Take home message

• Traditional blanket spraying methods for pre-emergent herbicides fail to consider in-field spatial variability, resulting in inefficiencies
• Customising pre-emergent herbicide placement and rates based on field zones can enhance performance, crop safety, and return on investment for growers
• Multi-point, direct injection technology offers a potential solution for variable, multi-product, and multi-rate application of pre-emergent herbicides across a paddock
• While further research and data analysis is ongoing, the trial work conducted so far has highlighted the potential benefits and value of applying variable rate pre-emergent herbicides.

Approach

Currently pre-emergent herbicide delivery is based on applying a single dose of one or more products in a tank mix. Typically, these are applied as a blanket spray, meaning the whole paddock is treated the same irrespective of soil characteristics or weed seed bank.

An inherent limitation with such an approach is that it locks in the rate and ratio of the individual or tank mixed products and assumes that all parts of the paddock require the same rate. This is not the case as many fields can be extremely heterogenous.

Although some herbicide labels may offer a range of rates that could be variably applied, the potential fit of variable rate pre-emergent herbicides in various systems needs to consider crop safety, plant back requirements and efficacy and the level of precision of application, spray rates and application methods.

This project develops a multifaceted streamlined workflow for precision agriculture technology, focusing on variable rate application of pre-emergent herbicides. It includes:

• Software for precision mapping
• Environmental and product interaction analysis
• Product and hardware integrations.

The success of a defined workflow also brings multiple benefits:

1. Enhanced adaptability: Growers and agronomists can fine-tune weed management programs using multiple technologies and products.
2. Targeted product utilisation: Precise application of herbicides optimises resource use and reduces environmental impact
3. Empowerment through information: Stakeholders provide insights on paddock variability, enriching the knowledge base
4. Tailored recommendations: Combining diverse data sources and expertise leads to accurate, customized herbicide recommendations.

This approach not only targets pre-emergent herbicide efficiency but also promotes more sustainable and informed agricultural practices.

Paddock characterisation (soil and weed seed bank)

The approach relies on observation and orientation of paddock variability through on-farm-knowledge and feedback to guide the data capture and digital layers for creating a paddock management zone. The zones are simplified to 2 to 3 zones based on diagnosed variability for soil characteristics and weed presence.

Grower/agronomist-led feedback

It is important to start with an open two-way conversation on trial opportunities, and soil and weed challenges. Some key steps for data sharing and knowledge transfer include:

1. Determine problematic weeds and seedbank locations
2. Understand herbicide use (programs, rates, tank mix)
3. Capture rainfall and temperature patterns during fallow periods
4. Review previous and planned future rotational crops
5. Discuss strategies and tasks for weed control
6. Identify products and rates for testing.
7. Identify paddock hotspots with varying soil type with emphasis on factors that may affect herbicide efficacy or persistence (e.g. organic matter, physical structure, pH, drainage)

With grower adoption of farm management software being slow and fragmented, often there are incomplete views of farm productivity which can complicate decision-making. Equipment manufacturers like John Deere and Case IH may have their own platforms, however these often require extra steps for setup and updates. Additionally, digitisation of farms is often deprioritised in favour of on-farm operational tasks. To boost engagement, the following should be considered.

• Close and continuous presence and support on the digital platform journey
• Finding simple solutions with clear benefits to grower and agronomist

Figure 1. Challenges with digital platforms partnering with growers.

Data exploration

After establishing access to farm management systems currently in use, an agreement is then made on what layers can be collected in addition to existing data. The usual recommendation for paddocks where soil is the key driver of weed distribution is Dual EM soil scans followed by strategic soil sampling based on management zones. Additional exploration of satellite data and analysis, fitness to model, and UAV (unmanned aerial vehicles = drones) multispectral paddock assessment of bare paddock when possible. Current examples of data capture and processing steps:

1. Historic data:
   1. Satellite remote sensing: Capture bare soil layers through different soil colours and vegetation index, identify weed scapes.
   2. Yield maps + as applied weed maps: capture zones of low yield and compare to as applied maps of spot-spraying records.
2. Dual EM or Veris Soil scanners: Run an EM soil scan across the paddock to identify varying soil texture, salinity (related to soil pH), soil moisture patterns.
3. Strategic soil sampling: Create a geotagged sampling plan with 1-2 samples per polygon/soil zone to compare soil survey and analysis.
4. UAV flights: Generate vegetation index/weed density maps by dividing paddocks into 12x12 m plots at 2.6 cm resolution. Post-process RGB and multispectral images into orthomosaic layers and plant health indexes, removing soil reflectance from bare soil assessment.

Prescription mapping with variable rate and product

Paddock diagnosis and zone creation

By stacking layers of topography, soil characteristics, crop, and weed density data into management zones, patterns within paddock’s can be identified. These layers then undergo geospatial analysis to understand their correlations, and then ground-truthing, before being consolidated into management zones.

Once management zones are defined, a prescription is generated using farm management software. This prescription includes a selection of pre-emergent herbicides to control target weed(s). The process is guided by testing a matrix of herbicides suggested by growers and agronomists, with local proposed chemistries providing case studies for regional feasibility on varying rates and application methods.

