Roundup Ready Flex cotton - friend or foe for grass weed control?

Author: Mark Congreve | Date: 01 Sep 2015

The introduction of glyphosate tolerant (Roundup Ready®) cotton in 2000 marked a major change in weed management for Australian cotton growers. The ability to apply glyphosate over the top of cotton gave managers the flexibility to achieve post-emergent weed control of a wide range of weeds.

15 years after the introduction of glyphosate tolerant cotton, it is pertinent to consider the upsides and downsides in relation to long term weed management.

Prior to the introduction of glyphosate tolerant cotton, growers were reliant on cultivation or pre- sowing applied knockdown herbicides to have paddocks completely clean at planting. Extensive use of pre-emergent herbicides was also the norm, as there were limited selective options for in-crop control (particularly for some broadleaf weeds). Inter-row cultivation was relatively common, and it was also not unusual to find teams of chippers physically removing weeds which had escaped herbicide control.

This created reasonable diversity in weed control tactics, which slowed the development of herbicide resistant weeds. A downside was the frequent use of cultivation favoured some larger seeded weeds that were adapted to germination after burial. The lack of glyphosate in the summer months, also meant that nutgrass was a far more widespread problem than it is today.

Glyphosate tolerant cotton provided a number of benefits to growers:

  • A completely clean fallow was not considered essential, as glyphosate could control weeds after emergence (the exception being volunteer glyphosate tolerant cotton). This can allow cotton to be planted earlier, as soon as soil temperature is adequate.
  • Some pre-emergent herbicides can set back early growth of the cotton plant, particularly where waterlogging occurs.
  • Growers have the flexibility of waiting to observe weed emergence, before instigating weed control as required
  • Many growers removed inter-row cultivation and chipping, reducing labour demand
  • Often the glyphosate program (herbicide + licence fee) was less expensive than the old system of knockdown sprays in fallow, pre and post-emergent herbicides, inter-row cultivation and chipping.

From a resistance perspective, growers adopting a ‘glyphosate only’ weed management program in cotton are effectively using a very similar program to a ‘glyphosate only’ summer fallow. The only significant difference is that in the cotton system, there would be some benefit from crop competition - particularly in solid plant cotton.

10-30 years of high reliance on ‘glyphosate only’ in summer weed control programs has resulted in significant changes to weed populations.

Species shift.

  • Weeds adapted to a cultivation system (e.g. nutgrass, larger seed weeds germinating from depth) are now often very difficult to find in these paddocks
  • Broadleaf weeds that were difficult to remove from cotton (e.g. bladder ketmia) are less significant, as they have been controlled by glyphosate
  • Surface germinating species have become more prevalent (e.g. feathertop Rhodes grass, barnyard grass, fleabane, sowthistle) with the removal of cultivation

In addition to these changes in the species present, the over reliance on glyphosate as the main control tactic has selected for glyphosate resistance or increased tolerance in a number of the key species (e.g. barnyard grass, fleabane, sow thistle, feathertop Rhodes grass, and liverseed grass).

So is glyphosate tolerant cotton still beneficial in today’s cotton farming system?

The answer to this largely depends upon how the technology is implemented.

It is increasingly common to find cotton fields such as those below where glyphosate is failing to provide clean paddocks.

cotton fields where glyphosate is failing to provide clean paddocks

Growers continuing to only rely on glyphosate will find their weed control programs rapidly becoming ineffective due to increasing tolerance to glyphosate.

However, when glyphosate tolerant cotton is used as part of a diverse weed control strategy, it can be an excellent tool to drive down the weed seedbank, in particular for the surface germinating summer grass weeds.

To achieve the goal of driving down the seedbank, growers need to urgently adopt combinations of multiple tactics:

  • Ensure the fallow is clean prior to planting. This will require cultivation or the use of a Group L knockdown (e.g. paraquat, paraquat + diquat or paraquat + amitrole (Group Q)). Do not rely on glyphosate alone.  If glyphosate is to be used, then ensure a following double knock application of Group L is always applied, ideally ~7 days later. This practice can also be used to control volunteer glyphosate tolerant cotton.
  • Incorporate a pre-emergent herbicide effective against the key weed seeds known to be in paddock in conjunction with the last knockdown application, at planting or post plant pre-emergent  e.g. diuron, prometryn, fluometuron, Terbyne® Xtreme (all Group C); pendimethalin (Group D); or metolachlor (Group K). Check product labels for specific application timings.
  • For paddocks with known weed problems, plant cotton on solid row configuration to maximise crop competition and reduce weed seed set from any herbicide escapes
  • Where grass weeds are a problem, consider replacing an early in-crop glyphosate application with a Group A herbicide (e.g. haloxyfop, propaquizafop), use maximum rates and the spraying oil recommended for the Group A herbicide.
  • Apply a lay-by residual herbicide effective against key target weeds before row-closure e.g. diuron, prometryn, fluometuron, Terbyne Xtreme (Group C). Nufarm Bouncer® (metolachlor – Group K) is now registered for over-the-top or directed lay-by application when applied alone or with a glyphosate application. Ideally use a different mode of action to the one applied at planting.
  • Include inter-row cultivation or chipping to stop weed escapes from setting seed.

The effective use of at least four tactics in an individual field should result in the cotton crop being relatively clean of grass weeds at harvest.

Maintaining a weed-free focus for the following two to three summers can effectively exhaust the seedbank of weeds such as fleabane, feathertop Rhodes grass and barnyard grass.

Further information

Mark Congreve, ICAN
0427 209 234

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