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  • Writer's pictureMichael Macpherson, Farmacist Senior Agronomist

Being proactive for grass control

Another planting season is upon us here in the Burdekin region and the Farmacist team have been out on-farm with growers actively managing the early season flush of weeds that inevitably occur when we get autumn rainfall and mild temperatures.

Management of early-germinating grasses in plant cane in this region has traditionally centred around the use of knockdowns with various ‘spikes’ for control of grass weeds at the spike-3 leaf stage of the crop. When relying on these knockdown style herbicides to do the heavy lifting early, timing is everything for good results - miss the window and options become limited and costly.

The long history of repeated knockdowns for grass control has also led to the development of a phenomenon known in weed science as ‘species shift’. This is where the species that are susceptible to a particular practice (like the use of knockdowns) disappear from the population over time, but species that are less responsive begin to dominate the population. In the Burdekin, we are seeing this trend occurring in our grass weed populations; we rarely talk to growers about summer grass (knockdown susceptible) these days, but we are seeing an increase in species such as Lovegrass, Itch Grass and Wild Sorghum (knockdown tolerant).

So what can we do as growers and advisors to manage this situation? The answer lies in taking a more holistic approach to our weed management strategies. In other words we need to keep our weeds confused! One of the key principles in weed management science is to use as many different tools as we can, as often as we can so our weeds don’t get ‘used to’ one individual management tool used repeatedly.

While there are many tools in our toolbox, in this article we want to talk about using our tools within our herbicide toolbox in a more strategic manner. The hardest thing to do from a bio-chemical (herbicide) point of view, is take an emerged weed grass out of a grass crop (in our case, sugarcane). Many grasses, crop and weed, have the same biochemical pathways that constitute their metabolic processes (the way they grow and reproduce). Typically, a herbicide works at a specific point in a biochemical process within the target weed that leads to its decline. Therefore when a crop can tolerate a herbicide, it means that the crop: 1) Does not have that same biochemical process for the chemical to act on; or 2) It has some mechanism to prevent that chemical from reaching the site of activity (it breaks it down before it does damage).

This ‘selectivity’ is easy (in relative terms) to do for a broadleaf weed to a grass crop because broadleaf weeds and grass crops have different biochemical processes that a herbicide can target. However, tropical grass weeds and sugarcane both use very similar biochemical processes to grow, hence the difficulty in developing a ‘selective’ product that is consistent and reliable that doesn’t check the sugarcane. The good news is that we DO have options outside of the knockdown group of products, namely the pre-emergent herbicides.

Because sugarcane emerges from a billet and is planted at depth, we have options to control grasses before they even germinate. These products target pathways that are related to seed germination and emergence – pathways that sugarcane doesn’t rely on to establish. Thankfully, these products are particularly strong on the weed species that are less susceptible to our current knockdown strategies such as Lovegrass. Farmacist has been conducting extensive investigation through various trials on these options over the last three years so we can advise our clients on the most useful options for their particular situation.

This season has provided an excellent opportunity for some of our clients with early plant cane to use one of the strongest pre-emergent herbicide products in the toolbox, Pendimethalin (Stomp® Xtra, Rifle®), in advance of the late April rain event in plantcane that had not yet received its first watering. All the blocks that Pendimethalin was applied to before the rain have returned excellent results for grass weed control, with upwards of 90% effectiveness on weeds such as Lovegrass. These blocks should now hold through the winter period without further grass pressure, only requiring a quick and cheap vine clean up to see them out to hill-up. Opportunistic application of Pendimethalin has returned an excellent result for our growers and has taken the selection pressure off of subsequent knockdown passes in these paddocks saving time and cash. If you would like to know more about how we proactively manage weeds in your sugarcane, get in touch with your Farmacist agronomist for a timely chat. Don’t let those weeds get away!

This article appeared in the Winter 2023 issue of Farmacist News. Download the PDF from the link below to view the original article or to print.

Being proactive for grass control MM Feb 2023
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