Vol. 54—2000 - NorthEastern Weed Science Society

ROUNDUPVS. LIBERTY:WHATHAVE WE LEARNED?
D.O. Lingenfelter, W.S. Curran, and E.L. Werner'
ABSTRACT
Experiments were conducted at Penn State University starting in 1995 with glyphosate and
glufosinate resistant crops. Soybean and com studies examined these herbicides alone. in
combinationwith other herbicides, postemergencefollowing soil-applied herbicides, and in
sequentialpostemergenceapplications. Soybeantrialswere conductedin seven to 15 inch rows
and in 3Q-inch row spacings. All com trials were conducted with 30 inch spaced rows.
Glyphosate was applied at 0.75 to 1.0 lb aiJA. Glufosinate was applied at 0.27 to 0.36 lb aiJA.
Post applications were applied between 3 and 6 weeks after planting. All studies were planted in
redueed tillage systems that generally included spring chisel plowing and disking once or twice.
Most experiments were consisted of 10 by 25 foot plots replicated threetimes.
Commonweeds throughoutthese studiesincludedcommon lambsquarters(Chenopodium
album L.), giant foxtail (Setaria faberi Herrm), smooth pigweed (Amaranthus Irybridus L.),
velvetleaf (Abutilon theophrasti Medic), common ragweed (Ambrosia artemisiifolia L.), and
yellow nutsedge (Cyperus esculentus L.). In addition, eastern black nightsbade (Solanum
ptycanthum Dun.) and burcucwnber(Sicyos angula/us L.) were present in a few experiments.
For Roundup Ready soybeans, row spacing did not greatly influence glyphosate performance.
Glyphosatealone consistentlycontrolledmost weeds in soybean. Tank-mixturesor sequential
treatmentswere generallynot neededunless weed infestationswere severe or underconditions
thatpreventedsoybean canopy closure. In RoundupReady com, single glyphosatetreatments
did not consistently control all weeds. Sequentialtreatmentsprovided the most consistent
control whether it was a soil applied product followed by glyphosate post or two applications of
glypbosate. Tank mixtures with glyphosate were slightly better than glyphosate alone, but not as
consistentas sequentialtreatments. LibertyLink com results were similarto RoundupReady
com results. Glufosinate alone was rarely effective for complete weed control. Sequential
treatmentsthat followed a soil applied treatmentprovided the most consistent control with
glufosinate. Tankmixturesandsplitpost applicationswere betterthanglufosinatealone, butnot
as effective as a sequential soil applied treatment followed by glufosinate post.
These resultsdemonstratethatglyphosateperformancein RoundupReadycom is different
from Roundup Ready soybean, where single applications of glyphosate have been very
consistent and effective. Com more often requirestwo separateapplications. Glufosinate
performance in Liberty Link com is sintilar to glyphosate performance in Roundup Ready corn,
butprobablyeven less likely to succeedin a single applicationapproach. Sequentialtreatments
that include a soil residualherbicideapplicationfollowed by a post applicationof glufosinate
havebeenmost successfulin LibertyLinkcom.
1 Asst. Ext. Agron.,Assoc. Prof.,andRes. Technol., Dept.of Agron.,PennStateUniv.,
University Park, PA 16802.
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