Comparative growth of six Amaranthus species in Missouri - Volume 51 Issue 3 - Brent A. Sellers, Reid J. Smeda, William G. Johnson, J. Andrew Kendig, Mark R. Ellersieck

Absorption and translocation of three commercial formulations of glyphosate, the isopropylamine salt formulated as Roundup Ultra™ (IPA1) and Roundup UltraMax™ (IPA2) and the diammonium salt formulated as Touchdown™ IQ (DA), were compared in three- to five-leaf velvetleaf, common waterhemp, and pitted morningglory. Absorption of 14C-glyphosate in velvetleaf was not significantly different among the three formulations up to 50 h after treatment (HAT). More absorption of 14C-glyphosate occurred in the IPA1 (26.0%) vs. the IPA2 (17.7%) formulation over 74 h. Of the total 14C-glyphosate absorbed, 20 to 35% was translocated from the treated leaf to the rest of the plant. Initial absorption of 14C-glyphosate was rapid in common waterhemp with the IPA1 (42.7%) and IPA2 (30.7%) formulations; both were higher compared with absorption of the DA formulation (11.5%) by 2 HAT. These differences continued up to 26 HAT, but no differences were evident by 74 HAT. Up to 65% of the 14C-glyphosate absorbed was translocated out of the treated leaf by 74 HAT, with roots the primary sink. Initial absorption of 14C-glyphosate was slow in pitted morningglory compared with the other species. More foliar absorption occurred in plants treated with the DA (13.6%) vs. the IPA2 formulation (4.9%) by 6 HAT. Absorption beyond 26 HAT was not different among the three glyphosate formulations. Translocation of 14C-glyphosate to roots was 27% greater as the DA salt than IPA1 and IPA2 by 74 HAT. The distribution pattern of glyphosate was similar in all species; phosphorimages demonstrated movement both acropetal and basipetal, with accumulation in roots greater than in any other plant parts. An efficacy study parallel to the 14C study showed no difference among the three glyphosate formulations on the species investigated at both 74 HAT and 2 wk after treatment.

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.

Field trials were conducted at two sites in both 1997 and 1998 to evaluate soybean response and weed control with glufosinate alone or combined with quizalofop, lactofen, imazethapyr, flumiclorac, or bentazon plus acifluorfen in narrow-row, glufosinate-resistant soybean. Soybean injury ranged from 0 to 21% at 2 wk after treatment (WAT) and from 0 to 5% by 4 WAT. Glufosinate alone at 0.29 and 0.4 kg ai/ha controlled velvetleaf, common waterhemp, common ragweed, morningglory species, and giant foxtail greater than 85% in all studies. Mixtures containing glufosinate and other herbicides controlled these species greater than 81% but did not improve control over glufosinate alone. Estimates of weed biomass closely reflected visual control evaluations. However, giant foxtail biomass was higher for mixtures of glufosinate plus lactofen, flumiclorac, or bentazon and acifluorfen, indicating possible antagonism of glufosinate activity. At both locations, soybean yields were similar among most treatments, but that of the glufosinate plus lactofen treatment was lower when compared with other treatments. Additional trials evaluated soybean response and weed control with a preemergence herbicide followed by glufosinate postemergence (POST), glufosinate applied once or twice POST, and mixtures of glufosinate plus imazethapyr or flumiclorac POST in wide-row soybean. Glufosinate applied twice controlled common waterhemp, morningglory species, prickly sida, common cocklebur, and giant foxtail up to 39% greater than did glufosinate applied once. The addition of imazethapyr, but not flumiclorac, to glufosinate improved weed control when compared with glufosinate alone. Nomenclature: Acifluorfen; bentazon; flumiclorac; glufosinate; imazethapyr; lactofen; quizalofop; common cocklebur, Xanthium strumarium L. # XANST; common ragweed, Ambrosia artemisiifolia L. # AMBEL; common waterhemp, Amaranthus rudis Sauer # AMATA; giant foxtail, Setaria faberi Herrm. # SETFA; morningglory species, Ipomoea spp. # IPOSS; prickly sida, Sida spinosa L. # SIDSP; velvetleaf, Abutilon theophrasti Medik. # ABUTH; soybean, Glycine max (L.) Merr. ‘Asgrow 2704 LL’. Additional index words: Herbicide mixtures, herbicide-resistant crop, soybean injury. Abbreviations: COC, crop oil concentrate; EPOST, early postemergence; LPOST, late postemergence; MPOST, midpostemergence; POST, postemergence; PRE, preemergence; WAT, weeks after treatment.

Biology and control of burcucumber - Volume 49 Issue 1 - Reid J. Smeda, Stephen C. Weller