SIGNIFICANT improvements for spray coverage in stubble retention systems can be achieved by following basic principles of measurement, stubble management and spray manipulation, according to an interim Hart Field Day Site project report.
Researcher Sarah Noack, who led the project team with other researchers, local consultants and growers, says growers need to keep in mind the three factors governing the behaviour of pre-emergents: solubility of the herbicide, its rate of breakdown in the soil and how tightly the herbicide is bound to stubble and soil components.
The three-year project, involving field trials with growers in the Mid North, looked at pre-emergent herbicide applications in a range of stubble heights and loads, manipulating spray set up (e.g. application volume, droplet size and position of the nozzle above the stubble row) and on-farm case studies.
Ms Noack said the use of water sensitive paper (WSP) had proven to be an extremely cost-effective method to assess spray coverage in a paddock.
“You can buy a packet of 50 cards for $60-$70 and it doesn’t take long to place a few pieces of card out in the field and spray over them,” she said. Using a free Snap Card app (Android, IOS), spray coverage could be easily reported in the field as a percentage of the card area stained by the herbicide.
Ms Noack said “rules of thumb” for spray coverage using WSP, were shared by Bill Gordon, spray specialist at one of the project’s spray workshops earlier in the year: fully translocated herbicides, more than 6-8%; contact herbicides, more than 10-12%; fungicides, more than 15%; pre-emergent herbicides, more than 15-20% (depending on product, soil moisture and rainfall). Using a packet of WSP growers could easily determine where they were within these ranges for a given herbicide application.
Ms Noack said one of the simplest changes to improve spray coverage in high stubble loads was increasing water rates.
Field research conducted in the project showed, on average for stubble heights below 30cm – baled, short and medium – spray coverage was increased from 13%, 20% and 33% for 50L/ha, 100L/ha and 150L/ha respectively. The second year of results showed a very similar trend with spray coverage increasing from 12%, 20% and 28% for the same volumes.
However, recent lab research from Western Australia had showed that a 5mm rainfall event was sufficient to move Sakura® from stubble to soil and that greater volumes – 10-20mm – were not needed to wash more off the stubble. Ms Noack said that if growers were spraying pre-emergents into heavy stubble loads, selecting a more soluble herbicide along with timing the application pre-rainfall to wash the herbicide from the stubble onto soil was the best scenario to still achieve effective weed control.
Ms Noack said that a lot of growers’ stubble management decisions were based around harvest conditions and ease of seeding operations, rather than their pre-emergent herbicide applications. Going into harvest this year, stubble loads would be far less compared with 2016.
“But one thing to keep in mind is that if you are harvesting at lower heights, you take a lot more material into the header, so there will be a more chaff being spread onto the soil surface to be intercepted by the herbicide applications,” she said.
“So you need to find a balance between stubble height and the amount of trash you’re returning to cover the soil.”