Vol. 54—2000 - NorthEastern Weed Science Society

INFLUENCE OF TIMING ON WEED MANAGEMENT IN GLYPHOSATE-RESIST ANT
CORN
F. J. Himmelstein and R. J. Durgy I
ABSTRACT
A 1999 field trial conducted in Lebanon, CT evaluated the influence of the time of
glyphosate (Roundup Ultra) application on the competitive effects of weedpopulations on com (Zea
mays L.) yield. Agway 637RR, a 107 day glyphosate-resistant com vatiety, was planted on May 13,
1999. Preemergence treatments (PRE) were applied on May, IS". Early postemergence treatments
(EP) were applied on June 10", when the com bad 4-to-5leaves, and S-to-14 inches in height. Mid
postemergence treatments (MP) were applied on June 16", when the com bad 5-to-6leaves, and 16­
to-21 inches in height. Late postemergence treatments wereapplied on June 24", when the com bad
g-to-c leaves, and 30-to-34 inches in height. Treatments included glyphosate alone at 0.5, 0.75, and
1.0 Ib ailA, all applied at the EP, MP, and LP timings. Other treatments included glyphosate at 1.0
lb aiJAEP in combinationwith EP applicationsof eitheratrazineat 0.75 IbaiJA,or acetochlor+
atrazine (Harness Xtra 5.6L) at 1.751b ailA, glyphosate EP followed by glyphosate LP, both at 1.0
ailA, and acetocblor + atrazine (Harness Xtra 5.6L) at 2.1 lb ailA PRE followed by glyphosate at 1.0
Ib ailA (MP). Comparative treatments included s-metolacblor + atrazine (Bicep 11Magnwn 5.5L)
at 2.91b ailA + pendimethalin (ProwI3.3EC) applied PRE, rintsulfuron + atrazine + nicosulfuron
(Basis Gold 90DF) at 0.791b ailA applied EP, and s-metolacblor(Dual11 Magnum 7.64EC) at 1.27
lb ailA applied PRE followed by prosulfuron+ printisulfuron (Exceed 57WG) at 0.036 lb ailA
applied EP. The study was a randomized complete block design with three replications. Herbicides
were applied with a CO 2 backpack sprayer delivering 20 gpa at 32 psi. Weed control was assessed
by visual ratingson July 3 M , and September2St. The dominantweed species were velvetleaf
[Abutjlon tbeQpbmstjMedik], giant foxtail [Setaria 1illlla:i Herrm.],andwild cucwnber (Echjnocystjs
1llhlIIlI (Micltx.). Common lambsquarters [CbenQpodium ll1bumL], redroot pigweed (Amarantbus
retroflexusL.),and large crabgrass IDiaitaria sanit!inalis (L.) Scop.] were also present in the study
area.
All herbicidetreatmentsresultedin excellentcontrolof all weedspecies by the time of
harvest with the exception of s-metolacblor + atrazine + pendimethalin applied PRE that bad a severe
infestationof wild cucumber.Boththattreatmentandthe untreatedcheck would have not been
harvestablewithstandardharvestingequipment.Com silageandgrainyields were similaramong
the EP treatments of glyphosate alone at all threerates and the other PRE and EP treatments
combinations. Both the overallcom silage andgrainyields werereducedby the extendedperiod
of weed competition when glyphosate treatments applied alone weredelayed. Average silage yields
were 30.3, 26.1, and 22.5 T/A, with the EP, MP, and LP treatments of glyphosate, respectively.
Average grain yields were 215, 189, and 167 bulA, with the EP, MP, and LP treatments of
glyphosate,respectively.This studyindicatesthatwhen glyphosatetreatmentsareappliedalone,
applications should he made prior to the fifth leaf stage of com in order to avoid potential yield loss
from the extendedperiodof weed competition.Whenpreemergencetreatmentsare applied,this
allows more flexibility in the timing of the postemergenceapplicationsto obtainmaximumcom
yields.
'Extension Educator-IntegratedCrop Management,and Res. Asst., respectively, University of
Coonecticut, Storrs, CT 06269
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