Giant ragweed was the first glyphosate resistant weed identified in Canada. It is a very competetive weed in row crop production and has been found to drastically reduce yields of soybean; therefore, control of this competitive weed is essential. The objective of this study was to determine effective control options for glyphosate resistant giant ragweed in soybean with herbicides applied preplant. Eighteen herbicide combinations were evaluated in field studies conducted in 2011 and 2012 at five locations with confirmed glyphosate resistant giant ragweed. Glyphosate plus 2,4-D ester or amitrole provided the best control of glyphosate resistant giant ragweed 4 WAA. Glyphosate plus 2,4-D ester provided 98 to 99% control and was equivalent to the weed free check at all locations. Glyphosate plus amitrole provided 90% to 93% control and was equivalent to the weed free check at 4 of 5 locations. Herbicides providing residual activity provided variable control across all locations. Of the herbicides with residual activity evaluated, glyphosate plus linuron provided the best control of glyphosate resistant giant ragweed; however, control was inconsistent across locations and years. Glyphosate plus linuron provided 23% to 99% controland was equal to the weed free check at one location 8 WAA.
Glyphosate is a nonselective herbicide that inhibits the enzyme 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) [1,2]. The inhibition of EPSPS stops plants from synthesizing certain aromatic acids that are essential for plant growth [
Glyphosate has limited mobility and is rapidly degraded in the soil. Glyphosate is rapidly inactivated when applied to the soil due to adsorption to clay and organic matter through the phosphonic acid moiety [
There has been rapid adoption of glyphosate resistant crops since the introduction of glyphosate resistant soybean in 1996 [
It was previously thought that glyphosate resistance was very unlikely to occur in weeds. Some of the reasons for the remote possibility of glyphosate resistant weeds included glyphosate’s unique mode of action, limited uptake from the soil and rapid degradation in the soil [
Ambrosia trifida L., commonly known as giant ragweed, is found in cultivated fields and orchards as well as non-cropped environments such as roadside ditches and river banks [
Glyphosate resistant giant ragweed is an increasing problem in glyphosate resistant cropping systems in Ontario. As of 2010 there were 48 locations confirmed with glyphosate-resistant giant ragweed in Ontario [
Field studies were conducted in 2011and 2012 at five locations with confirmed glyphosate resistant giant ragweed. The field sites were located near Windsor (L2 and L5), La Salle (L1 and L4) and Amherstburg (L3), Ontario. Two sets of experiments evaluating the effectiveness of glyphosate tankmixes with herbicides applied preplant were conducted which are referred to as “enhanced burndown” and “burndown plus residual”. Soil texture, soil organic matter content, soil pH, soybean cultivar, seeding date, seeding rate, row spacing, herbicide application date and giant ragweed height are presented in
Experiments were set up in a randomized complete block design with four replications. Each plot was 8 m long and 2.5 m wide. The first experiment (enhanced burndown) evaluated herbicides applied preplant (PP) that provided limited or no residual control. Herbicides evaluated in this study are listed in Tables 2-5. The second experiment (burndown plus residual) evaluated herbicides applied PP that provided burndown plus residual control. Herbicides evaluated in this study are listed in Tables 6-10. The herbicide rates used were the maximum labeled rate registered for use in Ontario. A weedy and weed-free check was included in each experiment. All weed-free check plots were maintained with 2,4-D ester (500 g a.e. ha−1) plus glyphosate (900 g a.e. ha−1) applied PP and subsequent hand hoeing as required.
Herbicide treatments were applied with a CO2-pressurized backpack sprayer equipped with ULD 120-02 flat fan nozzles (Hypro, New Brighton, MN) calibrated to deliver 200 L·ha−1 of water at 210 kPa. Herbicide treatments were applied with a 1.5 meter boom with four nozzles spaced 50 cm apart over the centre of the plot. Herbicide treatments were applied when giant ragweed reached 15 cm in height (
Percent weed control was rated 1, 2 and 4 WAA and 1
2, 4 and 8 WAA in the enhanced burndown and burndown plus residual experiments, respectively. Weed control was rated visually on a scale of 0% to 100%, where 0% was no control of giant ragweed compared to the weedy check and 100% was complete control of giant ragweed. At each control rating giant ragweed height and density (plants per two 0.25 m2 quadrats) were recorded. At 4 WAA, giant ragweed density and biomass was determined in each plot by counting giant ragweed plants in two 0.25 m2 quadrats. Giant ragweed plants were cut off at the soil surface from the two quadrats, placed in bags, dried at 60˚C to a constant moisture content and the dry weights were recorded. Soybean injury was rated 1, 2, 4 and 8 WAA. Soybean injury was rated visually on a scale of 0% to 100%, where 0% was no soybean injury and 100% was soybean death. At crop maturity, soybeans were hand harvested from 2 m of row from each plot at all locations. Soybeans were threshed in a stationary thresher and the weight and moisture were recorded. Yields were adjusted to 13.5% moisture.
