Does glyphosate formulation affect the control of glyphosate resistant weeds?

Take home messages

• Glyphosate resistance has been detected in annual ryegrass and sowthistle.
• Initial trials suggest significant differences in efficacy between glyphosate products.
• Treating younger plants at lower temperatures can improve glyphosate efficacy on resistant biotypes.
• Crop topping with glyphosate is not effective on glyphosate resistant ryegrass.

ɸ Extra technical comment by Protech Consulting Pty Ltd

Glyphosate resistance

The GRDC has invested in random weed surveys of cropping regions across WA, SA, VIC and NSW since 2005, to monitor for resistance levels in key weed species. In the latest round of weed surveys, glyphosate has been included in the suite of herbicides tested. The methodology involves collecting weed seeds from paddocks chosen randomly at pre-determined distances, at harvest. Weeds were tested in outdoor pot trials under natural growing conditions. The incidence of resistance to glyphosate identified in these surveys is presented in Figure 1.

Figure 1. Incidence of paddocks containing glyphosate resistant ryegrass. Resistance is defined as a sample where more than 20% plant survival was detected in a pot trial. Paddocks surveyed in WA = 500, SA = 700, Vic = 450 and NSW = 600.

Glyphosate resistance in ryegrass has also been detected in grower samples sent to commercial resistance testing laboratories. In most cases, testing requests have been to identify effective herbicides or verify a herbicide failure. Requests to test with glyphosate due to poor performance is common. Figure 2 presents test results from Plant Science Consulting in the last 12 months. It highlights that the level of glyphosate resistance is similar across the southern states and is approximately 10-fold greater than the figures identified in the random weed surveys (Figure 1).

Figure 2. Resistance to 1.5L/ha Glyphosate 540 confirmed in grower ryegrass samples (Seed and Quick-Test) in the past 12 months by Plant Science Consulting.

Differences between glyphosate products

Significant differences between glyphosate products have been identified in outdoor pot trials conducted in winter and summer annual weed species. Three undisclosed registered glyphosate products were compared in initial trials, with significant differences in weed control. Herbicide products Gly 1 and Gly 3 gave consistently greater control than Gly 2 on susceptible and resistant ryegrass (Figure 3). Surfactant differences between glyphosate products is likely to be a major factor determining final control. In the field, using glyphosate products with quality surfactants could be the difference between controlling ryegrass individuals with lower levels of resistance or allowing them to survive, cross-pollinate and increase the levels of glyphosate resistance.

Figure 3: Efficacy of three glyphosate products on susceptible and glyphosate resistant ryegrass populations, SLR77 with weak glyphosate resistance and SLR76 with strong glyphosate resistance.

Differences in the level of control between glyphosate products in another key weed species such as glyphosate-resistant milkthistle (sowthistle) from NSW has also been confirmed (Figure 4). This information highlights that significant differences in control between glyphosate formulations occur, not only on glyphosate sensitive, but also on glyphosate resistant individuals.

Figure 4. Efficacy of four glyphosate products on control of glyphosate resistant milkthistle as confirmed by outdoor pot trials by Plant Science Consulting.

Growth stage and glyphosate rate

Plant growth stage can play an important role in weed control. Even in resistant populations, improved control can be achieved at younger growth stages. Younger plants tend to have thinner cuticles than older plants, and therefore, herbicide movement into younger plants is generally quicker. The effect of growth stage and glyphosate rate was investigated in a field trial in NSW on a susceptible and two glyphosate resistant sowthistle populations by Tony Cook, DPI Tamworth (Table 1). Increased control of glyphosate resistant sowthistle was observed at younger growth stages.

Table 1. First cases of confirmed glyphosate resistant sowthistle from Liverpool plains. Data presented as percent biomass reduction at three growth stages. Fallow spray timings from early to late summer. Data courtesy of Tony Cooke, DPI, Tamworth.

