Mark L. Bernards

U.S. Grower Views on Problematic Weeds and Changes in Weed Pressure in Glyphosate-Resistant Corn, Cotton, and Soybean Cropping Systems

Abstract Corn and soybean growers in Illinois, Indiana, Iowa, Mississippi, Nebraska, and North Carolina, as well as cotton growers in Mississippi and North Carolina, were surveyed about their views on changes in problematic weeds and weed pressure in cropping systems based on a glyphosate-resistant (GR) crop. No growers using a GR cropping system for more than 5 yr reported heavy weed pressure. Over all cropping systems investigated (continuous GR soybean, continuous GR cotton, GR corn/GR soybean, GR soybean/non-GR crop, and GR corn/non-GR crop), 0 to 7% of survey respondents reported greater weed pressure after implementing rotations using GR crops, whereas 31 to 57% felt weed pressure was similar and 36 to 70% indicated that weed pressure was less. Pigweed, morningglory, johnsongrass, ragweed, foxtail, and velvetleaf were mentioned as their most problematic weeds, depending on the state and cropping system. Systems using GR crops improved weed management compared with the technologies used before the adoption of GR crops. However, the long-term success of managing problematic weeds in GR cropping systems will require the development of multifaceted integrated weed management programs that include glyphosate as well as other weed management tactics. Nomenclature: Glyphosate; foxtail, Setaria spp.; johnsongrass, Sorghum halepense (L.) Pers.; morningglory, Ipomoea spp.; pigweed, Amaranthus spp.; ragweed, Ambrosia spp.; velvetleaf, Abutilon theophrasti Medik.; corn, Zea mays L.; cotton, Gossypium hirsutum L; soybean, Glycine max (L.) Merr

Glyphosate interaction with manganese in tank mixtures and its effect on glyphosate absorption and translocation

Abstract Recent reports indicate that manganese (Mn), applied as a foliar fertilizer in tank mixtures with glyphosate, has the potential to antagonize glyphosate efficacy and reduce weed control. It was hypothesized that Mn2+ complexed with glyphosate in a similar manner to Ca2+, forming salts that were not readily absorbed and, thereby, reducing glyphosate efficacy. This study was conducted to confirm the interaction of Mn2+ and glyphosate and to measure the effect of Mn on glyphosate absorption and translocation in velvetleaf. In aqueous solutions, Mn2+ binds with solvent molecules and with chelating agents to form hexacoordinate complexes. The distribution of paramagnetic species, both the free manganous ion ([Mn{H2O}6]2+) and the Mn2+–glyphosate complex, in Mn–glyphosate solutions at various pH values were analyzed using electron paramagnetic resonance (EPR) spectroscopy. Glyphosate interaction with Mn appeared to increase as the pH was increased from spray solution levels (2.8 to 4.5) to levels common in the plant symplast (7.5). Growth chamber bioassays were conducted to measure absorption and translocation of 14C-labeled glyphosate in solution with four Mn fertilizers: Mn-ethylaminoacetate (Mn-EAA), Mn-ethylenediaminetetraacetate (Mn-EDTA), Mn-lignin sulfonate (Mn-LS), and Mn-sulfate (MnSO4). Mn-EDTA did not interfere with glyphosate efficacy, absorption, or translocation. However, both MnSO4 and Mn-LS reduced glyphosate efficacy, absorption, and translocation. Mn-EAA severely antagonized glyphosate efficacy, and although glyphosate in tank mixtures with Mn-EAA was absorbed rapidly, little was translocated from the treated leaf. The Mn-EAA fertilizer contained approximately 0.5% iron (Fe) not reported on the fertilizer label. Iron is presumed to be partially responsible for the very limited translocation of glyphosate from the treated leaf in Mn-EAA tank mixtures. Adding ammonium sulfate increased the efficacy, absorption, and translocation of glyphosate for each Mn fertilizer tank mixture. Nomenclature: Glyphosate; velvetleaf, Abutilon theophrasti Medicus. ABUTH.

