't given reliable control, choose the right rate of Roundup from the start."
Application and formulation
After choosing the correct rate it needs to reach the target. Step one is application which gets the spray onto the leaf. Lower water volumes are suitable for small grass weeds. Aim for a boom height of 0.5m above the target. Forward speed of no more than 12km/hour helps with boom stability, reduces the risk of drift and uneven deposition. Avoid applications in wind and rain.
Once it reaches the leaf, formulation comes into play. Roundup is a systemic herbicide so translocation to the growing points is vital. It's important that the initial spray doesn't stress the plant, so it continues to function as normal and translocates glyphosate effectively.
The surfactant system in Roundup branded glyphosate is optimised to minimise effects at the leaf surface and maximise uptake. They also improve speed of activity, rainfastness and reduce cultivation intervals.
"Roundup uses glyphosate based on a potassium salt (K-salt) form; this is a smaller molecule that the IPA-salt of many other glyphosates on the market. Consequently, there is more capacity to include surfactants and other adjuvants that improve performance of the formulation."
"Using Roundup reduces the risks of poor control at the vital pre-drilling timing. You can see the results in our trials, Roundup branded products offer more consistent control whereas, generic glyphosate products tend to be more variable. It's about peace of mind, knowing you have the best product available to do the job."
Roger adds that to get good results from Roundup, you still need to follow the best practice around application and timing.
Watch out for resistance
Roger reiterates the importance of resistance management following the announcement of glyphosate resistance in Italian rye-grass earlier this year. All the advice around rates, timing and so on are equally important for resistance management because surviving weeds are a resistance risk. "The only thing to add is that pre-drilling, avoid repeat doses of Roundup or any glyphosate to the same weeds, in practice that means cultivate between applications. Also avoid substituting a pre-drilling application with an application at the post-plant / pre-em timing which risks transplanted weeds surviving in an establishing crop."
