“We started spraying before sunrise — I mean it was dark when we got to the field. We sprayed glufosinate every hour until sunset, again it was dark when we left the field.

“The results were as surprising as most any I’ve seen in all my years of doing weed studies,” York says.

The first glufosinate application the North Carolina State research team applied didn’t look much different than the check plot and the next hour wasn’t much better.

By two hours after sunrise, control was close to 100 percent. They came back and sprayed the same plots with the same materials 14 days later, but waited until two hours after sunrise to start.

The second application masked the loss of efficacy from the first application, but it was still there, York says

Control didn’t vary much until the researchers got to within two hours of sunset. The results weren’t so dramatic the hour before sunset, but there were differences in control. “When we got closer to sunset, control dropped off dramatically,” York says.

“We were only looking at control of Palmer amaranth, so it’s hard to know whether the time of day of application will have a similar impact on other herbicides.

“It sure piqued our interest in looking at application time impact on other herbicides and on other weeds and grasses,” he adds.

Though grass wasn’t officially part of the study, there was some grass pressure in the North Carolina State test plots.

York says it appears there was less dramatic, but some reduction in control on grasses in these tests when glufosinate was sprayed early and late in the day.

The researchers ruled out humidity related impacts and dew as causes for reduced control early and late with glufosinate.

York says the loss of control is related to light, but exactly what the mechanism for causing the light-related loss of efficacy is not certain.

Time of application isn’t the only concern growers will have with some of the new transgenic crops and subsequently their resistance to long-used herbicides like 2,4-D and dicamba.