Testing optical spot-sprayers in a low weed environment

The latest weed-detection technology allows growers to find and spray weeds growing in fallow and within their crops, and early adopters are testing the practical and economic realities in the field.

Broden Holland says they have seen immediate benefits, mainly through optical spot-spraying in fallow situations on their 5000 ha mixed farming operation at Young, NSW.

“In 2021, we purchased a Goldacres G6 self-propelled sprayer with Weedetect cameras and Bilberry weed identification software to add spot-spraying to our weed control options,” he says. “We hadn’t done optical spot-spraying before, so it seemed sensible to start with equipment that gave us both green-on-brown and green-on-green capability.”

Detecting and spot-spraying weeds in their summer fallow has paid dividends in the last three (very wet) seasons. This technology has several good use cases, and having a second product tank would bring further value.

The cameras can detect weeds as small as a 10-cent coin, even when travelling at 23 km/hr. This capability reduces the tendency to compromise application timing.

“We are spraying more often and always targeting small weeds, which is a key to effective control of weeds that may harbour low-level resistance to our main herbicides,” says Broden. “Since purchasing the Weedetect cameras, we have spot-sprayed over 12,000 ha of fallow, and chemical use in the fallow is down by 60 per cent or more.”

Broden expects even more benefits in drier seasons and believes green-on-brown weed detection technology is a good fit for their operation going forward.

The cameras work well in high stubble loads (e.g. 5 t/ha wheat crop stubble), and Broden can adjust the settings to avoid re-spraying weeds that are already dying from an earlier application. The cameras also do an excellent job when implementing a double-knock treatment.

The Hollands have also spot-sprayed over 2000 ha using green-on-green weed detection to spray broadleaf weeds in cereals and grasses in canola.

“We are particularly pleased with the early signs of reduced crop damage when we spot-spray annual ryegrass in canola,” says Broden. “Depending on the weed density in the paddock, we are also saving between 20 and 80 per cent of our herbicide costs. The camera system also provides valuable weed mapping data that we can build on each year, and it’s easy to see the yield response in areas that previously had a higher weed burden.”

However, the accuracy of in-crop weed detection is not yet adequate enough for the Hollands to rely on across a large area of the farm each year. Until the detection accuracy improves, they will use this capability only where it is the best solution in specific situations.

For example, having green-on-green capability helped them avoid a potential blowout of resistant ryegrass and save yield in 2022 when they purchased a 180 ha block and soon discovered clethodim-resistant ryegrass in high numbers in their first canola crop.

“When the initial post-emergence blanket spray had minimal impact, we returned with a spot-spraying tank mix of 300 mL clethodim (Select) and 80 g butroxydim (Factor) before budding,” says Broden. “We were concerned about potential crop damage, so we dialled back the camera sensitivity to around 80–90 per cent. While this did not control all the resistant ryegrass, it had a significant suppressive effect, allowing the crop to gain the advantage.”

At the end of the season, they applied glyphosate over the top of the canola five days before windrowing the crop ahead of harvest. This killed the survivor weeds and minimised the quantity of seed returning to the soil.

At the end of the season, the crop yielded 2.5 t/ha, and they had averted a weed blow-out. The following year, they were able to plan a herbicide program to suit the resistance profile of the annual ryegrass population.

“Many of the chemicals that we use to spot-spray in-crop are relatively inexpensive, so the input cost savings are often not huge,” says Broden. “However, we know there are long-term benefits in reducing herbicide resistance risk and reducing the stress on the crop from broadacre herbicide application.”

The Hollands use many of the WeedSmart Big 6 tactics, including narrow row spacing (19 cm disc system) to achieve early crop competition in their crops. Having low weed numbers and strong competition can make it harder to justify green-on-green spot-spraying as the canopy closes over the inter-row quite early. Broden believes that green-on-green technology would quickly return savings to the grower with cheaper inter-row weed control in regions where wider rows are preferred.

The WeedSmart Big 6 strategy embraces herbicide and non-herbicide weed control tactics to keep weed numbers low in cropping systems.

In the Holland’s farming system, the additional costs associated with green-on-green spot-spraying are difficult to fully justify. However, Broden expects that future algorithms will allow them to target a broader range of weeds in their crops. They are particularly keen to be able to target black oats in wheat. A dual tank system would also help justify future use of the green-on-green technology by providing flexibility with each pass over a paddock.