Maximising Glyphosate Performance on Annual Ryegrass

At the GRDC Grains Research update in Adelaide; Peter Boutsalis discussed the rise of Glyphosate resistant annual ryegrass and the ways that we as consultants and growers can combat the ever-increasing issue across the medium and high rainfalls zones.

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Figure 1. Incidence of paddocks containing Glyphosate resistant ryegrass. Resistance is defined as a sample where >20% plant survival was detected in a pot trial

As seen in the figure above Glyphosate resistance is becoming more prevalent across the Western AG cropping region. Below are some factors that Peter Boutsalis highlighted which may reduce efficacy of Glyphosate and therefore selecting for resistance.

 

  1. Low quality Glyphosate and surfactants being used

  2. Antagonism - mixing too many products with Glyphosate causing antagonism especially in low water rates.

  3. Low quality HARD water (<200ppm) - Glyphosate is a weak acid and binds with the positive cations within the hard water.

  4. Applying Glyphosate in high temps and low humidity – Glyphosate does not stay in solution on the leaf long enough to be absorbed.

  5. Applying Glyphosate to stressed plants – reduces translocation.

  6. Shading effects – high stubble loads.

  7. Dust on leaves – Glyphosate binds to dirt readily, therefore not absorbing into the leaf.

  8. Spraying operation; wind speed, nozzle selection and boom height can affect Glyphosate coverage

 

As consultants and growers, we know that use of Glyphosate is not always performed in ideal conditions as much as we would love it to be. Therefore, we need to try and optimise the performance of Glyphosate where possible. At GRDC Peter Boutsalis outlined four different ways to try and increase the performance of Glyphosate as outlined below.

 

  1. Avoid spraying ryegrass in hot and dry conditions.

  2. Improve water quality for Glyphosate by adding ammonium sulphate (AMS).

  3. Ryegrass stage – do not spray at 1 leaf – as the plant translocates Glyphosate to the roots and since it is still using energy from seed it will not translocate to the roots.

  4. Double knock - Glyphosate followed by Paraquat 1-5 days later. It is important that it is not used after any longer than 5 days as Glyphosate starts to affect the uptake of the Paraquat as the plant becomes stressed.

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Figure 2. Effect of ammonium sulfate (AS) and wetter (BS1000) and ryegrass growth stage on Glyphosate performance

As shown in the left-hand figure 2, Glyphosate performance can be significantly improved by the addition of either or wetter or water conditioner. Ammonium Sulfate (AMS) has been shown to independently improve Glyphosate performance, as the ammonium ions can work with Glyphosate to increase leaf uptake. In a pot trial conducted with soft water, AMS was shown to significantly improve control of ryegrass with 222ml/ha (100g ai/ha) of Glyphosate 450 (Figure 2). The addition of a wetter resulted in a further improvement in control.

 

Glyphosate activity can vary at different growth stages of ryegrass. In a pot trial investigating the effect of Glyphosate at four ryegrass growth stages (1-leaf to 4-tiller), good control was achieved at the three older growth stages but not on 1-leaf ryegrass (Figure 2, right hand side). Most Glyphosate labels do not recommend application of Glyphosate on 1-leaf ryegrass seedlings because they are still relying on seed reserves for growth. Consequently, very little Glyphosate moves towards the roots. Therefore, spraying ryegrass <3 leaf stage should be avoided at all costs to maximise Glyphosate performance and subsequently increase ryegrass mortality.

 

Double Knockdown Strategy

A double knock strategy is defined as the sequential application of two weed control tactics to combat the same weed population. Double knock could be Glyphosate (Group M) followed by Paraquat (Group L) or Paraquat followed by Paraquat. In the presence of Glyphosate resistance, Paraquat applied one to five days following Glyphosate was shown to provide optimum control in trial work conducted by Dr Christopher Preston. Spikes can also be included with these to increase the modes of action and reduce possible resistance forming. There are a number of new group G spikes now available with ryegrass activity.

 

By acting now, we can ensure the long-term sustainable use of Glyphosate in Australian farming systems, by minimising the risk of weeds developing resistance to Glyphosate-based herbicides. It is vitally important to know the ryegrass chemical resistance levels on your farm. This can be done by sending ryegrass plants or seeds away for resistance testing- please consult your Western AG agronomist today if you wish to test ryegrass resistance levels on your farm.

Article produced by - Zach Reardon, Western AG Bordertown