Twenty-two field experiments (six maize (Zea mays L.) and five soybean [Glycine max (L.) Merr.] using low glyphosate doses to assess weed control and six maize and five soybean using high glyphosate doses to assess tolerance) were conducted from 2010 to 2012 at two locations in southern Ontario, Canada to compare the commercially available glyphosate formulations of Roundup Weather MAX?, Clearout?41 Plus, and Wise Up? (WeatherMAX, Clearout, and WiseUp, respectively). Inmaize and soybean, control of velvetleaf, pigweed species, common lambs quarters, and green foxtail 4 weeks after treatment (WAT) using 900 g·ae·ha-1 ranged from at least 85% to 99%, regardless of formulation. By 8 WAT with 900 g·ae·ha-1, control of these weeds generally declined, but still ranged from 82% to 97% across all formulations. At harvest, maize yields were similar to the weed-free control for 900 g·ae·ha-1 of glyphosate as WeatherMAX and Clearout; however, reduced weed control with WiseUp resulted in an 8.8% yield loss. For soybean, yields were similar to the weed-free control, regardless of formulation or dose. In the tolerance experiments, 2.1% and 2.8% injury was observed 4 WAT for maize treated with 3600 g·ae·ha-1 of glyphosate as WeatherMAX and WiseUp, respectively. However, maize yields were unaffected by glyphosate formulation or dose. In soybean, visible injury of 8.5%, 4.5%, and 3.7% was observed 1 WAT with 5400 g·ae·ha-1 of glyphosate as WeatherMAX, WiseUp, and Clearout, respectively; by 8 WAT, visible injury was similar to the untreated control, regardless of formulation or dose. The early injury from 5400 g·ae·ha-1 of glyphosate resulted in 8.5%, 4.6%, and 5.5% yield loss for the WeatherMAX, WiseUp, and Clearout formulations, respectively.
Within glyphosate-resistant maize and soybean production systems, glyphosate provides a broad spectrum of weed control, low crop phytotoxicity, and greater crop yields [
Recently, research in Ontario has explored the tolerance of glyphosate-resistant maize and soybean to glyphosate formulated as Roundup WeatherMAX® [
A total of twenty-two field experiments (six maize and five soybean using low glyphosate doses to assess weed control and six maize and five soybean using high glyphosate doses to assess tolerance) were conducted over a three-year period (2010 to 2012) at Ridgetown (42.4406˚N, 81.8842˚W) and Exeter (43.3500˚N, 81.4833˚W), Ontario, Canada. All experiments were designed as a randomized complete block, replicated four times. Treatments in the weed control and tolerance experiments conducted in both maize and soybean included glyphosate formulated as a potassium salt in WeatherMAX and glyphosate formulated as an isopropylamine salt in Clearout and WiseUp. In the weed control experiments, treatments included glyphosate applied at 225, 450, and 900 g·ae·ha−1 in addition to weedy and weed-free controls. In the tolerance experiments conducted in maize, treatments included glyphosate applied at 1800 and 3600 g·ae·ha−1 in addition to an untreated, weed-free control whereas in soybean, the glyphosate doses used were 2700 and 5400 g·ae·ha−1. In maize, 1800 g·ae·ha−1 repre- sented the maximum single application glyphosate dose [
In both the weed control and tolerance experiments, each treatment plot was 2 m wide by 8 m (Ridgetown) to 10 m (Exeter) long. Glyphosate-resistant maize hybrids were seeded 4 to 5 cm deep at a rate of 80,000 seeds·ha−1 in rows spaced 0.75 m apart. Glyphosate-resistant soybean cultivars were seeded 3 to 4 cm deep at a rate of 370,000 to 480,000 seeds·ha−1 in rows spaced 0.75 m apart. In the weed control experiments, glyphosate treatments were applied to maize at the 7- to 9-leaf stage (approximately June 13) and to soybean at the 3rd trifoliate leaf stage (approximately July 2). In the tolerance experiments, glyphosate treatments were applied to maize to at the 9- to 11-leaf stage (approximately June 22) and to soybean at the flower initiation stage (approximately July 11); both application timings were generally later than recommended [
In the maize and soybean weed control experiments, visible crop injury and weed control were estimated visually on a scale of 0% (no injury/control) to 100% (complete plant death) relative to untreated control plants. Maize and soybean injury was rated 1, 2, and 4 weeks after glyphosate application (WAT) whereas control of velvetleaf, pigweed species, common ragweed, common lambsquarters, and green foxtail was rated 4 and 8 WAT. In the maize and soybean tolerance experiments, visible crop injury was estimated visually on a scale of 0% (no injury) to 100% (complete plant death) relative to untreated control plants 1, 2, and 4 WAT in maize and 1, 2, 3, 4, and 8 WAT in soybean. At physiological maturity of maize, cob length was assessed by measuring ten fully exposed, randomly selected ears from end to end within each plot. The same cobs were rated for cob deformity on a scale from 1 (completely deformed) to 10 (no deformity) similar to methods used previously [
