Heat and Herbicides: Managing
Post-emergent Applications in Corn

Herbicide applications made when daytime air temperatures exceed 30?C will increase the potential for crop injury. Special care must be taken when applying certain post-emergent herbicides in corn, specifically Accent, Ultim and Option applied alone or with a tank-mix partner. Field experience has shown that applying these herbicides in the late afternoon or evening will reduce the potential for crop injury as the corn crop is generally under less stress, and is able to metabolize the herbicide more readily.

Real Life Example: Morning vs. Evening Herbicide Applications

To back up the above statements the following images illustrate the importance of evening applications to minimize heat-stress related crop injury. In Figure 1 and Figure 2, Accent + Pardner were applied in the morning of Monday June 6th while in Figure 3 and Figure 4 the same treatment was applied the same day but in the evening. In both cases there is noticeable crop injury, but the level of corn leaf burning occurring after the early morning application of Accent + Pardner is much greater than when the same treatment was applied in the evening. If we look at air temperatures before, during and after these two applications, it would appear that the early morning application put the crop in a situation where it was metabolizing the herbicide under stressful conditions, as air temperatures were above 28°C. Contrast this to the evening application, where nighttime temperatures were below 20°C.

Figure 1 - Accent + Pardner injury to corn 3 days after a morning application.
(Mike Cowbrough, OMAF, 2004)

Figure 2 - A close up of bromoxynil (Pardner) leaf burn after a morning application.
(Mike Cowbrough, OMAF, 2004)

Table 1. High and Low air temperatures (?C) before, during and after
an application of Accent + Pardner at Elora, ON.

Source: www.farmzone.com

Liberty: The exception to the rule

With almost every other post-emergent herbicide, spraying in the evening will minimize crop injury when air temperatures are above 28 °C. However, Liberty applied to Liberty Link corn, provides better weed control when sprayed in the heat of the day. Since Liberty Link hybrids are tolerant to the active ingredient in Liberty (glufosinate), crop safety is not an issue.

Figure 3 - Accent + Pardner injury to corn 3 days after an evening application.
(Mike Cowbrough, OMAF, 2004)

Figure 4 - A close up of bromoxynil (Pardner) leaf burn after an evening application.
(Mike Cowbrough, OMAF, 2004)

Figure 5 - Accent + Pardner injury to corn (left) verus an untreated strip (right).
(Mike Cowbrough, OMAF, 2004)

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