Robert E. Nurse

Corn competition alters the germinability of velvetleaf (Abutilon theophrasti ) seeds

Abstract Velvetleaf is a troublesome annual weed in many cropping systems of the United States and Canada. Differences in the growing environment of parent plants can influence the number, structure, germinability, and viability of seeds produced. Thus, the effects across a range of competitive environments and corn planting dates on velvetleaf seed production, germination, and seed coat weight were examined under field conditions. Seed production of velvetleaf increased with increasing biomass. Total velvetleaf reproductive output was reduced in competition with corn compared with monoculture stands. Corn planting date had no effect on the dormancy status of seeds, but increased competition from corn resulted in up to a 30% decrease in the proportion of seeds that were dormant. Seed and seed coat weights also decreased for plants of velvetleaf grown in competition with corn compared with those grown in monoculture. These findings suggest that velvetleaf plants growing in relatively noncompetitive environments, such as along field edges or in field areas with poor crop stands, are likely not only to produce a greater number of seeds but also a greater proportion of seeds that are dormant. This alteration in the dormancy status of velvetleaf seeds in the absence or presence of a crop provides unique opportunities for effective long-term management of the soil seedbank in this species, especially for velvetleaf individuals bordering fields or growing in fallow areas that might require more stringent control because of increased seed dormancy. Nomenclature: Velvetleaf, Abutilon theophrasti Medic. ABUTH; corn, Zea mays L. ‘Dekalb 520RR’.

Environment and Soil Conditions Influence Pre- and Postemergence Herbicide Efficacy in Soybean

Abstract Deciding on the most efficacious PRE and POST herbicide options and their ideal application timing can be challenging for soybean producers. Climatic events during the 14 d before and after herbicide application can further complicate decisions because of their influence on herbicide effectiveness. Nine field trials were conducted at three locations in southwestern Ontario from 2003 to 2006, to determine the most effective PRE and POST soybean herbicides for control of common lambsquarters, common ragweed, green foxtail, and redroot pigweed. When precipitation was low at least 7 d before and after herbicide application weed control was reduced in treatments that included imazethapyr (PRE or POST) or flumetsulam/S-metolachlor (a premix formulation) (PRE). Cumulative precipitation during the 12 d after PRE application that exceeded the monthly average by at least 60% reduced common lambsquarters control when metribuzin was applied and green foxtail control when imazethapyr was applied. Delaying application of imazethapyr + bentazon to a later soybean growth stage decreased control of common lambsquarters and green foxtail; however, environmental conditions appeared to influence these results. Precipitation on the day of application decreased control of common ragweed and redroot pigweed more with quizalofop-p-ethyl + thifensulfuron-methyl + bentazon compared with imazethapyr + bentazon. Soybean yield varied among POST herbicide treatments because of reduced weed control. This research confirms that environmental conditions pre- and postapplication, as well as application timing, influence herbicide efficacy and should be considered by growers when selecting an herbicide program. Nomenclature: Bentazon; cloransulam-methyl; flumetsulam; glyphosate; imazethapyr; linuron; metribuzin; quizalofop-p-ethyl; S-metolachlor; thifensulfuron-methyl; redroot pigweed, Amaranthus retroflexusL.; common ragweed, Ambrosia artemisiifolia L.; common lambsquarters, Chenopodium album L.; green foxtail, Setaria viridis (L.) Beauv.; soybean, Glycine max L.

Time of Day Impacts Postemergence Weed Control in Corn

Abstract Field trials were conducted from 2005 to 2007 at two locations in southwestern Ontario to investigate how weed control in corn was affected by the time of day that herbicides were applied. Weed control following the application of six POST herbicides (atrazine, bromoxynil, dicamba/diflufenzopyr, glyphosate, glufosinate, and nicosulfuron) at 06:00, 09:00, 12:00, 15:00, 18:00, 21:00, and 24:00 h was assessed. For many weed species herbicide efficacy was reduced when applications were made at 06:00, 21:00, and 24:00 h. Velvetleaf was the most sensitive to the time of day effect, followed by common ragweed, common lambsquarters, and redroot pigweed. Annual grasses were not as sensitive to application timing; however, control of barnyardgrass and green foxtail was reduced in some environments at 06:00 h and after 21:00 h. Only in the most severe cases was the grain yield of corn reduced due to decreased weed control. Daily changes in air temperature, relative humidity, and light intensity that cause species-specific physiological changes may account for the variation in weed control throughout the day. The results of this research suggest that there is a strong species-specific influence of ambient air temperature, light intensity, and leaf orientation on the efficacy of POST herbicides. These results should aid growers in applying herbicides when they are most efficacious, thus reducing costs associated with reduced efficacy. Nomenclature: Atrazine; bromoxynil; dicamba; diflufenzopyr; glufosinate; glyphosate; nicosulfuron; barnyardgrass, Echinochloa crus-galli (L.) Beauv.; common lambsquarters, Chenopodium album L.; common ragweed, Ambrosia artemisiifolia L.; green foxtail, Setaria viridis (L.) Beauv.; redroot pigweed, Amaranthus retroflexus L.; velvetleaf, Abutilon theophrasti L.; corn, Zea mays L.