Maize and soybean injury and weed control data were analyzed separately by crop within the weed control and tolerance experiments using PROC MIXED (SAS Ver. 9.2, SAS Institute Inc., Cary, NC). In the individual analysis of the experiments, variances were divided into fixed (glyphosate treatment) and random effects [block; environment (i.e., year or location-year combinations); block nested within environment; and the herbicide treatment × environment interaction]. Significance of the fixed effect was tested using F-test and random effects were tested using a Z-test of the variance estimate. The herbicide treatment × environment interactions in maize and soybean experiments were not significant and the data for each set of experiments were pooled across environments within each crop for the weed control and tolerance experiments. PROC UNIVARIATE in SAS was used to test data for normality and homogeneity of variance. For all crop injury and weed control ratings, the untreated control treatment (assigned a value of zero) was excluded from the analyses. However, all values were compared independently to zero to evaluate treatment differences with the untreated control. To satisfy the assumptions of the variance analyses, data were transformed with the appropriate transformation as needed to improve normality. Means were separated using Fisher’s Protected LSD at P < 0.05 and any data compared on the transformed scale were converted back to the original scale for presentation of results.
For maize and soybean, no crop injury symptoms were noted at any of the observation timings, regardless of glyphosate formulation or dose (data not shown), consistent with previous research [
In general, weed control in maize increased with glyphosate dose at 4 and 8 WAT (
Weed Control | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|
Treatment | Dose | ABUTH | AMASS | AMBEL | CHEAL | SETVI | |||||
g·ae·ha−1 | --------------------------------------------------------%-------------------------------------------------------- | ||||||||||
Weedy control | 0 | d | 0 | f | 0 | f | 0 | h | 0 | e | |
Weed-free control | 100 | a | 100 | a | 100 | a | 100 | a | 100 | a | |
WeatherMAX | 225 | 6 | d | 69 | e | 59 | e | 44 | fg | 61 | cd |
WeatherMAX | 450 | 50 | c | 91 | cd | 80 | cd | 80 | cd | 81 | bc |
WeatherMAX | 900 | 84 | ab | 97 | bc | 92 | b | 93 | bc | 95 | ab |
Clearout | 225 | 11 | d | 72 | e | 55 | e | 34 | g | 54 | d |
Clearout | 450 | 45 | c | 89 | d | 76 | d | 71 | de | 67 | cd |
Clearout | 900 | 86 | ab | 99 | ab | 92 | b | 89 | bc | 92 | b |
WiseUpc | 225 | 12 | d | 70 | e | 61 | e | 56 | ef | 70 | cd |
WiseUpc | 450 | 65 | bc | 91 | cd | 84 | c | 84 | bcd | 82 | bc |
WiseUpc | 900 | 87 | a | 98 | ab | 94 | b | 95 | b | 95 | ab |
aAbbreviations: ABUTH, velvetleaf; AMASS, pigweed species; AMBEL, common ragweed; CHEAL, common lambsquarters; SETVI, green foxtail; WAT, weeks after glyphosate treatment. bMeans followed by the same letter within a column are not significantly different according to Fisher’s Protected LSD (P < 0.05). cIncluded N-Tank at 0.5% v/v.
Weed control | |||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Treatment | Dose | ABUTH | AMASS | AMBEL | CHEAL | SETVI | Yield | ||||||
g·ae·ha−1 | -----------------------------------------------%----------------------------------------------- | T·ha−1 | |||||||||||
Weedy control | 0 | e | 0 | f | 0 | f | 0 | f | 0 | e | 7.6 | e | |
Weed-free control | 100 | a | 100 | a | 100 | a | 100 | a | 100 | a | 13.7 | a | |
WeatherMAX | 225 | 1 | e | 59 | e | 45 | e | 36 | e | 39 | d | 11.4 | cd |
WeatherMAX | 450 | 46 | cd | 85 | d | 73 | cd | 78 | bc | 65 | bcd | 12.3 | bcd |
WeatherMAX | 900 | 83 | ab | 95 | bc | 90 | b | 89 | b | 85 | b | 13.0 | ab |
Clearout | 225 | 2 | e | 59 | e | 44 | e | 30 | e | 39 | d | 11.2 | d |
Clearout | 450 | 33 | d | 87 | cd | 66 | d | 67 | cd | 60 | bcd | 12.5 | bc |
Clearout | 900 | 83 | ab | 97 | b | 89 | b | 87 | bc | 82 | bc | 12.9 | ab |
WiseUpc | 225 | 4 | e | 66 | e | 49 | e | 48 | de | 56 | cd | 11.9 | bcd |
WiseUpc | 450 | 62 | bc | 87 | cd | 77 | c | 81 | bc | 71 | bc | 12.5 | bc |
WiseUpc | 900 | 86 | a | 96 | b | 90 | b | 89 | b | 82 | bc | 12.5 | bc |
aAbbreviations: ABUTH, velvetleaf; AMASS, pigweed species; AMBEL, common ragweed; CHEAL, common lambsquarters; SETVI, green foxtail; WAT, weeks after glyphosate treatment. bMeans followed by the same letter within a column are not significantly different according to Fisher’s Protected LSD (P < 0.05). cIncluded N-Tank at 0.5% v/v.
