Success of total post weed control in northeastern U.S. corn. 2001. W. S. Curran, M. J. VanGessel, B. A. Majek, P. C. Bhowmik, E. S. Hagood, R. Hahn, F. J. Himmelstein, J. M. Jemison, R. L. Ritter, H. P. Wilson, T. E. Dutt, and D. J. Mayonado. Proc. NEWSS 55:30.

An experiment was conducted at multiple locations throughout the Northeast in 2000 to evaluate the effectiveness of a total POST weed control program over a broad range of conditions. The study included 14 treatments utilizing glyphosate-resistant corn. The series of treatments examined the need for residual herbicides when a total POST program is used. Glyphosate alone or glyphosate plus a prepackage mixture of acetochlor or alachlor plus atrazine were applied to approximately V-2 and V-4 corn. Glyphosate alone was also applied to V-6 corn and also in a sequential timing at V-2 followed by V-6. A prepackage mixture of rimsulfuron, nicosulfuron, and atrazine in combination with dicamba was included for comparison. A weedy and weed-free check were also included.
Study sites included no-till as well as conventional tillage and irrigated and non-irrigated locations. County agents helped conduct trials at some locations. 
Weed density and height, by species were determined prior to each POST application. Visional weed control ratings and additional emergence was recorded 2 and 4 weeks after treatment. In late summer or fall, weed biomass and grain or silage yield was collected. At the time of writing this abstract, weed biomass and corn yield data were still being processed at a number of locations. However, preliminary results showed that control varied across locations depending on weed species, severity, and rainfall. In general, PRE herbicide treatments were fairly effective at least early season with most areas receiving adequate rainfall. At some locations, late season control with PRE treatments alone declined. The effectiveness of POST control was strongly tied to weed species and severity. In Pennsylvania, good control of common lambsquarters (Chenopodium album L.) and other annual weeds was achieved with glyphosate alone, except at the early POST (V-2) timing, where weed emergence subsequent to the early application was a problem. In addition, certain weeds such as dandelion (Taraxacum officinale Weber in Wiggers) were not effectively controlled in no-till corn. In general, the mid POST application (V-4) regardless of treatment provided the best control compared with the other POST timings. Residual herbicides either improved or maintained control compared to glyphosate alone. Weed control in the late POST (V-6) timing varied depending on weed species and severity. As expected a sequential application of glyphosate was generally effective for control of most weeds.

Roundup vs. Liberty: What have we learned? 2000. D.D. Lingenfelter, W.S. Curran, and E.L. Werner. Proc. NEWSS 54:39.

Experiments were conducted at Penn State University starting in 1995 with glyphosate and glufosinate resistant crops. Soybean and corn studies examined these herbicides alone, in combination with other herbicides, postemergence following soil-applied herbicides, and in sequential postemergence applications. Soybean trials were conducted in seven to 15 inch rows and in 30-inch row spacings. All corn trials were conducted with 30 inch spaced rows. Glyphosate was applied at 0.75 to 1.0 lb ai/A. Glufosinate was applied at 0.27 to 0.36 lb ai/A. Post applications were applied between 3 and 6 weeks after planting. All studies were planted in reduced tillage systems that generally included spring chisel plowing and disking once or twice. Most experiments were consisted of 10 by 25 foot plots replicated three times.

Common weeds throughout these studies included common lambsquarters (Chenopodium album L.), giant foxtail (Setaria faberi Herrm.), smooth pigweed (Amaranthus hybridus L.), velvetleaf (Abutilon theophrasti Medic), common ragweed (Ambrosia artemisiifolia L.), and yellow nutsedge (Cyperus esculentus L.). In addition, eastern black nightshade (Solanum ptycanthum Dun.) and burcucumber (Sicyos angulatus L.) were present in a few experiments. For Roundup Ready soybeans, row spacing did not greatly influence glyphosate performance. Glyphosate alone consistently controlled most weeds in soybean. Tank-mixtures or sequential treatments were generally not needed unless weed infestations were severe or under conditions that prevented soybean canopy closure. In Roundup Ready corn, single glyphosate treatments did not consistently control all weeds. Sequential treatments provided the most consistent control whether it was a soil applied product followed by glyphosate post or two applications of glyphosate. Tank mixtures with glyphosate were slightly better than glyphosate alone, but not as consistent as sequential treatments. Liberty Link corn results were similar to Roundup Ready corn results. Glufosinate alone was rarely effective for complete weed control. Sequential treatments that followed a soil applied treatment provided the most consistent control with glufosinate. Tank mixtures and split post applications were better than glufosinate alone, but not as effective as a sequential soil applied treatment followed by glufosinate post.

These results demonstrate that glyphosate performance in Roundup Ready corn is different from Roundup Ready soybean, where single applications of glyphosate have been very consistent and effective. Corn more often requires two separate applications. Glufosinate performance in Liberty Link corn is similar to glyphosate performance in Roundup Ready corn, but probably even less likely to succeed in a single application approach. Sequential treatments that include a soil residual herbicide application followed by a post application of glufosinate have been most successful in Liberty Link corn.

Effectiveness of glyphosate and several other herbicides for managing weeds in roundup ready corn. 2000. 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. Proc. NEWSS 54:40.

With the recent introduction of Roundup Ready corn (Zea mays L.) hybrids, some speculation has been placed on herbicide mixtures with glyphosate and application timing to obtain effective weed control. The following research was designed to evaluate glyphosate alone at different application timings as well as to compare glyphosate mixed with other herbicides to traditional herbicide programs.