A Waterhemp (Amaranthus tuberculatus) Population Resistant to 2,4-D

Abstract A waterhemp population from a native-grass seed production field in Nebraska was no longer effectively controlled by 2,4-D. Seed was collected from the site, and dose-response studies were conducted to determine if this population was herbicide resistant. In the greenhouse, plants from the putative resistant and a susceptible waterhemp population were treated with 0, 18, 35, 70, 140, 280, 560, 1,120, or 2,240 g ae ha−1 2,4-D. Visual injury estimates (I) were made 28 d after treatment (DAT), and plants were harvested and dry weights (GR) measured. The putative resistant population was approximately 10-fold more resistant to 2,4-D (R∶S ratio) than the susceptible population based on both I50 (50% visual injury) and GR50 (50% reduction in dry weight) values. The R∶S ratio increased to 19 and 111 as the data were extrapolated to I90 and GR90 estimates, respectively. GR50 doses of 995 g ha−1 for the resistant and 109 g ha−1 for the susceptible populations were estimated. A field dose-response study was conducted at the suspected resistant site with 2,4-D doses of 0, 140, 280, 560, 1,120, 2,240, 4,480, 8,960, 17,920, and 35,840 g ha−1. At 28 DAT, visual injury estimates were 44% in plots treated with 35,840 g ha−1. Some plants treated with the highest rate recovered and produced seed. Plants from the resistant and susceptible populations were also treated with 0, 9, 18, 35, 70, 140, 280, 560, or 1,120 g ae ha−1 dicamba in greenhouse bioassays. The 2,4-D resistant population was threefold less sensitive to dicamba based on I50 estimates but less than twofold less sensitive based on GR50 estimates. The synthetic auxins are the sixth mechanism-of-action herbicide group to which waterhemp has evolved resistance. Nomenclature: 2,4-D; dicamba; waterhemp, Amaranthus tuberculatus (Moq.) Sauer var. rudis (Sauer) Costea and Tardif AMATU.

Effect of Weed Removal Timing and Row Spacing on Soil Moisture in Corn (Zea mays)

Glyphosate-resistant corn was grown in 38- and 76-cm row spacings at two locations in 2001 to examine the effect of weed competition and row spacing on soil moisture. Volumetric soil moisture was measured to a depth of 0.9 m in 18-cm increments. Glyphosate was applied when average weed canopy heights reached 5, 10, 15, 23, and 30 cm. Season-long weed interference reduced soil moisture compared with the weed free controls. At Clarksville, MI, where common lambsquarters was the dominant weed species, weed interference reduced soil moisture in the 0- to 18-cm soil depth from late June through early August and at the 54- to 72- and 72- to 90-cm depths from mid-July through the end of the season. At East Lansing, MI, where giant foxtail was the dominant weed species, weed interference reduced soil moisture at the 18- to 36-, 36- to 54-, and 54- to 72-cm soil depths from mid-June to the end of the season. Season-long weed competition reduced yields more than 90% at each location. Weeds that emerged after the 5-cm glyphosate timing reduced soil moisture and grain yield at both locations. Delaying glyphosate applications until weeds reached 23 cm or more in height reduced corn yield at both locations and soil moisture at East Lansing. Grain yields in the 10- and 15-cm glyphosate-timing treatments were equal to the weed-free corn, even though soil moisture was less during pollination and grain fill. Row spacing did not affect grain yield but did affect soil moisture. Soil moisture was greater in the 76-cm row spacing, suggesting that corn in the 38-cm row spacing may have been able to access soil moisture more effectively. Nomenclature: Glyphosate; common lambsquarters, Chenopodium album L. #3 CHEAL; giant foxtail, Setaria faberi Herrm. # SETFA; corn, Zea mays L. Additional index words: Narrow-row corn, glyphosate-resistant, weed interference, weed competition, time domain reflectometry, soil water. Abbreviations: DAP, days after planting; FC, field capacity; PRE, preemergence; PWP, permanent wilting point.

Glyphosate Efficacy is Antagonized by Manganese1

Michigan soybean producers have observed that glyphosate efficacy is sometimes reduced in tank mixtures with foliar manganese (Mn) fertilizers. The objectives of this study were to evaluate the effects of Mn formulation, Mn application timing, tank mixture adjuvants, and Mn rate on glyphosate efficacy. Three Mn formulations, manganese sulfate and ethylaminoacetate chelate (Mn-EAA), manganese sulfate and lignin sulfonate chelate (Mn-LS), and manganese sulfate monohydrate (MnSO4) reduced glyphosate efficacy in greenhouse and field bioassays, but Mn ethylenediaminetetraacetate (Mn-EDTA) did not. Mn-EAA applied less than 3 d before glyphosate reduced glyphosate efficacy on velvetleaf but not giant foxtail or common lambsquarters. The antagonism increased as the interval between treatment applications was shortened but did not appear when Mn was applied to velvetleaf 1 d or more after glyphosate. Including the adjuvants ammonium sulfate (AMS), EDTA, or citric acid in the glyphosate–Mn tank mixture increased control of giant foxtail and velvetleaf but only matched the efficacy of the glyphosate plus AMS control in three combinations: AMS with Mn-LS on velvetleaf, citric acid with MnSO4 on giant foxtail, and EDTA with Mn-EAA on giant foxtail. AMS increased the glyphosate efficacy in Mn tank mixtures as much as, or more than, citric acid and EDTA, with two exceptions: EDTA with Mn-EAA on giant foxtail and citric acid with MnSO4 on velvetleaf. Control of velvetleaf declined as the amount of Mn from Mn-EAA, Mn-LS, and MnSO4 in the tank mixture increased. Nomenclature: Glyphosate; common lambsquarters, Chenopodium album L. #3 CHEAL; giant foxtail, Setaria faberi Herrm. # SETFA; velvetleaf, Abutilon theophrasti Medicus. # ABUTH; soybean, Glycine max L. Additional index words: Fertilizer, hard-water antagonism, herbicide interaction, micronutrient, split application. Abbreviations: AMS, ammonium sulfate; EDTA, ethylenediaminetetraacetate; glyphosate-IPA, isopropylamine salt of glyphosate; glyphosate-K, potassium salt of glyphosate; Mn-EAA, manganese sulfate and ethylaminoacetate chelate; Mn-EDTA, manganese ethylenediaminetetraacetate; Mn-LS, manganese sulfate and lignin sulfonate chelate; MnSO4, manganese sulfate monohydrate.

