Eliminating weeds with glyphosate pre-drilling prepares the way for the residual herbicide campaign to control the next wave of weed germination in the crop. But effective control shouldn't be taken for granted as experience earlier this year shows.

Bayer agronomist Roger Bradbury tells us what we learned last spring and how to get the best results this time around.

Difficult spring showed importance of correct glyphosate application

"Without wanting to retread old ground, the weather earlier this year was dreadful and it had consequences for glyphosate performance. We received a few of reports of control well below expectations. On investigation we found that application conditions, timing and rates all contributed to poor control."

"Firstly, spray days were few and far between so weeds grew significantly between the time of the recommendation and application meaning the rate was too low. Of course, with limited spray opportunities, farmers may have applied in marginal conditions or gone well above the recommended 12km/h to cover the ground in limited time.

"Also, the exceptionally wet field conditions in early spring meant that target plants were under stress affecting plant growth hence translocation of glyphosate within the weed."

Mr Bradbury adds that some applications were delayed until stem extension when glyphosate is translocated to the growing tip but not to the roots leading to potential regrowth. However, this is not relevant to autumn applications.

Wake up call for autumn

Mr Bradbury emphasises that correct rate and conditions are equally relevant to autumn. "Make sure the rate is sufficient to kill the target, use Roundup delivering at least 720g/ha glyphosate for black-grass and 1080g for brome, ryegrass and couch."

Trying to cover ground too quickly is probably the biggest danger. Speeds in excess of 12km/h lead to more turbulence and drift, these effects are multiplied at higher boom heights and on rougher ground. The result is inconsistent spray distribution with some areas receiving less than the intended dose.

"The target weed is usually quite small in autumn so it's easy to miss some plants if the application isn't right. I think go low, go slow was the message one of our competitors gave for a different herbicide but it's the right advice for Roundup too, and any residuals you use this autumn."

Mr Bradbury urges farmers to monitor results of all glyphosate applications to ensure they are getting good control.

Eye on stewardship

Achieving good immediate control with Roundup also helps secure the long-term efficacy of the product. Sub-optimal doses of glyphosate increase the risk of resistance developing.

"As mentioned, use the right rate for the target, and apply it properly so the correct rate reaches the target. If you are making two applications in the same field, ensure there is cultivation in between as an alternative form of weed control."

"One thing we're particularly concerned about is farmers reducing rates and adding citric or fulvic acid to compensate. Our trials show control is reduced and it's a stewardship risk."