Field and growth chamber experiments determined the efficacy of temporal glyphosate applications on velvetleaf. Glyphosate was applied postemergence to velvetleaf periodically before and during light and after dark. In 1999, glyphosate at 840 g ae/ha applied before sunrise and after midday provided 54 and 100% velvetleaf control, respectively. In 2000, glyphosate at 840 g/ha applied before sunrise, midday, and after sunset provided 69, 100, and 37% velvetleaf control, respectively. In the growth chamber, glyphosate at 840 g/ha applied before or after light reduced velvetleaf biomass 15 to 20% or 32 to 47%, respectively, and reduced velvetleaf height 24% or 45 to 54%, respectively. Velvetleaf control was consistently greater with glyphosate applications during light compared with dark, regardless of constant air temperature and relative humidity (growth chamber), dew absence or presence (field and growth chamber), or leaf blade orientation (growth chamber) with natural light–dark movements or a fixed horizontal position.
Nomenclature: Glyphosate; velvetleaf, Abutilon theophrasti Medic. #3 ABUTH.
Additional index words: Application timing, differential response.
Abbreviations: DAT, days after treatment; POST, postemergence; PPFD, photosynthetic photon flux density; RH, relative humidity.