Does time of day have an influence on how well Liberty herbicide controls Palmer amaranth?

That is a question we had as a result of some field observations last year. Typically we see good control of Palmer pigweed that is less than 6 inches tall with Liberty. However, from time to time we have noted inconsistent control of Palmer pigweed greater than 6 inches tall where time of application seem to be the only difference. 

Therefore a study was constructed in Jackson, Tenn., as well as Tifton, Ga., to see if time of day may affect Liberty efficacy on Palmer pigweed control and crop injury to cotton. 

The study was conducted based off of sunrise (5:56 a.m.) and sunset (7:48 p.m.) timings, where applications of Liberty were applied on 5 inch Palmer pigweed in PHY 375 WRF cotton 1 hour before sunrise, ½ hour before sunrise, at sunrise, ½ hour after sunrise, 1 hour after sunrise, 2 hours after sunrise, 4 hours after sunrise, and 6 hours after sunrise. 

Shortly after the sunrise applications, treatment timings based off of sunset commenced. These started at 6 hours before sunset and followed the same trend mentioned earlier, ending at 1 hour after sunset. 

The resulting Palmer control and cotton injury 14 days after application suggest that adequate control of Palmer pigweed was not achieved until ½ after sunrise and continued to be good throughout the day running onto after sunset. 

Prior to sunrise, Palmer control ranged from 50 percent to 73 percent with cotton injury ranging from 11 percent to 14 percent. 

After sunrise, percent weed control and percent crop injury increased significantly. Control of Palmer pigweed was greater than 90 percent and crop injury ranged from 10 percent to 29 percent. The highest amount of crop injury (29 percent) was seen at the 6 hour before sunset application.

Keep in mind this is Widestrike cotton. We would not expect injury with LibertyLink cotton.

At this point we are not sure what the exact reason/s are, for the difference in Palmer amaranth control, but the results speak for themselves.

Perhaps the difference is due to Liberty being applied on wet (i.e. dewy) leaf surfaces and not getting into the Palmer. Or maybe, there is some impact on efficacy if Palmer has been in the dark for a length of time.

It is interesting to note the results of this study from south Georgia and west Tennessee were very similar. This study would indicate that on large Palmer pigweed the best time to spray starts 30 minutes after sunrise through sunset. 

Of course an early morning application when winds are calm is the best time to spray to avoid drift. However, from a Palmer control standpoint it might be better to wait and have one more cup of coffee before you go to spray.