Herbicide Options for Control of Palmer Amaranth (Amaranthus palmeri) and Common Waterhemp (Amaranthus rudis) in Double-Crop Soybean

Double-crop soybean after winter wheat is a component of many cropping systems across eastern and central Kansas. Until recently, control of Palmer amaranth and common waterhemp has been both easy and economical with the use of sequential applications of glyphosate in glyphosate-resistant soybean. Many populations of Palmer amaranth and common waterhemp have become resistant to glyphosate. During 2015 and 2016, a total of five field experiments were conducted near Manhattan, Hutchinson, and Ottawa, KS, to assess various non-glyphosate herbicide programs at three different application timings for the control of Palmer amaranth and waterhemp in double-crop soybean after winter wheat. Spring-POST treatments of pyroxasulfone (119 g ai ha–1) and pendimethalin (1065 g ai ha–1) were applied to winter wheat to evaluate residual control of Palmer amaranth and waterhemp. Less than 40% control of Palmer amaranth and waterhemp was observed in both treatments 2 wk after planting (WAP) double-crop soybean. Preharvest treatments of 2,4-D (561 g ae ha–1) and flumioxazin (107 g ai ha–1) were also applied to the winter wheat to assess control of emerged Palmer amaranth and waterhemp. 2,4-D resulted in highly variable Palmer amaranth and waterhemp control, whereas flumioxazin resulted in control similar to PRE treatments that contained paraquat (841 g ai ha–1) plus residual herbicide(s). Excellent control of both species was observed 2 WAP with a PRE paraquat application; however, reduced control of Palmer amaranth and waterhemp was noted 8 WAP due to subsequent emergence. Results indicate that Palmer amaranth and waterhemp control was 85% or greater 8 WAP for PRE treatments that included a combination of paraquat plus residual herbicide(s). PRE treatments that did not include both paraquat and residual herbicide(s) did not provide acceptable control.

Tolpyralate Applied Alone and With Atrazine for Weed Control in Corn

Tolpyralate, an HPPD (4-hydroxyphenyl-pyruvate dioxygenase) inhibitor, is a relatively new herbicide for weed control in corn. Field studies were conducted in 2015 and 2016 to evaluate the effective dose of tolpyralate applied alone or mixed with atrazine for weed control in corn. The treatments included seven rates (0, 5, 20, 29, 40, 50 and 100 g ai ha-1) of tolpyralate applied alone or mixed with a constant rate (560 g ai ha-1) of atrazine. The evaluated weed species were common waterhemp (Amaranthus rudis Sauer), common lambsquarters (Chenopodium album L.), velvetleaf (Abutilon theophrasti Medik), henbit (Lamium amplexicaule L.) and green foxtail (Setaria viridis L.). Overall, POST-application of tolpyralate resulted in 58-94% visual weed control when applied alone; whereas, addition of atrazine provided 71-100% control of same species. Calculated dose of 19-31 g ai ha-1 (ED90) of tolpyralate applied alone provided 90% visual control of waterhemp, lambsquaters, henbit, and velvetleaf. Whereas, addition of atrazine resulted in significantly lower dose of 11-17 g ai ha-1 for the same level of control, suggesting synergy between the two herbicides.

Control of Glyphosate-Resistant Common waterhemp (Amaranthus rudis) in Three New Herbicide-Resistant Soybean Varieties in Ontario