Response of Nebraska Kochia (Kochia scoparia) Accessions to Dicamba

Abstract Kochia is a troublesome weed in the western Great Plains and many accessions have evolved resistance to one or more herbicides. Dicamba-resistant soybean is being developed to provide an additional herbicide mechanism of action for POST weed control in soybean. The objective of this study was to evaluate variation in response to dicamba among kochia accessions collected from across Nebraska. Kochia plants were grown in a greenhouse and treated when they were 8 to 12 cm tall. A discriminating experiment with a single dose of 420 g ae ha−1 of dicamba was conducted on 67 accessions collected in Nebraska in 2010. Visual injury estimates were recorded at 21 d after treatment (DAT) and accessions were ranked from least to most susceptible. Four accessions representing two of the most and least susceptible accessions from this screening were subjected to dose-response experiments using dicamba. At 28 DAT, visible injury estimates were made and plants were harvested to determine dry weight. An 18-fold difference in dicamba dose was necessary to achieve 90% injury (I90) between the least (accession 11) and most susceptible accessions. Approximately 3,500 g ha−1 of dicamba was required in accession 11 to reach a 50% dry weight reduction (GR50). There was less than twofold variation among the three more susceptible accessions for both the I90 and GR90 parameters, suggesting that most kochia accessions will be similarly susceptible to dicamba. At 110 DAT, accession 11 had plants that survived doses of 35,840 g ha−1, and produced seed at doses of 17,420 g ha−1. The identification of one resistant accession among the 67 accessions screened, and the fact that dicamba doses greater than 560 g ha−1 were required to achieve GR80 for all accessions suggest that repeated use of dicamba for weed control in fields where kochia is present may quickly result in the evolution of dicamba-resistant kochia populations. Nomenclature: Dicamba; kochia; Kochia scoparia (L.) Schrad. KCHSC; soybean; Glycine max L. Merr. GLYMX. Resumen Kochia scoparia es una maleza problemática en el oeste de las Grandes Planicies y muchas accesiones han evolucionado resistencia a uno o más herbicidas. Se está desarrollando soya resistente a dicamba para proveer un mecanismo de acción adicional para el control de malezas POST en soya. El objetivo de este estudio fue evaluar la variación en la respuesta a dicamba entre accesiones de K. scoparia colectada a lo largo de Nebraska. Plantas de K. scoparia fueron crecidas en un invernadero y tratadas cuando tuvieron 8 a 12 cm de altura. Se realizó un experimento de discriminación con una sola dosis de 420 g ae ha−1 de dicamba con 67 accesiones colectadas en Nebraska en 2010. Estimaciones visuales de daño se realizaron 21 días después del tratamiento (DAT) y las accesiones fueron ordenadas de menor a mayor susceptibilidad. Cuatro accesiones representando dos de las accesiones más y menos susceptibles en la evaluación fueron sometidos a experimentos de respuesta a dosis usando dicamba. A 28 DAT, se realizaron las estimaciones visuales de daño y las plantas fueron cosechadas para determinar su peso seco. Una diferencia de 18 veces en la dosis de dicamba fue necesaria para alcanzar 90% de daño (I90) entre la accesión menos susceptible (accesión 11) y las más susceptibles. Se necesitó aproximadamente 3,500 g ha−1 de dicamba para reducir en 50% el peso seco (GR50) de la accesión 11. Hubo una variación de menos de dos veces en los valores de los parámetros I90 y GR90 entre las tres accesiones más susceptibles, lo que sugiere que la mayoría de las accesiones de K. scoparia serán similarmente susceptibles a dicamba. A 110 DAT, la accesión 11 tenía plantas que sobrevivieron la dosis de 35,840 g ha−1, y produjeron semillas a dosis de 17,420 g ha−1. La identificación de una accesión resistente entre 67 accesiones evaluadas, y el hecho de que las dosis de dicamba mayores a 560 g ha−1 fueron necesarias para alcanzar GR80 para todas las accesiones sugiere que el uso repetido de dicamba para el control de malezas en campos donde K. scoparia está presente podría resultar rápidamente en la evolución de poblaciones de esta maleza resistentes a dicamba.

