Environmental Conditions Have the Greatest Impact on Crop Injury from Herbicides
Why Reading Herbicide Lable Matters
By Allan Kaastra - MSc. Candidate, University of Guelph and Mike Cowbrough and Ian McDonald - OMAFRA, Guelph

Applying herbicides under extreme environmental conditions, especially periods of fluctuating day and night temperatures can cause significant yield losses. Reading the herbicide label will help ensure that such yield losses are avoided.

Ultim® herbicide was applied to four replicated research trials during the first three weeks of June in 2006. Although this product provided excellent grassy weed control, two of the trials suffered extensive corn injury as shown in Figures 1 and 2.
Figure 1 – Ultim® injury 14 days after application
Figure 2 – Ultim® injury 28 days after application

The recommendations found on the Ultim® label indicate that “rapid fluctuations in temperatures greater than 20 °C within 24-36 hours of application will stress the corn crop.” It also suggests to “only apply Ultim® when the temperatures in the 24 hours before and after application range between 5 and 28 °C, temperatures beyond this range increase the potential for crop injury.”

TABLE 1. Four Ultim® trial applications including temperatures and times corresponding to visual injury and yield loss
Trial #
Leaf Stage at Appln.
Extremes (C)
Temperature Swing (C)
Appln. Timing
Visual Injury %
% Yield Loss
5-6 leaf
8 pm
7-8 leaf
9 am
7-8 leaf
9 pm
no yield loss
5-6 leaf
10 am
no yield loss

*Temperature within 36 hour window before and after application
**The same hybrid was used in all four trials.

As seen in Table 1, the two trials that suffered Ultim® injury and yield loss were applied when temperatures were above 28 °C, and exceeded the 20 °C swing range stated on the label. Additionally, the minimum temperatures experienced in those trials were approaching the temperature at which injury from low temperature effects can be seen. These were clearly off-label applications. When the applications
were made while temperatures were below 28 °C and within a 20 °C swing range, regardless of the leaf stage (Ultim® can only be applied up to the 6 leaf stage of corn) and the time of day at which the applications were made, no injury or yield loss was observed. Evening applications, which are often recommended during times of hot daytime temperatures, did not eliminate the potential for injury as the trial showing the most injury was sprayed in the evening.

In order to avoid this potential for injury from extreme air temperature fluctuations, the Ultim® label suggests “waiting 48-72 hours before application to allow the corn plants to acclimatize.” If you cannot wait the recommended 48-72 hours because the crop stage may exceed the label restrictions or poor weather conditions are forecast, then other herbicide options should be considered. Accent® and OptionTM were also
applied in all four trials and under the same environmental conditions. Both of these herbicides provided grassy weed control equivalent to Ultim®, with no visual injury or yield loss observed.

Key findings from this research:
• Off label applications of any herbicide will increase the risk of crop injury and yield loss.
• Applying Ultim® during periods of air temperature fluctuations greater than 20 °C resulted in crop injury and yield loss.
x The product label warns against such applications.
• Applying Ultim® past the maximum labeled corn leaf stage did not appear to have as great an influence on crop injury as air temperature fluctuations did.
• Always check the herbicide label before use.

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