An analysis of variance was conducted on all data using the PROC MIXED procedure in SAS (Ver. 9.2, SAS Institute Inc., Cary, NC). Variances were separated into the random effects of location (year and location), replication (at each location) and location by treatment. Herbicide treatment was considered the fixed effect. The significance of the random effects (location, replication and location by treatment) and their interaction with fixed effects was tested using the Z-test of the variance estimate. The significance of the fixed effects was tested using the F-test. Significant location by treatment interactions were found for all variables; therefore, locations were analyzed according to their interaction and presented accordingly. To ensure the assumptions (errors are independent, homogenous and normally distributed) of the variance analysis were met; residual plots were examined. Data were tested for normality using the Shapiro-Wilk statistic as generated by the UNIVARIATE procedure in SAS. If necessary, a transformation of the data (natural log, square root or arcsine square root) was applied and chosen based on the highest Shapiro-Wilk statistic generated. The means between treatments were produced and separated using Fisher’s protected LSD at P < 0.05.
The herbicides evaluated in the enhanced burndown experiment did not cause any injury in soybean (data not shown).
For control 1 WAA, data were analyzoped separately in groups L1, L2, L3, L4, and L5. At 1 WAA, all the herbicides evaluated provided better control of glyphosate resistant giant ragweed compared to glyphosate alone. Control with glyphosate ranged from 31% to 43% at 1 WAA (
For control 2 WAA, data was analyzed separately in groups L1, L2, L3, L4, and L5. Glyphosate provided only 29% to 35% control (
At 4 WAA L2 and L5, L3 and L4 could be combined while L1, was analyzed separately. Glyphosate provided 26% to 39% control 4 WAA (
zL1, LaSalle; L2, Windsor; L3, Amherstburg; L4, LaSalle; L5, Windsor; WAA, days after herbicide application; yIncluded merge at 1.0% vol/vol; xIncluded merge at 1.0% vol/vol; wIncluded Agral 90 at 0.25% vol/vol plus UAN 28%; a-kMeans followed by the same letter are not significantly different according to fisher’s protected LSD at P < 0.05.
zL1, LaSalle; L2, Windsor; L3, Amherstburg; L4, LaSalle; L5, Windsor; WAA, days after herbicide application; yIncluded merge at 1.0% vol/vol; xIncluded merge at 1.0% vol/vol; wIncluded Agral 90 at 0.25% vol/vol plus UAN 28%; a-kMeans followed by the same letter are not significantly different according to fisher’s protected LSD at P < 0.05.
zL1, LaSalle; L2, Windsor; L3, Amherstburg; L4, LaSalle; L5, Windsor; WAA, days after herbicide application; yIncluded Merge at 1.0% vol/vol; xIncluded merge at 1.0% vol/vol; wIncluded Agral 90 at 0.25%vol/vol plus UAN 28%; a-hMeans followed by the same letter are not significantly different according to fisher’s protected LSD at P < 0.05.
mid-p at 245 g a.i. ha−1, respectively applied alone 4 weeks after corn emergence. Glyphosate plus carfentrazone, glufosinate, chlorimuron-ethyl, flumioxazin, or chlorimuron-ethyl plus flumioxazin provided up to 76%, 77%, 51%, 44% and 46% control, respectively (
For giant ragweed shoot dry weight all data were combined and analyzed. Glyphosate reduced giant ragweed shoot dry weight by 13% (
For soybean yield L1, L2, and L4 were combined and L3 and L5 were combined and analyzed. Giant ragweed interference caused a reduction in soybean yield of 43% to 87% across all sites (
zL1, LaSalle; L2, Windsor; L3, Amherstburg; L4, LaSalle; L5, Windsor; yIncluded merge at 1.0% vol/vol; xIncluded merge at 1.0% vol/vol; wIncluded Agral 90 at 0.25% vol/vol plus UAN 28%; a-iMeans followed by the same letter are not significantly different according to fisher’s protected LSD at P < 0.05.