Environmental Triggers of Winter Annual Weed Emergence in the Midwestern United States

Abstract Winter annual weeds are becoming prolific in agricultural fields in the midwestern United States. The objectives of this research were to understand the roles of soil temperature (daily average and fluctuation) and moisture on the emergence of nine winter annual weed species and dandelion and to develop predictive models for weed emergence based on the accumulation of modified thermal/hydrothermal time (mHTT). Experiments were established at Lincoln, NE; Mead, NE; and at two sites (irrigated and rainfed) near Clay Center, NE, in 2010 and 2011. In July of each year, 1,000 seeds of each species were planted in 15 by 20 by 6-cm mesh baskets installed between soybean rows. Soil temperature and water content were recorded at the 2-cm depth. Emerged seedlings were counted and removed from the baskets on a weekly basis until no additional emergence was observed in the fall, resumed in late winter, and continued until emergence ceased in late spring. Weather data were used to accumulate mHTT beginning on August 1. A Weibull function was selected to fit cumulative emergence (%) on cumulative mHTT (seven base temperature [Tbase] by six base water potential [&PSgr;base] by three base temperature fluctuation [Fbase] candidate threshold values  =  126 models); it was also fit to days after August 1 (DAA1), for a total of 127 candidate models per species. The search for optimal base thresholds was based on the theoretic-model comparison approach (Akaike information criterion [AIC]). All three components (Tbase, &PSgr;base, and Fbase) were only important for Virginia pepperweed. For downy brome and purslane speedwell, including Tbase and &PSgr;base resulted in the best fit, whereas for dandelion including Tbase and Fbase resulted in the best fit. A model including only Tbase resulted in the best fit for most species included in this study (Carolina foxtail, shepherds-purse, pinnate tansymustard, henbit, and field pansy). For field pennycress, the model based on DAA1 resulted in the best fit. Threshold values were species specific. Soil temperature was the major environmental factor influencing winter annual weed emergence. Even though soil moisture and often temperature fluctuation are essential for seed germination, &PSgr;base and Fbase were not as critical in the predictive models as initially expected. Most seedlings (> 90%) of downy brome, pinnate tansymustard, Carolina foxtail, henbit, and field pansy emerged during the fall. Virginia pepperweed, purslane speedwell, dandelion, shepherds-purse, and field pennycress seedlings emerged during both fall and spring. The results of this research provide robust information on the prediction of the time of winter annual weed emergence, which can help growers make better management decisions. Nomenclature: Carolina foxtail, Alopecurus carolinianus Walt. ALOCA; dandelion, Taraxacum officinale G.H. Weber ex Wiggers TAROF; downy brome, Bromus tectorum L. BROTE; field pansy, Viola bicolor Pursh VIORA; field pennycress, Thlaspi arvense L. THLAR; henbit, Lamium amplexicaule L. LAMAM; pinnate tansymustard, Descurainia pinnata (Walt.) Britt. DESPI; purslane speedwell, Veronica peregrina L. VERPG; shepherds-purse, Capsella bursa-pastoris (L.) Medik. CAPBP; Virginia pepperweed, Lepidium virginicum L. LEPVI; Glycine max (L.) Merr.

Effect of Planting Depth and Isoxaflutole Rate on Corn Injury in Nebraska

Field experiments were conducted at five sites in Nebraska in 2000 and 2001 to determine the effect of planting depth and isoxaflutole rate on the response of an isoxaflutole-sensitive corn hybrid, ‘Pioneer 33-G’ across variable environments. Corn was planted at depths of 2.5 and 5.0 cm, and isoxaflutole was applied PRE at the recommended (1×) and twice the recommended (2×) rate. The effects of planting depth and herbicide rate on injury varied considerably across site–years. When injury was evident, it was generally greater at the high rate of isoxaflutole (2×) and at the shallow planting depth (2.5 cm). In most site–years, corn recovered from early season injury, and yields were not reduced, except at Scottsbluff, NE, and North Platte, NE, where soils were lower in organic matter and higher in pH. Isoxaflutole rates should be carefully selected for soils with low organic matter and high pH. Nomenclature: Isoxaflutole, corn, Zea mays L