Weed seed sterilisation

Crop-topping is a procedure aimed at controlling weed seed set at pre-harvest timings with non-selective herbicides. One of the most commonly used practices is applying glyphosate pre-harvest to prevent seed set by flowering ryegrass. Only two glyphosate products (Nufarm Weedmaster DST^® and Roundup Ultramax^®ɸa) are registered for this practice in wheat, barley^ɸb, canola and some pulse crops. A field trial was conducted in 2016 to investigate the effect of crop-topping a glyphosate resistant ryegrass population with 2.8L/ha and 4.1L/ha of Weedmaster DST at two timings (flowering and milky dough). Additionally, laboratory testing confirmed that this population was not target site resistant, therefore resistance is due most likely to the reduced translocation mechanism. This is the most common glyphosate resistance mechanism identified in ryegrass.

Viability testing of the seed after maturation revealed that the reduction in seed germination was between 9% to 22%, indicating that at least 80% of the seed remained viable. Glyphosate was therefore not effective in sterilising glyphosate resistant ryegrass. In addition, glyphosate resistance can increase if susceptible ryegrass is sterilised leaving only resistant individuals to cross-pollinate with each other.

^ɸaRoundup Ultramax is no longer registered but Pintobi Attack has these uses on its label; commercial applicators must use registered products. ^ɸbProducts listed are not registered for use in barley, their use is for research purposes only.

Figure 5. Reduction in viability of ryegrass seed after crop-topping with Weedmaster DST at two timings, F - flowering and MD = milky dough. Trial conducted at Roseworthy SA in 2016.

Effect of temperature

Temperature has been identified as playing a major role in glyphosate efficacy. Significant differences were identified in wild oat control with the same glyphosate product in plants sprayed in outdoor summer or winter pot trials in South Australia (Figure 6). Complete control of a wild oat population was not achieved in summer even at higher than label rates (1600g ai/ha glyphosate) whereas in winter trials 400g ai/ha glyphosate resulted in complete control. These large differences suggest that controlling wild oats in summer fallows can be affected by high temperatures.

Figure 6. Control of wild oats with the same glyphosate product in outdoor summer and winter pot trials.

A current study is investigating the effect of temperature on control of glyphosate resistant sowthistle from NSW. Initial trials have confirmed greater control with glyphosate at lower temperatures, particularly of resistant biotypes (Table 2). These findings suggest that applying glyphosate at lower temperatures can improve control of glyphosate resistant sowthistle. At lower temperatures glyphosate remains in liquid form on plant surfaces longer leading to greater uptake, particularly at higher humidity. Maximising glyphosate uptake is therefore likely to improve weed control and factors such as lower temperature and higher humidity influence uptake.

Table 2. Effect of temperature in control of four biotypes of sowthistle with Glyphosate 540. Data is LD50= dose required to kill 50% of the population.

Conclusion

In the southern cropping zone glyphosate resistance in ryegrass is becoming increasingly common. Significant differences between registered glyphosate products have been identified on several weed species with some products more effective than others. Differences in the control of glyphosate resistant ryegrass and sowthistle biotypes with different glyphosate products were observed. Products with quality surfactants can be expected to more effective than products with poor quality surfactants, particularly on stressed weeds. Treating younger plants under cooler temperatures using robust rates can improve weed control of susceptible and some glyphosate resistant individuals.

These initial findings have identified that there are several factors that influence glyphosate efficacy including product choice. A better understanding of glyphosate formulations could improve weed control and delay glyphosate resistance. Further investigation of glyphosate products is recommended.

Acknowledgements

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC, the author would like to thank them for their continued support under project UCS00020. Additional thanks Tony Cook from DPI Tamworth and to Sinochem Australia for permission to use trial data.

Contact details

Peter Boutsalis
University of Adelaide
Waite Campus, Glen Osmond SA 5064
peter.boutsalis@adelaide.edu.au

Herbicide resistance testing
Peter Boutsalis
Plant Science Consulting
0400 66 44 60
@PBoutsalis