Pre-Harvest Herbicide Use: Selective Spray Topping & Crop Topping

What do we mean by spray topping and crop topping? Aren’t they the same thing? Well yes. The aim of the game is applying herbicide…
October 10, 2018Agronomics Back to All

What do we mean by spray topping and crop topping? Aren’t they the same thing?

Well yes. The aim of the game is applying herbicide late in the season to try and help manage weeds. But in practice, the two strategies are; either spray topping with a selective herbicide, or crop topping with a non-selective herbicide.

Selective spray topping is when you give weeds a whack of selective herbicides during their reproductive growth stages to prevent seed set, without affecting the crop.
This practice is best suited for targeting broadleaf weeds, especially brassica weeds, in crops that you don’t want to desiccate. For example, using amine or ester on late wheat. Sharpen has also been registered for this purpose in cereals and pulses.
Make sure you have a good idea of any resistance issues you might have with your paddock. Resistance will limit the effectiveness of the herbicide group you are using. Also, take note or get advice on crop timings for late applications and be aware of any harvest with holdings post application.

Crop topping is the use of non-selective herbicides to desiccate weeds and accelerate/even-up ripening of crops for harvest. The timing of this is when the target weed is flowering or in its early grain fill, and the crop is in its late mature stage.
Applications at the correct stage of development have been seen to have a reduction of more than 90% of seed set in annual ryegrass. But you need to balance this out with potential crop yield loss.
Hitting this balance can be hard. The physiological difference between crops and weeds is not always as wide as required, and yield losses can occur. Remember that if the grain is intended for use as seed or for sprouting, crop topping before physical maturity can cause the grain to be less viable. Nevertheless, it is a proven, effective strategy against resistant weeds.


What to use in crop topping situations:

Wheat: Diquat, Sharpen and Glyphosate (check individual glyphosate labels as not all are registered).

Barley (not malt): Diquat, Sharpen, Weedmaster DST and Crucial.

Canola: Diquat, Weedmaster DST.

Pulses: Diquat, Sharpen, Glyphosate, and Paraquat.
(Note: When applying Sharpen to Lentils, DO NOT retain seed for the following season as some reduction to vigour is possible.)

(Note: that paraquat + diquat products (i.e. Spray Seed) are NOT registered for pre-harvest use in ANY crop.)
(Also note there are different registrations for under-cutter-bars and applications prior to hay cuts. Contact our agronomy team for more information on these.)

As you know, Glyphosate can be applied to feed barley, although this is ONLY Nufarm’s Crucial and Weedmaster DST glyphosates. *No malt barley will be accepted if glyphosate has been applied late to the crop.*
This registration will apply for wheat as well.
Look to apply glyphosate once the grain has reached maximum size and is beginning to dry down. This is most commonly known as the firm dough stage, with the wheat/barley grain being hard to squeeze between two fingers and no moisture coming out when squeezed. If glyphosate is applied too early yield can be lost and there will most likely be an increase in screenings.

Sharpen is also available for late application in Wheat, Barley, and Triticale. This will allow for a reduction of seed set and viability of wild radish seeds. The big advantage of this is that it can be applied earlier than Glyphosate and other options. Sharpen WG can be applied from watery ripe stage (Z71) and can somewhat sterilize weed seeds even if they are able to form to normal size, and even if weeds themselves don’t completely die. When applied at the correct timing, no yield or quality reductions are seen. However, some minor scoring on the wheat stems and grain heads may be visible. But this should only be cosmetic.

As always, make sure you consult labels and permits for harvest withholdings with all herbicide usage. If keeping seed for next years sowing, be very cautious with late-season herbicides. Grain handlers and marketers regularly conduct surveillance tests to make sure grain is within Maximum Residue Limits (MRLs). So make sure you comply with registered labels.

Come in and see us for more information on what tactics will work best on your specific case.