Evaluation of postemergence weed control strategies in herbicide-resistant isolines of corn (Zea mays)

Herbicide-resistant corn hybrids offer additional options for POST weed control in corn, and growers may benefit from information on the consistency of these weed-control strategies. Studies were conducted in Indiana, Illinois, Michigan, and Ohio, in 2000 and 2001, to evaluate weed control among herbicide strategies for imidazolinone-resistant, glufosinate-resistant, glyphosate-resistant, and conventional corn. Isogenic hybrids were utilized to minimize variation in growth and yield potential among hybrids. The glyphosate-resistant corn postemergence (glyphosate-POST) treatment provided more consistent control of giant foxtail than the PRE, conventional corn postemergence (conventional-POST), glufosinate-resistant corn postemergence (glufosinate-POST), and imidazolinone-resistant corn postemergence (imi-POST) treatments. All four POST treatments were more consistent and provided greater control than the PRE treatment of the large-seeded broadleaf weeds velvetleaf, giant ragweed, common cocklebur, and morningglory species. Conventional-POST and imi-POST were more consistent than glufosinate-POST and glyphosate-POST treatments in controlling giant ragweed. There were no statistical differences in the variability of PRE or POST treatments for control of common lambsquarters, common ragweed, and redroot pigweed. Corn yield varied among locations and years. The glyphosate-POST treatment did not reduce yield relative to the weed-free treatment, the imi-POST and glufosinate-POST treatments each reduced yield in one of eight locations, and the conventional-POST treatment reduced yield in three of eight locations. Nomenclature: Metolachlor; atrazine; nicosulfuron; rimsulfuron; dicamba; imazethapyr; imazapyr; glufosinate; glyphosate; common cocklebur, Xanthium strumarium L. #3 XANST; common lambsquarters, Chenopodium album L. # CHEAL; common ragweed, Ambrosia artemisiifolia L. # AMBEL; giant foxtail, Setaria faberi (L.) Herrm. # SETFA; giant ragweed, Ambrosia trifida L. # AMBTR; morningglory species, Ipomea spp. # IPOSS; redroot pigweed, Amaranthus retroflexus L. # AMARE; velvetleaf, Abutilon theophrasti Medicus # ABUTH; corn, Zea mays L. # ZEAMX. Additional index words: Corn yield, herbicide-resistant crop, herbicide system, Ipomea species, conventional corn, imidazolinone. Abbreviations: AMS, ammonium sulfate; crm, comparative relative maturity; DAP, days after postemergence.

Inbred corn response to acetamide herbicides as affected by safeners and microencapsulation

Corn inbreds are often more sensitive to herbicides than hybrids. Field experiments were conducted with three corn inbreds to (1) evaluate inbred sensitivity to the acetamide herbicides acetochlor, dimethenamid, flufenacet, and metolachlor, (2) compare the effects of various crop safeners in combination with acetochlor and metolachlor, and (3) measure the effect of herbicide microencapsulation on acetochlor injury. Herbicides were applied preemergence at the registered rate and at two, three, or four times the registered rate in corn. Injury ratings, plant population, and the percentage of plants showing acetamide injury symptoms were used to measure herbicide effect. The inbreds ‘Mo17’ and ‘Great Lakes 15’ (GL15) were sensitive to acetamide injury. Reductions in plant population and increases in the injury rating and the percentage of injured plants were caused by acetochlor, dimethenamid, flufenacet, metolachlor, and flufenacet + metribuzin when applied at three times the registered rate. The inbred ‘B73’ was not injured. The safeners benoxacor and dichlormid reduced injury caused by metolachlor. The percentage of plants injured by metolachlor 15 days after treatment (DAT) was lower when benoxacor was the safener compared to dichlormid. By 28 DAT, plants treated with safeners recovered from injury, and there were no differences between the treatments. The safeners dichlormid and furilazole reduced, but did not always eliminate, injury caused by acetochlor applied at three times the registered rate. Microencapsulation of acetochlor reduced injury to GL15. When the safeners dichlormid or furilazole were included in an acetochlor formulation, microencapsulation did not further reduce corn injury. Nomenclature: Acetochlor; benoxacor; dichlormid; dimethenamid; flufenacet; furilazole; metolachlor; metribuzin; corn, Zea mays L. Additional index words: Controlled release formulation, herbicide antidote, microencapsulation, safener. Abbreviations: DAT, days after treatment; GL15, ‘Great Lakes 15’.