Plant health top tips - October: Why should we not overuse glyphosate for weed control?

Roundup Ready® cotton has been grown in Australia since 2002 and since then glyphosate has become a firm favourite: a bit like your footy team, biscuit, or car.

Glyphosate is a once-in-a-generation herbicide - we are unlikely to see a more robust product with this longevity ever again.

But its great strength has also become its greatest weakness! Growers have had so much success controlling problem weeds with glyphosate that they have reduced the use of other modes of action, especially in non-crop areas around the farm. Repeated application of glyphosate along fence lines, roadways and channels has led to the evolution of glyphosate resistant weeds.

We are now seeing many of the common cropping weeds develop resistance to glyphosate. This is especially concerning in the cotton farming system where the building block of the industry is the Roundup Ready Flex® trait. Whilst industry surveys report that glyphosate in most cases is still working effectively, there are increasing levels of herbicide resistance appearing. Recent surveys funded by CRDC and NSW DPI have reported concerning levels of glyphosate resistance in fleabane (> 90 per cent), sowthistle (25 per cent), windmill grass (> 50 per cent), feathertop Rhodes grass(40 per cent) and awnless barnyard grass (60 per cent).

In the case of weed management, if you are on a good thing, DON’T stick to it. Diversity is the key to managing weeds in all locations on the farm and especially in-crop.

The Australian cotton industry has been very proactive in this space and developed the Herbicide Resistance Management Strategy (HRMS) to delay for as long as possible the development of widespread resistance to glyphosate. Alarming stories of herbicide resistance and ‘super weeds’ appearing in the US mainly from the overuse of glyphosate has seen the HRMS focus on diversity and a more integrated approach to weed management. The key to the HRMS is to not overuse glyphosate: 2 non-glyphosate tactics in fallow and 2 non-glyphosate tactics in-crop, and NO survivors.

So, where does that leave us with our in-crop weed control? Fortunately, we have several options available, both chemical or non-chemical, to control weeds. New registrations and formulations of some common modes of action have become available in the last five years, allowing growers the flexibility to spray over the top of cotton.

An integrated approach with residual herbicides applied at planting and in-crop allows ongoing weed control throughout the season. As one mode of action runs out of steam another is layered on top to extend the window for control. Growers then have a couple of options for late in-crop weed control. Layby applications of residual herbicides to control late germinating weeds or a non-selective applied through a shielded sprayer are good tools to include prior to canopy closure where crop competition will help suppress weeds.

Targeted tillage is the final piece in the puzzle for weed management. Adding an in-crop tillage operation to remove any survivors can be a standalone pass or part of a late side dressing of nitrogen. Either way it provides an important opportunity to control any survivors or late germinating weeds.