Testing soil characterisation through mapping zones on herbicide performance

A series of trials were planned from 2023-2025 to gain insight into soil variability by mapping paddocks with available technology and comparison treatments comprising of grower-led indication of a standard herbicide matrix, yet at variable rate or product, based on zones.

In QLD and northern NSW, problematic grass weed patches are often associated with lighter soil zones. This is common knowledge but has been validated/measured through UAV and satellite layers where we are testing products such as Dual Gold^®, Valor Eze^®, and Terbyne^® associated chemistries for control of a range of summer grass and broad-leafed weeds at variable rates associated with label in fallow paddocks followed by crops such as sorghum or chickpeas (as allowed by product labels). Current trials are located in Grassdale, Capella have been deployed in the fallow after sorghum. Herbicide performance and crop safety are evaluated using a combination of digital capture by UAV and ground-truth weed assessments at 6 and 10 weeks after application, and post-planting crop response assessments at 3,6 and 10 weeks after sowing.

In southern Australia, small plot trials targeting annual ryegrass (ARG) in wheat were conducted near Crystal Brook, and Gulnare South Australia, respectively in 2023 and 2024. These trials addressed soil pH as the primary soil variability factor and driver of weed pressure . To select trial locations, pH surveys of paddocks were created using a Veris^® soil sensor that scans paddock for pH, organic matter/carbon and EC (electroconductivity).

Figure 2. Preliminary pH Zone Mapping of trials in SA with Veris^®.
Survey by Trengrove Consulting.

Seasonal conditions across 2023 and 2024 varied.  2024 was a very dry season with 190mm rainfall average in the growing season versus over 230 mm at Crystal Brook. In 2023, dry conditions severely impacted yield. However, the trials produced comparable results with regards to weed control in both years. In 2024, minimal differences in crop response were observed between herbicides per zone. However, Overwatch^® treatments scored an average crop response of 7.95 on a general phytotoxicity scale 1-9 (1 plant dead, 9 healthy plant).

Despite several products demonstrating similar ARG control across different pH zones, significant differences in herbicide performance across products and pH were noted (mainly low and high pH zones).  There was no significant effect on the interaction between product and pH zones for 2024, while some products performed similarly across pH zones (e.g. Boxer Gold^® and Sakura^®) whereas others did not show consistent control over different pH zones.

Table 1. P-Values Prob > F - General Factorial Anova (LSD) % Weed control.

Figure 3. Main effects plot on percent Annual Ryegrass (ARG) Control for 2023 (42 days after application,  DAA) and 2024 (55 days after application, DAA). This plot shows the mean percentage weed control of each treatment, with the impact of pH Zone displayed on the left, and the impact of different herbicide products on the right.

Whilst the soil pH screening with Veris Equipment was the preliminary driver of mapping management zones, in both years, the allocation of trial zones (low, mid and high) smoothed the pH variability within the paddock. Differences between soil analysis results and Veris could be due to timing of Veris Screening and growers practice on liming. Sampling could have introduced another source of variation in the results, as many samples collected at 0-10 cm depth were composited. Sampling occurred prior to trial and post liming.  However, low pH zones, weed pressure was consistently higher, and control was lowest, for these observations reflect the zone variability found in the Veris screening.

Additionally, 2024 soil results indicated differences in low and mid pH in the Cation Exchange Capacity, PBI (phosphorus buffering capacity) and DGT-P, which measures plant available phosphorus. In the mid pH soil in 2024 with higher PBI values, may bind phosphorus tightly, and with a lower DGT-P (69) than the low pH soil (97) may be reducing phosphorus availability and potentially limiting ARG growth. Conversely, lower PBI soils retain more available phosphorus, supporting ryegrass proliferation. This correlation with different pH sites becomes less evident when examining some of the trial results across the two seasons, specifically when assessing ARG pressure and control per treatment in low and high pH zones across years. This is particularly true for Avadex Xtra^® and in combination with Overwatch^® treatments over both years as seen in Table 2.

Table 2. Pre-sowing results - Soil Physico-chemical analysis by APAL.

Table 3. Mean percent weed control on multiple products in Small Plot Trials in SA, 2023 and 2024.

Table 4. 2024 ANOVA - ARG counts per meter square 55 DAA means comparison with LSD test.

In Western Australia trials were conducted for canola-wheat rotation in both 2023 and 2024 to assess the impact of soil type on product efficacy and crop safety. The trials targeted volunteer canola and wild radish in wheat, across three different soil types: white sand, yellow sand, and gravel. While the results from the 2024 trials are still being analysed, the 2023 results showed significant differences in herbicide efficacy by soil zone. White sand enabled the best weed control for most herbicides, but the inverse was true for crop health (see Figure 4 and 5 below).

Figure 4. Herbicide efficacy (% control) on wild radish 85 days after application (DAA) in a small plot pre-emergent trial at Walkaway, WA 2023. Products: Callisto^® @ 100/150/200 mL/ha; Diuron 900 @ 275/550 g/ha; Voraxor^® @ 150/200 mL/ha; Terrain^® Flow @ 125 mL/ha; Brodal^® Options @ 200 mL/ha.

Figure 5. Wheat crop emergence 31 days after planting (DAP) in a small plot pre-emergent trial at Walkaway, WA 2023.