Weed Control and Yield Response to Foramsulfuron in Corn

Foramsulfuron has recently been registered for weed control in corn in Ontario, but there is very little information on the rate of foramsulfuron required to obtain at least 90% weed control. Our objective was to determine the foramsulfuron rates giving at least 90% weed control while maintaining crop yield loss due to weed interference and injury at less than 5%. Ten field trials were conducted at five Ontario locations (Exeter, Harrow, Ridgetown, Woodslee, and Woodstock) in 2001 and 2002 to evaluate the effectiveness of foramsulfuron at rates ranging from 8.75 to 140 g ai/ha. To obtain a reduction in biomass of 90% (I90) at 78 d after treatment (DAT), foramsulfuron must be applied to common lambsquarters at 68 g/ha and to common ragweed at 86 g/ha, respectively. For green foxtail a foramsulfuron rate of 25 g/ha was required to achieve 90% control. The application of foramsulfuron caused injury to corn at 7 DAT at Ridgetown and Woodstock only, but did not exceed a rating of 10%; by 14 and 28 DAT no corn injury was recorded at any location. Corn yield of at least 95% of a weed-free check was obtained at Woodstock when foramsulfuron was applied at 70 g/ha. At Exeter and Woodslee yield was 90% of the weed-free check at a foramsulfuron rate of 35 g/ha. Finally, at Harrow and Ridgetown, corn yield was lowered at all foramsulfuron rates because of broadleaved weed interference. Tank-mixing foramsulfuron with dicamba plus prosulfuron improved common lambsquarters and common ragweed control and final corn yield was improved by more than 20% when compared with an application of foramsulfuron alone. Thus, these results show that weed control with foramsulfuron is species specific and that tank mixtures with a broadleaf herbicide may be required for broad-spectrum weed control and to protect the full yield potential of corn. Nomenclature: Foramsulfuron; common ragweed, Ambrosia artemisiifolia L. AMBEL; common lambsquarters, Chenopodium album L. CHEAL; green foxtail, Setaria viridis (L.) Beauv. SETVI; corn, Zea mays L.

Response of Pinto and Small Red Mexican Bean to Postemergence Herbicides

Four field trials were conducted over a 2-yr period at Exeter (2005, 2006), Harrow (2006) and Ridgetown (2006), Ontario to evaluate the tolerance of pinto and small red Mexican (SRM) bean to the POST application of bentazon, imazethapyr, or their combination. Bentazon applied once or twice (to simulate a spray overlap in the field) at 840 g ai/ha and imazethapyr applied at 37.5 g/ha caused minimal injury (6% or less) in pinto and SRM bean and had no adverse effect on plant height, shoot dry weight, seed moisture content, and yield. Imazethapyr applied twice at 37.5 and all single and repeat applications containing 75 or 150 g/ha caused 15 to 44% injury to dry bean. These injuries were persistent and reduced plant height by as much as 21% and shoot dry weight by as much as 34%, but caused no adverse effect on maturity and yield, except for imazethapyr applied twice at 150 g/ha, which delayed maturity and reduced yield 16%. The addition of bentazon to imazethapyr applied as a tankmix reduced injury by as much as 23%. Imazethapyr at 37.5 or 75 g/ha combined with bentazon at 840 g/ha applied once or twice caused 3 to 23% injury but had no adverse effect on plant height, shoot dry weight, maturity, or yield. Two applications of imazethapyr at 150 g/ha plus bentazon at 840 g/ha reduced plant height 16% and shoot dry weight 28%. Nomenclature: Bentazon, imazethapyr, pinto bean, small red Mexican bean, Phaseolus vulgaris L

Weed Control, Environmental Impact, and Economics of Weed Management Strategies in Glyphosate-Resistant Soybean

