Earlier this year in the Irish Farmers Journal, Vijaya Bhaskar detailed research examining different glyphosate products and rates.

In summary, the research found that not all glyphosate was the same. Different formulations were performing differently in the glasshouse trial.

There was a big difference between the generic products and the Monsanto-Bayer products, especially when dealing with grass weeds already resistant to other herbicides.

Generic products are copies of the original, but they can vary from the original formulation.

This means that the rate must match the product, weed species and the target weed size and one glyphosate product may need to be higher than another in order to achieve the same level of control.

Glasshouse research

Teagasc has compiled a table (see below) to guide farmers on what rates to use based on glasshouse research. The rates specified in the table are for controlling annual grass weeds growing from seeds in a pre-sowing context only. Scutch should be controlled using a higher rate based on scutch density and the label rate.

Applying the correct rate is essential to keep problem grass weeds under control, particularly those that are more difficult to control due to resistance to come chemistry, like blackgrass, which is becoming almost impossible to control.

The highlighted rates in the table indicate that a higher rate of glyphosate is necessary when targeting smaller grass-weed populations.

Farmers should assess weed types, population size, plant size, etc, to determine effective rates and be careful not to exceed the maximum total dose (1,440g/ha or 4l/ha of 360g glyphosate) outlined on the label.

Teagasc’s top tips on glyphosate use

  • An insufficient glyphosate product rate for the specific weed species or weed size can increase problems with grass weed control.
  • Not all products work equally, and 540g/ha glyphosate (1.5l/ha of 360g/l glyphosate) may not be adequate in all situations. Reduced sensitivity or tolerance to glyphosate is developing in our Italian ryegrass and blackgrass populations that are resistant to ACCase/ALS modes of action.
  • These plants need higher product rates (>720g/ha) depending on target size. Unlike target-site resistance, which is mode of action-specific, metabolism-based, non-target-site resistance affects multiple herbicide modes of action, including glyphosate, and needs further investigation.
  • A total of 1,080g/ha of any product is effective for grass weed control of all populations when sprayed on larger tillering plants. At that rate and weed size, no weed control differences between products were noted.
  • Growers should note, if we continuously use rates that leave survivors, these are the plants that could develop into resistant populations, which could make any glyphosate product ineffective in the future. Glyphosate use must be supplemented with cultivation and herbicide applications (pre- and, where necessary, post-emergence) to eradicate any possible survivors.