Glyphosate-resistant (GR) common waterhemp (CW) is a localized weed in Ontario and one of the most problematic weeds in the US Corn Belt. First confirmed in Ontario in 2014, GR CW has now been confirmed in forty fields in three counties in Ontario as of 2015. Historically, the primary POST herbicides used for the control of CW in soybean were glyphosate, acifluorfen and fomesafen, but resistance to all three has been confirmed in many US states. Research was conducted in 2015 and 2016 to determine the control of GR CW with some of the new herbicide-resistant soybean technologies including glufosinate (LibertyLink), 2,4-D and glyphosate (Enlist), and isoxaflutole, mesotrione, and glufosinate (HPPD-resistant). Glyphosate-resistant CW was controlled (≥90%) all season with a two-pass weed control system across all herbicide-resistant soybean technologies evaluated. The two-pass weed control system in this research is defined as a PRE herbicide followed by a POST herbicide. At 12 WAA, the two-pass programs in LibertyLink, Enlist, and HPPD-resistant systems controlled GR CW up to 98, 98, and 92%, respectively, and reduced GR CW densities to 0 to 2% of the weedy control at 4 WAA. The two-pass programs provided greater GR CW control than PRE or POST herbicides alone. This study found that the use of two-pass weed control programs in glufosinate-resistant, glyphosate DMA/2,4-D choline-resistant and HPPD-resistant soybean can provide excellent control of GR CW, and can be valuable tools to reduce the selection intensity for herbicide-resistant weeds. Through the rotational use of different technologies, growers may be able to better manage their weed populations in reducing the risk of resistance when compared to the use of one herbicide repeatedly.

Control of Glyphosate-Resistant Common Waterhemp (Amaranthus rudis) in Glufosinate-Resistant Soybean

Glyphosate-resistant (GR) common waterhemp has become a significant problem weed in Nebraska and several Midwestern states. Several populations of GR common waterhemp are also resistant to acetolactate synthase (ALS)-inhibiting herbicides, making them difficult to control with POST herbicides in GR soybean. Glufosinate-resistant (GFR) soybean is an alternate system for controlling GR common waterhemp, justifying the need for evaluating glufosinate-based herbicide programs. The objectives of this study were to compare POST-only herbicide programs (including one-pass and two-pass POST programs) with PRE followed by (fb) POST herbicide programs for control of GR common waterhemp in GFR soybean and their effect on common waterhemp density, biomass, and soybean yield. Field experiments were conducted in 2013 and 2014 near Fremont, NE in a grower’s field infested with GR common waterhemp. Glufosinate applied early- and late-POST provided 76% control of GR common waterhemp at 14 d after late-POST (DALPOST) compared with 93% control with a PRE fb POST program when averaged across treatments. The PRE application of chlorimuron plus thifensulfuron plus flumioxazin,S-metolachlor plus fomesafen or metribuzin, saflufenacil plus dimethenamid-P fb glufosinate provided ≥95% control of common waterhemp throughout the growing season, reduced common waterhemp density to ≤2.0 plants m─2, caused ≥94% biomass reduction, and led to 1,984 to 2,210 kg ha─1soybean yield. Averaged across treatments, the PRE fb POST program provided 82% common waterhemp control at soybean harvest, reduced density to 23 plants m─2at 14 DALPOST, and caused 86% biomass reduction and 1,803 kg ha─1soybean yield compared with 77% control, 99 plants m─2, 53% biomass reduction, and 1,190 kg ha─1yield with POST-only program. It is concluded that PRE fb POST programs with multiple effective modes of action are available for control of GR common waterhemp in GFR soybean.

Comparison of Herbicide Programs for Season-Long Control of Glyphosate-Resistant Common Waterhemp (Amaranthus rudis) in Soybean

The evolution of glyphosate and acetolactate synthase (ALS) inhibitor-resistant common waterhemp in the Midwestern United States has reduced the number of effective POST herbicide options for management of this problem weed in glyphosate-resistant soybean. Moreover, common waterhemp emerges throughout the crop growing season, justifying the need to evaluate herbicide programs that provide season-long control. The objectives of this study were to compare POST-only and PRE followed by (fb) POST herbicide programs for control of glyphosate-resistant common waterhemp in glyphosate-resistant soybean. Field experiments were conducted in 2013 and 2014 in Dodge County, NE, in a field infested with glyphosate-resistant common waterhemp. Programs containing PRE herbicides resulted in ≥83% control of common waterhemp and densities of ≤35 plantsm–2at 21 d after PRE (DAPRE). Post-only herbicide programs resulted in <70% control and densities of 107 to 215 plants m–2at 14 d after early-POST (DAEPOST) treatment. PRE fb POST herbicide programs, including saflufenacil plus imazethapyr plus dimethenamid-P, sulfentrazone plus cloransulam, orS-metolachlor plus metribuzin, fb fomesafen plus glyphosate;S-metolachlor plus fomesafen fb acifluorfen plus glyphosate resulted in >90% control of glyphosate-resistant common waterhemp throughout the growing season, reduced density to ≤7plantsm–2, ≥92% biomass reduction, and soybean yield >2,200kg ha–1. Averaged across herbicide programs, common waterhemp control was 84%, and density was 15 plants m–2with PRE fb POST herbicide programs compared with 42% control, and density of 101 plants m–2with POST-only herbicide programs at harvest. Results of this study indicated that PRE fb POST herbicide programs with effective modes of action exist for season-long control of glyphosate-resistant common waterhemp in glyphosate-resistant soybean.

