Velvetleaf plants have diurnal leaf movements, which may result in decreased interception of herbicides when applications are made near sunset. However, it is not known if leaf angle alone accounts for diurnal fluctuations in efficacy. Greenhouse experiments were conducted to determine the effect of time of day (TOD) of application and velvetleaf leaf angle on glufosinate efficacy and spray interception. Glufosinate at 90, 180, and 360 g ai/ha was applied to 10-cm-tall plants at 4:00, 6:00, 7:00, 7:30, and 8:00 p.m., respectively. Leaf angles were either manipulated physically to −90° or the plant's natural 2:00 p.m. leaf angle (approximately −10°) or were allowed to exhibit their natural leaf movements. Plant dry weight 3 wk after treatment revealed that TOD effects were observed for all leaf angle treatments after glufosinate application at 90 g/ha. At 180 g/ha glufosinate, there was no TOD effect for plants with 2 p.m. leaf angles, whereas there was a TOD effect for plants with −90° and natural leaf angles. At 360 g/ha glufosinate, biomass for the −90° leaf angle plants was similar to that for the natural and the 2:00 p.m. leaf angle plants when glufosinate was applied at 4:00 p.m. but was significantly different at or after 6:00 p.m.. This suggests that at least 4 h of light is needed to provide optimum herbicide activity when spray interception is reduced as a result of leaf movements. Leaf angle decreased by as much as 70% from 4:00 to 8:00 p.m., which resulted in approximately 50% less spray interception at 8:00 p.m. than at 4:00 p.m. These data provide evidence that leaf angle plays a pivotal role in reducing glufosinate efficacy when applications are made near sundown. However, leaf angle is not the sole reason for reduced efficacy because TOD effects were observed at different leaf angles with 4 h of light, after an application of 360 g/ha glufosinate.
Nomenclature: Glufosinate; velvetleaf, Abutilon theophrasti Medicus #3 ABUTH.
Additional index words: Application timing, glufosinate, herbicide interception, time of day.
Abbreviations: GS, glutamine synthetase; POST, postemergence; TOD, time of day; WAT, weeks after treatment.
