Vol. 54—2000 - NorthEastern Weed Science Society

EFFECTIVENESS OF GLYPHOSATE AND SEVERAL OTHER HERBICIDES FOR
MANAGING WEEDS IN ROUNDUP READY CORN
D. G. Voight, W. S. Curran, R. L. Hockensmith, G. J. Hostetter, J. M. Hunter, D. D.
Lingenfelter, D. T. Messersmith, and J. E. Rowehl'
ABSTRACT
With the recent introduction of Roundup Ready com (Zea mays L.) hybrids, some
speculation bas been placed on herbicide mixtures with glyphosate and application timing to
obtaineffectiveweedcontrol. Thefollowingresearchwas designedto evaluateglyphosatealone
atdifferentapplicationtimingsas well as to compareglyphosatemixedwith otherherbicidesto
traditionalherbicideprograms.
In 1999, field studies wereconducted at Lawrence, Crawford, Juniata, York, Lebanon,
and Wayne counties Pennsylvania. Roundup Ready com wasplanted during May and the
following herbicide treatments were evaluated: (1) s-metolachlortatrazine plus pendimetbalin,
PRE (2.2lb and 1.24lb aiJA respectively); (2) acetochlortatrazine, PRE (2.8Ib aiJA) followed
by glyphosate, POST, (lib aiJA); (3) glyphosate, EPOST, (lib/A); (4) glyphosate, POST, (I
Ib/A); (5) glyphosate, LPOST, (lib/A); (6) glyphosate plus atrazine, EPOST, (lIb plus I lb
aiJA); and (7) glyphosate, EPOST (lIb/A) followed by glyphosate, LPOST, (0.751b/A).
Although weed apecies and severity differed across locations, giant foxtail (Setariafabert
Herrm.), common lambsquarters (Chenopodium album L.), velvetleaf(Albulilon theophrosti
Medic.), smooth pigweed (Amaranthus_hybridus L.), common ragweed(Ambrosia artemisiifolia
L.), Pennsylvania smartweed(Polygonum pensylvanicum L.), and yellow nutsedge (Cyperus
esculentus L.), weremostoftenpresent.The York.Countysite wasdominatedbyJohnsongrass
(Sorghum holepense (L.) Pers.). A randomized complete block design with threereplications
was used for all studies. Herbicides were applied with a CO,-backpack sprayer that delivered 20
gpa, Above-ground weed biomass and com grain yield were collected at the end of season.
Sampleswereovendriedandcom yield wasconvertedto hulAat 15.5%moisture.
Locationby treatmentinteractionswereabsentfor fourof six counties,so weed biomass
datawerecombinedoverLawrence,Wayne,Lebanon,andJuniatacounties. At these locations,
all herbicide treatments effectively controlled the weeds,while weed biomass in the untreated
plot averaged 22 I9 lb/A. At Crawford County, the LPOST application of glyphosae was
ineffective probably due to the larger weedsand poor spray coverage. At York County, s­
metolachlor+atrazinepluspendimethalinwastheonlytreatmentnotdifferentfromthecontrol;
this wasprimarily due to the presence of Johnsongrass that was not controlled by the PRE
treatment.
Com yield ranged from an average low of 40 buiA in Wayne County to a high of 160
butA in Lawrence.Comyielddatawas combinedacrosslocationsbecauseof the lackof
locationbytreatmentintemctions.Acrosslocationscom yield in theuntreatedplots averaged47
bulA while the herbicide treatments ranged from 86 to 97 bulA.
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