For soybean, weed control increased with glyphosate dose and excellent control was observed 4 and 8 WAT when 900 g·ae·ha−1 was applied at the 3rd trifoliate leaf stage for nearly every weed species tested, regardless of glyphosate formulation (
At all observation timings, visible injury levels of maize treated with 1800 g·ae·ha−1 of glyphosate applied at the 9- to 11-leaf stage were similar to the untreated control, regardless of formulation (
Weed Control | |||||||||
---|---|---|---|---|---|---|---|---|---|
Treatment | Dose | ABUTH | AMASS | CHEAL | SETVI | ||||
g·ae·ha−1 | ----------------------------------------------------%---------------------------------------------------- | ||||||||
Weedy control | 0 | f | 0 | e | 0 | f | 0 | e | |
Weed-free control | 100 | a | 100 | a | 100 | a | 100 | a | |
WeatherMAX | 225 | 28 | ef | 72 | d | 58 | de | 83 | d |
WeatherMAX | 450 | 52 | cd | 95 | b | 90 | bc | 95 | bc |
WeatherMAX | 900 | 85 | ab | 100 | a | 97 | b | 99 | ab |
Clearout | 225 | 28 | ef | 79 | cd | 51 | e | 82 | d |
Clearout | 450 | 41 | de | 94 | b | 68 | d | 94 | bc |
Clearout | 900 | 89 | ab | 100 | a | 93 | bc | 98 | ab |
WiseUpc | 225 | 36 | de | 84 | c | 62 | de | 86 | cd |
WiseUpc | 450 | 69 | bc | 98 | ab | 88 | c | 97 | ab |
WiseUpc | 900 | 93 | a | 100 | a | 96 | bc | 99 | ab |
aAbbreviations: ABUTH, velvetleaf; AMASS, pigweed species; CHEAL, common lambsquarters; SETVI, green foxtail; WAT, weeks after glyphosate treatment. bMeans followed by the same letter within a column are not significantly different according to Fisher’s Protected LSD (P < 0.05). cIncluded N-Tank at 0.5% v/v.
Weed Control | |||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Treatment | Dose | ABUTH | AMASS | CHEAL | SETVI | Moisture | Yield | ||||||
g·ae·ha−1 | -----------------------------------------------%----------------------------------------------- | T·ha−1 | |||||||||||
Weedy control | 0 | f | 0 | f | 0 | e | 0 | e | 17.0 | c | 2.70 | c | |
Weed-free control | 100 | a | 100 | a | 100 | a | 100 | a | 14.6 | a | 4.00 | ab | |
WeatherMAX | 225 | 30 | ef | 79 | e | 63 | cd | 84 | d | 15.8 | b | 3.84 | b |
WeatherMAX | 450 | 60 | cd | 98 | ab | 91 | b | 95 | bc | 15.0 | a | 4.01 | ab |
WeatherMAX | 900 | 89 | ab | 100 | a | 97 | ab | 99 | ab | 14.7 | a | 3.96 | ab |
Clearout | 225 | 31 | ef | 83 | de | 52 | d | 85 | cd | 15.3 | ab | 3.84 | b |
Clearout | 450 | 48 | de | 96 | bc | 69 | cd | 95 | bc | 15.3 | ab | 3.92 | ab |
Clearout | 900 | 90 | ab | 100 | a | 95 | b | 99 | ab | 15.0 | a | 3.98 | ab |
WiseUpc | 225 | 46 | de | 90 | cd | 70 | c | 88 | cd | 15.0 | a | 3.95 | ab |
WiseUpc | 450 | 74 | bc | 98 | ab | 90 | b | 97 | ab | 14.8 | a | 4.04 | a |
WiseUpc | 900 | 94 | ab | 100 | a | 96 | b | 99 | ab | 14.6 | a | 4.04 | a |
aAbbreviations: ABUTH, velvetleaf; AMASS, pigweed species; CHEAL, common lambsquarters; SETVI, green foxtail; WAT, weeks after glyphosate treatment. bMeans followed by the same letter within a column are not significantly different according to Fisher’s Protected LSD (P < 0.05). cIncluded N-Tank at 0.5% v/v.