Abstract With the number of glyphosate-resistant weed species increasing in North America and a lack of new herbicide chemistries being developed, growers are shifting toward using older herbicides that are more expensive and may be less environmentally friendly. Therefore, to determine which weed management strategies are most cost effective and have the lowest impact on the environment we evaluated the efficacy, environmental impact, and the profitability of several weed management strategies in glyphosate-resistant soybean over a 3-yr period (2007 to 2009) at three locations in southwestern Ontario, Canada. No visible injury to soybean was observed with the herbicide treatments evaluated. A sequential application of glyphosate consistently provided high levels of weed control (99 to 100%) at 56 d after treatment in comparison with one- or two-pass herbicide programs. Soybean yield did not differ between the two-pass herbicide programs and glyphosate applied early POST; however, a yield benefit was found with a sequential application of glyphosate or a PRE herbicide followed by glyphosate compared with glyphosate applied only at late POST. The two-pass herbicide programs had higher environmental impact (EI) (> 23) than the one-pass herbicide programs (< 15), except when imazethapyr was followed by or tank-mixed with glyphosate, which had an equivalent EI (∼ 14) to the one-pass herbicide programs. Not surprisingly because of the low purchase price of glyphosate, gross margins were highest for treatments that included glyphosate. However, to reduce the selection pressure on glyphosate-resistant weed biotypes, to reduce environmental impact, and to increase gross margins a combination of glyphosate with another mode of action would be most beneficial. In this study glyphosate + imazethapyr was the best alternative to a sequential two-pass glyphosate program. Nomenclature: Flumetsulam; glyphosate; imazethapyr; s-metolachlor; soybean, Glycine max L.; common lambsquarters, Chenopodium album L. CHEAL; common ragweed, Ambrosia artemisiifolia L. AMBEL; green foxtail, Setaria viridis (L.) Beauv. SETVI; redroot pigweed, Amaranthus retroflexus L. AMARE; velvetleaf, Abutilon theophrasti Medic. ABUTH.

Response of Corn to Simulated Glyphosate Drift Followed by In-Crop Herbicides

Abstract Thirteen field experiments were conducted in Illinois, Indiana, Ohio, and Ontario from 2005 to 2007 to determine the effects of simulated glyphosate drift followed by in-crop applications of nicosulfuron/rimsulfuron plus dicamba/diflufenzopyr or foramsulfuron plus bromoxynil plus atrazine on nontransgenic corn injury, height, stand count, shoot dry weight, and yield. Simulated glyphosate drift at 100 and 200 g/ha, resulted in 11 to 61% visual crop injury and a 19 to 45% decrease in corn height. Simulated glyphosate drift at 200 g/ha caused a reduction in shoot dry weight by 46%, stand count by 28% and yield by 49 to 56%. Generally, simulated glyphosate drift followed by the in-crop herbicides resulted in an additive response with respect to visual crop injury, height, stand count, shoot dry weight, and yield. Nomenclature: Atrazine; bromoxynil; dicamba/diflufenzopyr; foramsulfuron; glyphosate; nicosulfuron/rimsulfuron; corn, Zea mays L

