Warm weather disrupts spraying programs

Western Australian spray application specialist Bill Campbell, of Campbell Ag, told a recent South West WA Drought Hub dry season resources podcast that logistics, weather patterns, machinery set up and herbicide mixes were key considerations when spraying this autumn.

He said overall spraying aims should be targeted at conserving soil moisture in the lead-up to seeding.

"Conditions in summer and autumn make life difficult because there are limited hours in the day when it is ideal to spray," Mr Campbell said.

"So, growers are restricted from a logistics point of view."

Mr Campbell said weeds would continue to grow, regardless of soil moisture - or lack thereof.

"When you spray a weed it needs to be growing, and happily growing," he said.

"When it is really hot - as it has been - and plants are stressed, you get poor efficacy of herbicides because the plants won't take up chemicals."

Mr Campbell said there were guidelines for spraying, including minimal wind and a Delta T index (an index of temperature and humidity) of 10-12 or below.

"When it is a Delta T of 10-12 or less, there are ideal droplet survival conditions and when we spray in ideal conditions, the plant can take the herbicide up," he said.

Mr Campbell said herbicide mixes needed to be adjusted, while adhering to label rates, according to the size of the weed and the species present.

"If hard-to-kill weeds, such as button grass or windmill grass, are emerging, they will be difficult to control even when small and will require the high label rate of glyphosate in the tank mix," he said.

"Glyphosate mixes need sunlight for the plant to take them up.

"But if you have a 2,4-D or Ester mix, you can spray at night.

"So, if glyphosate is in the mix, you have only about three to four hours per day that you can spray.

"It is really this time of the year that is so crucial to get spraying conditions right.

"If we get to late autumn and conditions are good, we don't have to consider stresses as much or how plants take up herbicides."

Mr Campbell said a lot of summer spraying programs were finished and much of the spraying left was fallow paddocks, valleys floors where there was moisture or areas that have had summer thunderstorms.

"In mid-February and March we are really tidying up loose ends where weeds have come back," he said.

"It is a continuing process over the autumn period.

"We are closely monitoring weather systems now, and when we can see weather patterns that are conducive to spraying, we make the most of it.

"A lot of growers have optical sprayers, so they can spray with high doses and high water rates to improve efficacy.

"Spray drift is a big risk when spraying at night if there are inversions."

Mr Campbell recommends not spraying if wind speeds are high.

He said recently, inversion conditions had not been too strong, but it was vital to monitor changing conditions.

This year, interval and plant-back period considerations were also crucial, according to Mr Campbell.

"A big question is whether we need residual herbicides, such as Sakura and 2,4-D, in the system," he said.

"We have come off a very dry year, where many farmers have had only 85-160 millimetres of rain.

"When we have dry soils and haven't achieved 200mm, there could be issues with plant-backs.

"It is advisable to do an audit of what was used last year and do a risk profile of each paddock and work out what the effects will be.

"Study your weather maps, try to pick the best conditions, look at the weed species and time of day/night to spray.

"If conditions, set-up or chemical mix are not right, then don't spray."

Dowerin-Goomalling farmer Jemma Sadler crops wheat, barley, canola and lupins and runs sheep.

She runs a self-propelled boomsprayer which she said was better for dust management, along with chaff decks.

"We know the Delta T message," Ms Sadler said.

"You need to keep an eye on it and when the weather is right you get to it."

Ms Sadler said she used a lower water rate when spraying this year, which was more efficient because you could get more hectares from the tank - without compromising efficacy.

She said this summer and autumn there were not a lot of weeds on the family property, but there were random weeds everywhere.

"Our main challenge this year with summer spraying has been the wind because it is always windy and we have to be very mindful of (chemical) drift," she said.

Ms Sadler said she was also concerned about plant-back periods this season.

"Things are not dissolving in the paddocks and we just have to hope for big thunderstorms before the season break," she said.