Visible Crop Injuryc | |||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Treatment | Dose | 1 WAT | 2 WAT | 4 WAT | Cob Length | Cob Deformity | Yield | ||||||
g·ae·ha−1 | -------------------------%------------------------- | cm | T·ha−1 | ||||||||||
Untreated | 0 | a | 0 | a | 0 | a | 19.6 | a | 10.0 | a | 14.4 | a | |
WeatherMAX | 1800 | 0.3 | a | 0.2 | a | 0 | a | 19.8 | a | 9.8 | ab | 14.4 | a |
WeatherMAX | 3600 | 2.3 | bc | 1.7 | b | 2.1 | b | 19.8 | a | 9.3 | cd | 14.2 | a |
Clearout | 1800 | 0.1 | a | 0 | a | 0 | a | 19.6 | a | 9.8 | ab | 14.6 | a |
Clearout | 3600 | 0.4 | a | 0.2 | a | 0.1 | a | 19.8 | a | 9.4 | bcd | 14.6 | a |
WiseUpd | 1800 | 1.0 | ab | 0.7 | ab | 0.1 | a | 19.8 | a | 9.7 | abc | 14.4 | a |
WiseUpe | 3600 | 3.8 | c | 3.7 | c | 2.8 | b | 19.3 | a | 9.1 | d | 13.8 | a |
aAbbreviation: WAT, weeks after glyphosate treatment. bMeans followed by the same letter within a column are not significantly different according to Fisher’s Protected LSD (P < 0.05). cRidgetown 2011 (4 WAT) and Exeter 2012 (1, 2, and 4 WAT) injury data were not included in the analysis as no injury was detected. dN-Tank at 0.5% v/v. eN-Tank at 1.0% v/v.
as WeatherMAX and WiseUp, respectively (
For soybean, visible injury 1 WAT with 2700 g·ae·ha−1 of glyphosate applied at the flower initiation stage was 3.2%, 1.4%, and 1.0% for WeatherMAX, WiseUp, and Clearout formulations, respectively (
With the proliferation of glyphosate-containing herbicides available since the expiration of the glyphosate patent [
Visible Crop Injury | |||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Treatment | Dose | 1 WAT | 2 WAT | 3 WAT | 4 WAT | 8 WAT | Moisture | Yield | |||||||
g·ae·ha−1 | ------------------------------------------------%------------------------------------------------ | T·ha−1 | |||||||||||||
Untreated | 0 | a | 0 | a | 0 | a | 0 | a | 0 | a | 14.3 | ab | 4.33 | a | |
WeatherMAX | 2700 | 3.2 | c | 2.5 | c | 1.2 | cd | 0.1 | a | 0.4 | a | 14.3 | ab | 4.23 | ab |
WeatherMAX | 5400 | 8.5 | e | 7.5 | d | 4.9 | e | 1.9 | c | 0.8 | a | 14.5 | b | 3.96 | c |
Clearout | 2700 | 1.4 | b | 0.6 | b | 0.3 | b | 0 | a | 0 | a | 14.2 | a | 4.30 | a |
Clearout | 5400 | 4.5 | d | 3.6 | c | 1.8 | d | 0.5 | ab | 0.6 | a | 14.3 | ab | 4.13 | b |
WiseUpc | 2700 | 1.0 | b | 0.9 | b | 0.5 | bc | 0.3 | a | 0.3 | a | 14.2 | a | 4.36 | a |
WiseUpd | 5400 | 3.7 | cd | 3.1 | c | 1.8 | d | 0.9 | b | 0.6 | a | 14.4 | b | 4.09 | bc |
aAbbreviation: WAT, weeks after glyphosate treatment. bMeans followed by the same letter within a column are not significantly different according to Fisher’s Protected LSD (P < 0.05). cIncluded N-Tank at 0.5% v/v. dIncluded N-Tank at 1.0% v/v.
Clearout and WiseUp and the application of WeatherMAX resulted in a greater yield loss than Clearout. For the WiseUp and Clearout formulations, less injury was observed which translated into lower yield reductions compared to the WeatherMAX formulation.
The authors acknowledge Lynette Brown and Todd Cowan for their technical assistance in these studies.
WAT, weeks after treatment.