Vol. 54—2000 - NorthEastern Weed Science Society

Perennial and Annual Weed Control In Glyphosate Tolerant Crops
S. Glenn and P. A. Kalnay'
ABSTRACT
Multiple-yearstudieswere initiatedin 1998withtheobjectivesof evaluatingweed controland
weed shifts following POST applications of glyphosate in glyphosate-tolerant crops. Studies
were conducted at Keedysville, MD on Hagerstown silt loam soils with 2. I to 2.4% OM and pH
of6.5 to 6.9. In one study, POST applications of 0.75, 1.0, and 1.5 Ihs ai/A glyphosate were
made to soybeans in the V-2, V-4, and V-8 stage of growth. Control of giant foxtail (Setaria
faber; Herrm.) And common lamhsquarters (Chenopodium album L.) was better following V-4
and V-8 application (93 to 100"10)than with V-2 applications ofglyphosate (53 to 60%). This
was dueto lateemergenceof these annualweeds.Hempdogbane(ApocynumcannabinumL.)
control following POST applications ofO.75lhslA was greater with V-2 applications (98%)
compared to later applications (60 to 73%). Hemp dogbane control was similar at all application
stages with 1.0 and 1.5 IhslA glyphosate (83 to 97%). Hemp doghane had initiated reproductive
growth by the later application stages which apparently made hemp dogbane less susceptible to
0.751bsiA glyphosate. Pokeweed (Phytolacca americana L.) control was excellent (97 to 100%)
for all rates and stages of application of glyphosate. Soybean yield was lower with V-8
glyphosate applications than earlier applications suggesting that early weed competition reduced
yield. A separate study was designed to examine long-term weed control in glyphosate-tolerant
com and soybean rotations. Glyphosate applied at 1.0 IbslA to 5-inch com and V-4 soybeans
provided90%or greater controlof hemp dogbane,giantfoxtail,andcommonlambsquarters8
weeks after treatment. Similar control wasobtained in year 2 regardless of the crop rotation. In
both com and soybeans three weeds were observed that were not controlled by glyphosate
applications. These weeds were honeyvine ntilkweed [Ampelamus albidus (Nutt.) Britt.], smooth
groundcherry (Physalis subglabrata Mackenz. & Bush).
'Assoc. Prof. and Research Assoc., Univ. of Maryland, College Park, MD 20742.
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