80%, 82%, and 83% and were equivalent to the weedy control (
The herbicides evaluated in the burndown plus residual experiment did not cause soybean injury (data not shown).
For control 1 WAA, data was statistically analyzed for each site separately except for L4 and L5 which were combined and analyzed. One WAA all the herbicides evaluated provided better control of glyphosate resistant giant ragweed compared to glyphosate alone. Control with glyphosate ranged from 35% to 65% (
For control 2 WAA, all data were analyzed separately. Control was generally higher with all herbicides evaluated for group L1 compared to L2, L3, L4 and L5 and may be due to smaller giant ragweed (up to 9 cm in height) at the time of application and lower giant ragweed density (
zL1, LaSalle; L2, Windsor; L3, Amherstburg; L4, LaSalle; L5, Windsor, WAA, days after herbicide application; a-iMeans followed by the same letter are not significantly different according to fisher’s protected LSD at P < 0.05.
zL1, LaSalle; L2, Windsor; L3, Amherstburg; L4, LaSalle; L5, Windsor; WAA, days after herbicide application; a-kMeans followed by the same letter are not significantly different according to fisher’s protected LSD at P < 0.05.
phosate plus saflufenacil/dimethenamid-P provided 79% to 92% control and was consistent with control ratings 1 WAA. Glyphosate plus metribuzin or cloransulam-methyl provided 57% to 94% and 68% to 88% control respectively (
At 4 WAA L2 and L5 could be combined and L1, L3 and L4 were analyzed separately. Glyphosate provided 19% to 45% control 4 WAA (
At 8 WAA L4 and L5 could be combined and L1, L2 and L3 were analyzed separately. Glyphosate provided less than 31% control across all sites 8 WAA (
zL1, LaSalle; L2, Windsor; L3, Amherstburg; L4, LaSalle; L5, Windsor; WAA, days after herbicide application; a-kMeans followed by the same letter are not significantly different according to fisher’s protected LSD at P < 0.05.
zL1, LaSalle; L2, Windsor; L3, Amherstburg; L4, LaSalle; L5, Windsor; WAA, days after herbicide application; a-gMeans followed by the same letter are not significantly different according to fisher’s protected LSD at P < 0.05.
to 98% control 8 WAAwith glyphosate plus linuron applied at 900 g a.e. ha−1 + 2250 g a.i. ha−1. Glyphosate plus cloransulam-methyl provided 45% to 90% control (
For giant ragweed shoot dry weight L2, L3, L4, and L5 could be combined while L1 was analyzed separately.There was a greater reduction in giant ragweed shoot dry weight at L1 (
zL1, LaSalle; L2, Windsor; L3, Amherstburg; L4, LaSalle; L5, Windsor; a-jMeans followed by the same letter are not significantly different according to fisher’s protected LSD at P < 0.05.
[
Soybean yield at L2, L3, L4 and L5 could be combined and L1 was analyzed separately. Giant ragweed interference caused a reduction in soybean yield by 80% to 86% across all sites (
In summary, glyphosate plus 2,4-D ester or amitrole provided the best control of glyphosate resistant giant ragweed. Herbicides providing residual activity provided variable control across all locations. Out of the burndown plus residual herbicides evaluated, glyphosate plus linuron provided the best control of glyphosate resistant giant ragweed. Although this herbicide provided the highest control of glyphosate resistant giant ragweed, control was not always acceptable across all locations. This research shows that control of glyphosate resistant giant ragweed is needed early in the season when plants are small. Future research should look at other herbicide tankmixes with multiple modes of action.
The authors acknowledge Chris Kramer for his expertise and technical assistance in these studies. Funding for this project was provided in part by Monsanto Canada Inc., the Grain Farmers of Ontario and the Agricultural Adaptation Council through the Canadian Agricultural Adaptation Program.