Vol. 54—2000 - NorthEastern Weed Science Society

GLYPHOSATE ALONE AND PRECEDED BY PREEMERGENCE HERBICIDES IN
GL YPHOSATE-TOLERANT CORN
B. F. Johnson, H. P. Wilson, T. E. Hines, and D. H. Poston
ABSTRACT
Annual grass and broadleaf weed control in com has traditioually been achieved
withpreemergence(PRE)combinationsof triazine andchIoroacetamideherbicides.
Follow-up postemergence (pOSn herbicide applications are occasioually required to
achieveseason-longcontrol,however the use of herbicide-tolerant com (Zea mays L.)
mayreduceor eliminatethe need forPREherbicides.
Field experiments were conducted in 1998 and 1999 at the Eastern Shore
AgriculturalResearchandExtensionCenternearPainter,VA to evaluateweedand crop
response to POST glyphosate applied at various timings and to evaluate the potential
benefit of PRE herbicides in Roundup Ready~ com. In 1998, POST glyphosate (1.0 Ib
ailA) was applied at 2, 3, 4, 5 and 3 plus 5 wk after planting (WAP). Glyphosate was
applied alone and preceded by s-metolacblor (0.78Ib ailA) plus atrazine (1.0 Ib ailA). In
1999, POST glyphosate applications were made approximately 2, 3, 7, or 3 plus 7 WAP.
As a comparison,s-metoiachIorplusatrazinewas includedin the study.
In 1998, com injury from glyphosate was minimal(S 10%) with and without s­
metolacblor plus atrazine PRE. Late-season (9 WAP) weed control and com yields were
generally greatest when POST glyphosate applications were made 4, 5, or 3 plus 5 WAP
comparedto 2 Or3 WAP. Com yield andweed controlwere generallysimilarwith PRE
s-metolacblor plus atrazine and POST glyphosate at 4, 5, or 3 plus 5 WAP. POST
glyphosate applications following PRE s-metolacblor plus atrazine did not improve
broadleafweedcontrol9WAP or com yield comparedto PREs-metolachlorplus atrazine
alone. However, POST glyphosate applications made3, 4, 5, or 3 plus 5 WAP following
PREs-metolachlorplus atrazineimprovedannualgrasscontrolcomparedto s­
metolachlorplus atrazinealone.
Efficacyof PRE s-metolachlorplus atrazinein 1999was reducedby very dry
growing conditions. Annual grass and broadleaf weed control 9 WAP with POST
glyphosateapplications(62 to 97%)was significantlygreaterthanwith PREmetolachlor
plus atrazine(37 to 47%).Theseobviousdifferencesin weed controlshouldhave been
reflectedin com yields butyields were similarwith all treatmentsandextremelyvariable.
Variability in yields was likely due to bird dsmage early in the growing season, drought
stress,andpoorcom standabiJity.
I GraduateResearchAssistant,Professor,ResearchSpecialistSenior,andFaculty
ResearchAssociate,respectively,VirginiaPolytechnicInstituteandStateUniversity,
Eastern Shore Agricultural Research and Extension Center, Painter, VA 23420-2827.
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