Does glyphosate formulation affect the control of glyphosate resistant weeds?

Call to action/take home messages

• Glyphosate products differ in their effectiveness.
• Some glyphosate products are more effective on certain weed species than others.
• The choice of wetter can improve efficacy of low surfactant loaded products.
• Treating young weeds with quality glyphosate products under cooler conditions at robust label rates can improve control of glyphosate resistant individuals.

Glyphosate- background

The aim of this paper is to discuss findings from initial trials that have identified significant differences between glyphosate products. The composition of salts and surfactants between some glyphosate products vary significantly. Glyphosate is a weak acid and is combined with a positively charged salt to produce a stable commercial formulation. Salts include isopropylamine (IPA), dimethylamine, trimesium and potassium.

The salt type can affect the maximum glyphosate loading. For example, potassium salts enable higher load formulations compared to IPA salts. Salt composition can also affect compatibility with tank-mixed agricultural products. Another major component of glyphosate products are surfactants such as tallow amines, quaternary amines, betaines and polyglucosides. Surfactant choice can alter the rate of diffusion of glyphosate through leaf cuticles and impact entry rates via stomata.

Glyphosate products can be categorised as “fully loaded” or “partially loaded” depending on the surfactant concentration. A fully loaded product contains at least 15% surfactant. Partially loaded products contain lower concentrations of surfactants and may require additional surfactant (wetter) to improve weed control in some situations. More information can be found in the GRDC Adjuvants Booklet.

Glyphosate resistance

Increased glyphosate resistance is being detected in key weed species in the northern and southern cropping zones. In the northern cropping zone, the most common glyphosate resistant weed species include barnyard grass (108 confirmed cases), fleabane (51), sowthistle (23), and liverseed grass (4). The actual number of resistant cases is likely to be much higher. More information should be available soon from recent GRDC random weed surveys conducted in the northern zone. In the southern zone, between 2-5% of the 2250 paddocks recently surveyed as part of GRDC random weed surveys contain glyphosate resistant ryegrass.

Differences between glyphosate products

Significant differences in efficacy between glyphosate products have been identified in pot trials conducted on winter and summer annual weed species in outdoor growing conditions. 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 1). In the field, this could be the difference between controlling ryegrass individuals with low-level resistance and allowing them to survive, cross-pollinate and increase the levels of glyphosate resistance.

Figure 1. Efficacy of three glyphosate products on susceptible and glyphosate resistant ryegrass populations; SLR77 with weak glyphosate resistance and SLR76 with strong glyphosate resistance

The first case of glyphosate resistance in sowthistle was reported by Tony Cook, NSW DPI Tamworth (Cook et al 2014). Differences in the level of control between glyphosate products were observed on a glyphosate resistant sowthistle population investigated at Waite (Figure 2). These results highlight the significant differences in weed control between glyphosate formulations. Such differences in efficacy could affect the rate of glyphosate resistance development.

Figure 2. Efficacy of four commercial glyphosate products in controlling of glyphosate resistant sowthistle from NSW as confirmed by outdoor pot trials by Plant Science Consulting

Differences between glyphosate formulations were also detected on healthy fleabane, feathertop Rhodes grass and windmill grass in outdoor summer pot trials (Figure 3). Large differences were evident at lower rates that can represent similar activity to label rates on stressed weeds in the field. These results suggest that large differences in efficacy between glyphosate products can occur. At 200g ai/ha glyphosate, product Gly 1 was more active on fleabane whereas it was not on windmill grass or feathertop Rhodes grass. This indicates that one product was not consistently more active on all three weed species.

The role of two common surfactants (wetters) in a glyphosate 450 g ai/L formulation was also investigated in feathertop Rhodes grass, fleabane and windmill grass. The addition of wetter 1 to a glyphosate 450 g ai/L product produced significantly greater control of all three species. Surfactant choice can therefore be critical in improving weed control of partially loaded glyphosate formulations.

Figure 3. Efficacy of different glyphosate products on the control of fleabane, windmill grass and Feathertop Rhodes grass 5 weeks after treatment in outdoor summer pot trials

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 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, NSW DPI Tamworth (Table 2). Increased control of glyphosate resistant sowthistle was observed at younger growth stages.

Table 2. 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 Cook, NSW DPI, Tamworth.

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 4). 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 complete control was achieved with 400g ai/ha glyphosate. These large differences suggest that controlling wild oats in summer fallows can be affected by high temperatures.

Figure 4. Control of wild oats with the same glyphosate product in outdoor summer and winter pot trials.

Temperature effects on barnyard grass control with glyphosate have also been identified (Nguyen et. al. 2016). At higher temperatures reduced control of some resistant biotypes was detected.

Table 3. Glyphosate dose (g ai/ha) required to control 50% (ED₅₀) susceptible and resistant barnyard grass populations at 20^oC vs 30^oC in two trials. R = glyphosate resistant, S = susceptible.

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 4). 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 for a longer time which leads to a greater uptake into plants, especially at higher humidity.

Maximising glyphosate uptake is likely to result in greater weed control. While, lower temperature has been identified as improving glyphosate uptake, it is likely that other factors such as humidity are also important in glyphosate uptake

Table 4. Effect of temperature in control of four biotypes of sowthistle with glyphosate 540. Data is LD₅₀= dose required to kill 50% of the population (J. Malone, University of Adelaide).

Conclusion

In the northern cropping zone, glyphosate resistance in several species including barnyard grass, fleabane, sowthistle and liverseed grass is becoming increasingly problematic.

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.

Choice of surfactant in partially loaded formulations can also influence weed control. Treating younger plants under cooler temperatures at the top-end of the label 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 trail 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 NSW DPI Tamworth and to Sinochem Australia for permission to use trial data.

References

Cook, T., Davidson, B. and Miller, R. (2014), A new glyphosate resistant weed species confirmed for northern New South Wales and the world: common sowthistle (Sonchus oleraceus). Nineteenth Australasian Weed Conference. 206-209.

Nguyen, T. H., Malone, J. M., Boutsalis, P., Shirley, N. and Preston, C. (2016), Temperature influences the level of glyphosate resistance in barnyardgrass (Echinochloa colona). Pest. Manag. Sci., 72: 1031–1039. doi:10.1002/ps.4085

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
Plant Science Consulting, herbicide resistance website
0400 66 44 60
@PBoutsalis