Understanding Delta T and timing

“As part of our state trials program we decided to run a test to compare the effectiveness of weed control applications made at varying Delta T conditions,” said John Both, Nufarm research and development officer in South Australia.

“As one of the technical components that needs to be carefully considered when making decisions about the timing of crop protection applications in conjunction with local weather conditions, it generated a lot of discussion between those growers and consultants who visited the trial site.”

The difference between dry bulb (normal thermometer) and wet bulb (thermometer with moist cloth), Delta T is the relationship between temperature and humidity. It is used by applicators, growers and agronomists to help to decide the most suitable spraying conditions when applying pesticides, particularly during the warmer summer months.

“The conventional wisdom is that a Delta T range of 2 to 10 is the ideal for spraying. A Delta T value above 10 is associated with higher temperatures and lower humidity, with potential impact on both droplet survival and the target weeds, which would be under greater environmental stress,” John explained.

“Measuring Delta T often tells us that overnight or early morning spraying is the preferred timing for applying herbicides to summer weeds. One problem with this is that these can also be times of low wind or calm conditions which are when inversions are likely to develop – an absolute no-go zone when spraying.”

The replicated trial was conducted in the upper south east of South Australia over the 2009/10 summer. Young, actively growing weeds on good soil moisture levels were sprayed at a range of Delta T readings at approximately 1.5 to 2 hour intervals. Application was made with Air Mix 110-01 nozzles (MEDIUM spray quality) and LI700®, a registered drift reducing agent and penetrant, was added to 750mL/ha Weedmaster DST®.

The results indicated that even up to a Delta T of 13.8, good efficacy was achieved. In line with other trial work, the Weedmaster DST achieved rapid brownout with near total control of potato weed (heliotrope) and caltrop in seven days.

“It was felt that avoiding a FINE spray quality would minimise droplet evaporation at the higher Delta Ts (lower relative humidity), therefore appropriate nozzles were selected and LI700 adjuvant was used to help reduce the likelihood of drift – all tactics that applicators should use as dictated by the conditions they are working with,” John said.

“The results also emphasised the importance of spraying summer weeds early – within the first three weeks of germinating rains while they are young, fresh and prior to establishing. If older and/or stressed weeds were present, the results of this trial may well have been different.

“This work highlights the importance of treating young, actively growing weeds using the correct spray quality to achieve the best possible control outcomes.”

% of Brownout with 750 mL/ha Weedmaster DST at 7DAA

(Applied at 5 intervals between 8am and 1.45pm)

New Picture (2)

New Picture (3)

Potato weed and caltrop on day of spraying

Footnote: In choosing a day to conduct this trial (that is, a day when a wide range of Delta T readings would be achieved over a reasonable time period), the Spraywise Decisions website was used. The predicted Delta Ts, temperatures and humidity readings on the day the trial was applied were accurately predicted by Spraywise Decisions. For more information visit