Herbicide Time of Day Application Effects on Crop and Weed Injury.

Effects related to time of day (TOD) herbicide applications can lead to decreased herbicide activation and reduced efficacy, which are major contributors to herbicide resistance issues.  TOD effects have been well documented for protoporphyrinogen oxidase (PPO) and glutamine synthetase inhibitor (GSI) herbicides, but not for other photosynthesis dependent herbicides such as 4-hydroxyphenylpyruvate dioxygenase (HPPD) and photosystem II modes of action (MOA).  This is suspected to be a result of not only the light activation requirement for these herbicides, but also the relative biological activity and metabolism of the target plant.  All herbicides examined are key drivers to successful herbicide control programs related to herbicide resistance.  Field trials were conducted to examine the TOD effects for several photosynthesis related herbicide MOA applied at different times over a 24 hour period. The trial was designed as a 4x4 factorial arrangement of treatments, with factor A (4 levels: 06:00, 12:00, 18:00 and 00:00 CST) and Factor B (4 levels: HPPD (topramazone), photosystem II (bromoxynil), PPO (saflufenacil) and GSI (glufosinate) MOA) in a randomized complete block design, with 3 replications. All applications were completed in a 24 hour period where optimal photosynthesis conditions existed. Treatments were applied to a multispecies screen of crop (wheat, barley, canola, flax, oats and sunflowers) and weeds (common lambsquarters, red root pigweed and buckwheat).  Data were collected 7 and 15 days after treatment for weed efficacy and crop injury.  Results showed varying levels of TOD induced herbicide effects on all species tested. As light levels increased during applications, herbicide efficacy increased proportionally.