The Effect of Time of Day on the Activity of Postemergence Soybean Herbicides

Abstract The effect of time of day (TOD) on the activity of six common POST herbicides was investigated in field trials from 2007 to 2009 at two locations in southwestern Ontario. Percentage weed control was assessed following application of bentazon, chlorimuron-ethyl, fomesafen, glyphosate, imazethapyr, or quizalofop-p-ethyl applied at 3-h intervals from 6:00 A.M. to midnight, when weeds averaged 15 cm tall. The effect of time of day varied with weed species, but weed control was generally reduced when herbicides were applied at 6:00 A.M., 9:00 P.M., and midnight. Herbicide activity on velvetleaf was most frequently reduced, especially for chlorimuron-ethyl, glyphosate, and imazethapyr. Control of common ragweed with glyphosate and imazethapyr was also affected by the timing of application, and pigweed species only showed an effect with glyphosate. Variation in temperature, relative humidity, and dew presence/absence at different times of the day, as well as morphological/physiological characteristics such as weed size at time of application and diurnal leaf movement in response to light intensity, may account for the variation in weed control at different times of the day. Significant soybean yield loss was not observed in this study, but may occur if herbicide efficacy is severely reduced by application at inappropriate times of day. These results provide valuable information for growers, and suggest that POST herbicides are most effective when applied midday, rather than in the early morning or late evening. Nomenclature: Bentazon; chlorimuron-ethyl; fomesafen; glyphosate; imazethapyr; quizalofop-p-ethyl; common ragweed, Ambrosia artemisiifolia L. AMBEL; pigweed species, Amaranthus sp; velvetleaf, Abutilon theophrasti Medic., ABUTH; soybean, Glycine max (L.) Merr. Resumen El efecto del momento de aplicación durante el día (TOD) en la actividad de seis herbicidas POST comunes fue investigado en experimentos de campo desde 2007 a 2009 en dos localidades del suroeste de Ontario. El porcentaje de control de malezas fue evaluado después de la aplicación de bentazon, chlorimuron-ethyl, fomesafen, glyphosate, imazethapyr, o quizalofop-p-ethyl, aplicados en intervalos de 3 horas desde 6:00 A.M. hasta medianoche, cuando las malezas tuvieron una altura promedio de 15 cm. El efecto del momento de aplicación durante el día varió dependiendo de la especie de malezas, pero el control de malezas fue generalmente reducido cuando los herbicidas se aplicaron a 6:00 A.M., 9:00 P.M., y medianoche. La actividad herbicida se redujo más frecuentemente en Abutilon theophrasti, especialmente con chlorimuron-ethyl, glyphosate, e imazethapyr. El control de Ambrosia artemisiifolia con glyphosate e imazethapyr también fue afectado por el momento de aplicación, y las especies del género Amaranthus solamente mostraron efectos con glyphosate. Variaciones en temperatura, humedad relativa, y la presencia/ausencia de rocío en diferentes momentos del día, además de las características morfológicas/fisiológicas, tales como el tamaño de las malezas al momento de aplicación, y el movimiento diario de hojas en respuesta a la intensidad lumínica, podrían explicar la variación en el control de malezas en diferentes momentos del día. En este estudio, no se observaron pérdidas significativas en el rendimiento de la soya, pero estas podrían ocurrir si la eficacia del herbicida es reducida severamente debido a aplicaciones en momentos inapropiados durante el día. Los resultados brindan información valiosa para los productores, y sugieren que los herbicidas POST son más efectivos cuando son aplicados al mediodía, en lugar de las aplicaciones temprano en la mañana o tarde al final del día.

Weed Control in White Bean with Various Halosulfuron Tankmixes

Four field trials were conducted over a three-year period (2011–2013) in southwestern Ontario to evaluate the level of weed control provided by various halosulfuron tankmixes applied preplant incorporated (PPI) in white bean. Trifluralin, s-metolachlor, halosulfuron, and imazethapyr applied alone or in combination caused 4% or less visible injury 1 and 4 weeks after emergence (WAE) in white bean. Trifluralin, s-metolachlor, halosulfuron, and imazethapyr applied PPI provided 80–96%, 84–95%, 83–100%, and 75–92% control of redroot pigweed; 19–28%, 30–40%, 97–99%, and 73–84% control of common ragweed; 94–96%, 63–82%, 96–100%, and 96–100% control of common lambsquarters; 14-15%, 12–35%, 100%, and 96–97% control of wild mustard; and 96–97%, 95–97%, 53–56%, and 80–82% control of green foxtail, respectively. The two- and three-way tankmixes of halosulfuron with trifluralin, s-metolachlor, or imazethapyr provided 85–100% control of redroot pigweed, 90–98% control of common ragweed, 97–100% control of common lambsquarters, 100% control of wild mustard, and 93–98% control of green foxtail. Weed density, weed biomass and white bean seed yields reflected the level of visible weed control.

Acetyl‐CoA carboxylase overexpression in herbicide‐resistant large crabgrass (Digitaria sanguinalis)

BACKGROUND The occurrence of herbicide-resistant weed biotypes is increasing and this report of an acetyl-CoA carboxylase (ACCase) inhibitor-resistant Digitaria sanguinalis L. Scop. from southwestern Ontario is another example. The identified weed escaped control in an onion and carrot rotation in which graminicides were used for several consecutive years. Our goal was to characterize the level and mechanism of resistance of the biotype. RESULTS The biotype was resistant to all five ACCase inhibitor herbicides tested. Gene-expression profiling was performed because none of the mutations known to confer resistance in the ACCase gene were detected. RNASeq and quantitative reverse-transcriptase PCR (qRT-PCR) results indicated that transcription of ACCase was 3.4-9.3 times higher in the resistant biotype than the susceptible biotype. ACCase gene copy number was determined by qPCR to be five to seven times higher in the resistant compared with the susceptible biotype. ACCase gene overexpression was directly related to the increase of the ACCase gene copy number. CONCLUSION Our results are consistent with the hypothesis that overexpression of the herbicide target gene ACCase confers resistance to the herbicide. This is the first reported case of target gene duplication conferring resistance to a herbicide other than glyphosate.