In 1999, field studies were conducted at Lawrence, Crawford, Juniata, York, Lebanon, and Wayne counties Pennsylvania. Roundup Ready corn was planted during May and the following herbicide treatments were evaluated: (1) s-metolachlor+atrazine plus pendimethalin, PRE (2.2 lb and 1.24 lb ai/A respectively); (2) acetochlor+atrazine, PRE (2.8 lb ai/A) followed by glyphosate, POST, (1 lb ai/A); (3) glyphosate, EPOST, (1 lb/A); (4) glyphosate, POST, (1 lb/A); (5) glyphosate, LPOST, (1 lb/A); (6) glyphosate plus atrazine, EPOST, (1 lb plus 1 lb ai/A); and (7) glyphosate, EPOST (1 lb/A) followed by glyphosate, LPOST, (0.75 lb/A). Although weed species and severity differed across locations, giant foxtail (Setaria faberi Herrm.), common lambsquarters (Chenopodium album L.), velvetleaf (Albutilon theophrasti Medic.), smooth pigweed (Amaranthus hybridus L.), common ragweed (Ambrosia artemisiifolia L.), Pennsylvania smartweed (Polygonum pensylvanicum L.), and yellow nutsedge (Cyperus esculentus L.), were most often present. The York County site was dominated by Johnsongrass (Sorghum halepense (L.) Pers.). A randomized complete block design with three replications was used for all studies. Herbicides were applied with a CO₂-backpack sprayer that delivered 20 gpa. Above-ground weed biomass and corn grain yield were collected at the end of season. Samples were oven dried and corn yield was converted to bu/A at 15.5% moisture.

Location by treatment interactions were absent for four of six counties, so weed biomass data were combined over Lawrence, Wayne, Lebanon, and Juniata counties. At these locations, all herbicide treatments effectively controlled the weeds, while weed biomass in the untreated plot averaged 2219 lb/A. At Crawford County, the LPOST application of glyphosae was ineffective probably due to the larger weeds and poor spray coverage. At York County, s-metolachlor+ atrazine plus pendimethalin was the only treatment not different from the control; this was primarily due to the presence of Johnsongrass that was not controlled by the PRE treatment.

Corn yield ranged from an average low of 40 bu/A in Wayne County to a high of 160 bu/A in Lawrence. Corn yield data was combined across locations because of the lack of location by treatment interactions. Across locations corn yield in the untreated plots averaged 47 bu/A while the herbicide treatments ranged from 86 to 97 bu/A.

In summary, application timing for glyphosate was generally not as critical in 1999 as it may be in other years due to dry weather and a lack of subsequent weed emergence. Only with a LPOST application, where herbicide deposition may have been impacted because of larger corn, were differences in timing observed.

Glyphosate vs. glufosinate: What have we learned? 1999. W. S. Curran, D. D. Lingenfelter, and E. L. Werner. Proc. NCWSS 54. (In Press).

Experiments were conducted at Penn State University starting in 1995 with glyphosate- and glufosinate-resistant crops. Soybean and corn studies examined these herbicides alone, in combination with other herbicides, postemergence following soil-applied herbicides, and in sequential postemergence applications. Soybean trials were conducted in seven to 15-inch rows and in 30-inch row spacings. All corn trials were conducted with 30-inch spaced rows. Glyphosate was applied at 0.75 to 1 lb ai/A. Glufosinate was applied at 0.27 to 0.36 lb ai/A. Post applications were applied between 3 and 6 weeks after planting. All studies were planted in reduced tillage systems that generally included spring chisel plowing and disking once or twice. Most experiments were consisted of 10 by 25 foot plots replicated three times.

Common weeds throughout these studies included common lambsquarters (Chenopodium album L.), giant foxtail (Setaria faberi Herrm.), smooth pigweed (Amaranthus hybridus L.), velvetleaf (Abutilon theophrasti Medic), common ragweed (Ambrosia artemisiifolia L.), and yellow nutsedge (Cyperus esculentus L.). In addition, eastern black nightshade (Solanum ptycanthum Dun.) and burcucumber (Sicyos angulatus L.) were present in a few experiments. For glyphosate-resistant soybeans, row spacing did not greatly influence glyphosate performance. Glyphosate alone consistently controlled most weeds in soybean. Tank-mixtures or sequential treatments were generally not needed unless weed infestations were severe or under conditions that prevented soybean canopy closure. In glyphosate-resistant corn, single glyphosate treatments did not consistently control all weeds. Sequential treatments provided the most consistent control whether it was a soil applied product followed by glyphosate post or two applications of glyphosate. Tank mixtures with glyphosate were slightly better than glyphosate alone, but not as consistent as sequential treatments. Glufosinate-resistant corn results were similar to glyphosate-resistant corn results. Glufosinate alone was rarely effective for complete weed control. Sequential treatments that followed a soil applied treatment provided the most consistent control with glufosinate. Tank mixtures and split post applications were better than glufosinate alone, but not as effective as a sequential soil applied treatment followed by glufosinate post.

These results demonstrate that glyphosate performance in glyphosate-resistant corn is different from glyphosate-resistant soybean, where single applications of glyphosate have been very consistent and effective. Corn more often requires two separate applications. Glufosinate performance in glufosinate-resistant corn is similar to glyphosate performance in glyphosate-resistant corn, but probably even less likely to succeed in a single application approach. Sequential treatments that include a soil residual herbicide application followed by a post application of glufosinate have been most successful in glufosinate-resistant corn.