Confirmation and Control of Glyphosate-Resistant Common Waterhemp (Amaranthus rudis) in Nebraska

Glyphosate-resistant common waterhemp is a difficult-to-control annual broadleaf weed that has become a serious management challenge for growers in Nebraska and other states in the United States. The objectives of this study were to confirm glyphosate-resistant common waterhemp in Nebraska by quantifying level of resistance in a dose-response study, and to determine the sensitivity and efficacy of POST soybean herbicides for controlling suspected glyphosate-resistant common waterhemp biotypes. Seeds of suspected glyphosate-resistant common waterhemp biotypes were collected from seven eastern Nebraska counties. Greenhouse dose-response experiments were conducted to evaluate the response of common waterhemp biotypes to nine rates of glyphosate (0 to 16×). Common waterhemp biotypes were 3- to 39-fold resistant to glyphosate depending on the biotype being investigated and the susceptible biotype used for comparison. Results of the POST soybean herbicides efficacy experiment suggested that glyphosate-resistant biotypes, except a biotype from Pawnee County, had reduced sensitivity to acetolactate synthase (ALS)–inhibiting herbicides (chlorimuron-ethyl, imazamox, imazaquin, imazethapyr, and thifensulfuron-methyl). Glufosinate and protoporphyrinogen oxidase (PPO)–inhibiting herbicides (acifluorfen, fluthiacet-methyl, fomesafen, and lactofen) provided ≥ 80% control of glyphosate-resistant common waterhemp at 21 d after treatment (DAT). This study confirmed the first occurrence of glyphosate-resistant common waterhemp in Nebraska, and also revealed reduced sensitivity to ALS-inhibiting herbicides in most of the biotypes tested in this study.

Glyphosate-Resistant Giant Ragweed (Ambrosia trifida) and Waterhemp (Amaranthus rudis) Management in Dicamba-Resistant Soybean (Glycine max)

Field experiments were conducted across two locations during 2011 and 2012 to evaluate herbicide options for the control of glyphosate-resistant (GR) giant ragweed and GR waterhemp in dicamba-resistant (DR) soybean. All herbicide treatments provided 91 to 100% control of GR giant ragweed 3 wk after treatment (WAT). Flumioxazin plus dicamba plus glyphosate applied preplant provided greater control and density reduction of GR giant ragweed than flumioxazin plus 2,4-D plus glyphosate. When flumioxazin plus dicamba plus glyphosate were applied preplant, the addition of dicamba to glyphosate at either the early-postemergence (EPOST) or mid-postemergence (MPOST) timing provided greater control and density reduction of GR giant ragweed than glyphosate alone. Regardless of the preplant treatment, delay of EPOST dicamba to the MPOST timing did not influence GR giant ragweed control or density reduction. In the GR waterhemp experiment, dicamba plus glyphosate applied sequentially provided 88 to 89% control and 90% density reduction at the EPOST and MPOST timings compared to only 24% control and 42% density reduction in response to glyphosate applied sequentially. Control and GR waterhemp density reduction did not improve with the addition of acetochlor to either the EPOST or late-postemergence (LPOST) timings. Flumioxazin plus chlorimuron applied PRE followed by dicamba plus glyphosate or dicamba plus glyphosate plus acetochlor provided greater control of GR waterhemp than glyphosate plus fomesafen or glyphosate alone applied EPOST. Results from this research indicate that dicamba applied once or sequentially and when timed appropriately to match the biology of the weed species can be utilized as a component of an integrated program for the management of GR weeds like giant ragweed and waterhemp in DR soybean.