Optimum crop-topping timing – getting better ryegrass control

Take home messages

• Crop-topping is an effective strategy to reduce ryegrass seed viability with herbicides such as glyphosate and paraquat.
• The greatest reduction in ryegrass seed viability is when the herbicide is applied between the early flowering to milky dough stage.
• To ensure sufficient crop safety, the developmental stage of the crop should be more advanced than the ryegrass.
• Glyphosate is not effective in sterilising ryegrass if the plants are glyphosate resistant.
• DO NOT use glyphosate on crops intended for seed production or sprouting

Pre-harvest non-selective herbicide use for ryegrass seed sterilisation

Crop-topping is the application of non-selective herbicides, such as glyphosate or paraquat, prior to harvest to promote more even maturation of the crop and to sterilise developing weed seeds in species such as ryegrass, particularly when applied between the flowering to early grain fill (soft dough) stage. To determine the stage of ryegrass seed development, test the seed located at the middle of the seed head. Remove the seed and squeeze between your fingers. If the contents are soft and pliable, this is the most advanced stage to apply the herbicide at. If the contents are ‘watery’ or ‘milky’, these are optimum growth stages. If the seed hardly compresses upon squeezing, it is too late to crop-top. The presence of ryegrass late in the season can result from later germination or surviving herbicide resistant plants. The selectivity of crop-topping is dependent on there being a sufficient gap in physiological maturity between crop and weed. There are several factors that determine the effectiveness including developmental stage, environmental conditions, product choice and herbicide resistance. It is important to be aware of the maximum residue levels (MRLs) that vary for different crops and different markets. Growers are urged to check with their grain handlers before undertaking crop-topping.

Herbicide vs crop choice

Wheat

Paraquat is not registered for crop-topping in wheat whereas glyphosate is registered. In Australia, there are hundreds of glyphosate products. While most products can be used, the maximum registered rate is 975g ai/ha, whereas higher rates are registered for Roundup Ultra® Max (3.4 L/ha = 1938g ai/ha), Weedmaster® DST® (4.1 L/ha = 1927g ai/ha), Weedmaster® Argo® (3.6 L/ha = 1944g ai/ha) and Crucial® (3.2 L/ha = 1920g ai/ha). In addition, the withholding period of the three latter products is 5 days compared to 7 days for other glyphosate products.

Barley (except malting barley)

Paraquat is not registered for crop-topping in barley . No glyphosate products are approved for use on malt barley, only for feed barley on crops at the late dough stage. As for wheat, Crucial, Weedmaster DST, Weedmaster Argo® and Roundup Ultra Max are registered in feed barley. As the maturation stage between barley and ryegrass tends to be greater than wheat, the opportunity for crop-topping is greater with less risk in reducing the crop yield.

Canola

Paraquat is not registered in canola for any use. Four glyphosate products are registered in canola, Roundup Ultra Max, Weedmaster DST, Weedmaster Argo and Crucial. Apply to mature standing crop from early senescence prior to windrowing or direct harvest. Application can also be applied at the time of windrowing, utilising windrow equipment fitted with spray booms.. Additionally, Crucial, Weedmaster DST and Weedmaster Argo have a 5-day withholding period whereas there is no withholding period for Roundup Ultra Max when used as per the label.

Pulses

Paraquat is registered to reduce seed set of annual ryegrass between 400mL/ha – 800mL/ha (Paraquat 250EC). The optimum stage to spray ryegrass is when the majority of the seed heads at the bottom of the plants have emerged and the majority are between flowering and no later than soft dough stage, usually between October to November. Using the higher registered rate provides a greater reduction in seed set. In seasons where the crop is less advanced than the ryegrass, a yield reduction may occur, therefore crop-topping may not be practical every season. Diquat is also registered in pulses for crop desiccation but not for ARG seed set control. Note that not all glyphosate formulations are registered for crop desiccation and crop-topping, therefore refer to the specific product labels for more detailed information.

Crop-topping timing field trials with glyphosate

Field trials were conducted at Blyth and Spalding in canola in 2010 after paddocks had been windrowed. Glyphosate was applied at three rates at the time of windrowing. Seeds were collected from tillers and tested for germination to determine the effectiveness (Table 1). Although ryegrass seed was slightly more advanced at Blyth (late milky dough to early soft dough), spraying conditions were much better at Blyth (dT = 7) compared to Spalding site (dT = 6-11) where temperatures exceeded 30°C (Table 2). This indicates that, as well as the developmental stage of the ryegrass, the weather conditions at crop-topping play an important role. Targeting ryegrass at the optimum developmental stage during cooler conditions is likely to result in the greatest reduction in ryegrass seed viability.

Table 1: Results are the % of Nil for visual viability x germination. Means with different letters are significantly different at P=0.05.
Ryegrass stage at Blyth: oldest tillers - late milky to early soft dough stage
Ryegrass stage at Spalding: oldest tillers - seed at milky dough stage, youngest tillers anthers not emerged yet, but could be seen when glumes were peeled off.

Table 2: Weather conditions at each trial site.

Crop-topping with glyphosate resistance

A field trial was conducted in 2016 at Roseworthy to investigate the effect of crop-topping a glyphosate resistant ryegrass population with 2.8 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 most likely was via the more prevalent reduced translocation mechanism. Viability testing of the seed after maturation revealed that the reduction in seed germination was 9–22% indicating that at least 80% of the seed remained viable (Figure 1). Glyphosate, therefore, had very little effect in reducing the viability of glyphosate resistant ryegrass. A point to consider is that glyphosate resistance could increase quicker if susceptible ryegrass is controlled leaving only resistant individuals to preferentially cross-pollinate with each other.

Figure 1. 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.

In a recent GRDC investment a random weed survey in the lower Eyre Peninsula in 2020, 15% of ryegrass samples from 66 collected seed samples tested were confirmed resistant (defined as survival in pots of >20%) to glyphosate and 16% developing resistant (survival in pots of 1% to 19%). Resistance testing to determine whether glyphosate resistance is present in a paddock is therefore very important for establishing effective paddock strategies in subsequent season(s).

Conclusion

Crop-topping is an opportunistic strategy to sterilise ryegrass seeds from plants present at the end of the season, particularly where the maturation of the ryegrass is delayed compared to the crop. Glyphosate and paraquat are the most commonly used herbicides. Paraquat is only registered in pulse crops whereas glyphosate is registered in wheat, barley (not malting barley), canola and pulse crops. The presence of glyphosate resistance can reduce the effectiveness of this practice.

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.

Useful resources

Pre-harvest herbicide use fact sheet

Contact details

Peter Boutsalis
University of Adelaide
Waite Campus, Glen Osmond SA 5064
peter.boutsalis@adelaide.edu.au
Plant Science Consulting
www.plantscienceconsulting.com.au
0400 66 44 60
@PBoutsalis