Isoxaflutole-resistant soybean is currently in development for commercialization in North America. Proposals to use isoxaflutole + metribuzin as the main herbicide tank-mixture raise concerns as there is limited grass control with these herbicides. Strategies are needed to improve grass control with isoxaflutole + metribuzin. Nine experiments were conducted over a two-year period (2017, 2018) to determine the efficacy of isoxaflutole + metribuzin (52.5 + 210 g a· i· ha- 1 ) applied alone and co -applied with pendimethalin, dimethenamid-P, pethoxamid, pyroxasulfone or S-metolachlor applied preemergence (PRE). Comparisons were made with isoxaflutole + metribuzin at a low rate (52.5 + 210 g a· i· ha- 1 ), medium rate (79 + 315 g a· i· ha- 1 ) and a high rate (105 + 420 g a· i· ha- 1 ). Eight weed species were evaluated including common lambsquarters, green and redroot pigweed, common ragweed, velvetleaf, green and giant foxtail, yellow foxtail, barnyardgrass and witchgrass. All herbicides were affected by amount of rainfall following application; less rainfall resulted in reduced weed control. The addition of pendimethalin, dimethenamid-P, pethoxamid, pyroxasulfone or S-metolachlor to the low rate ofisoxaflutole + metribuzin provided equivalent control of all weed species evaluated compared toisoxaflutole + metribuzin at the low, medium, or high rate.
New hydroxyphenylpyruvate dioxygenase (HPPD) transgenic soybean cultivars are in development with resistance to a suite of herbicides including isoxaflutole and glyphosate; isoxaflutole, glyphosate and glufosinate; and isoxaflutole, mesotrione and glufosinate. Once commercialized, one weed management program will be the application of isoxaflutole + metribuzin applied preplant (PP) or preemergence (PRE) for residual control of annual grass and broadleaf weeds. These two herbicides, when used together, have complementary activity for the control of glyphosate-resistant (GR) Canada fleabane (Conyza canadensis L. Cronq.) [
Isoxaflutole is an HPPD-inhibiting herbicide; this enzyme catalyzes the production of tocopherols and plastoquinone; a cofactor essential for carotenoid biosynthesis and an electron transporter in the electron transport chain [
Grass competition with soybean can cause a yield reduction; the amount of yield loss is influenced by weed species, density, relative time of weed and crop emergence, weather patterns, soil nutrient status and time of removal. Populations of Johnsongrass (Sorghum halepense L. Pers.) of 16 plants per 10 m of soybean row caused a 48% soybean yield loss; increases in weed density caused 88% yield loss [
Herbicides applied preemergence (PRE) with grass activity mitigates yield limiting soybean stress from grass weed competition early in the season. Soil applied grass herbicide families for soybean include dinitroaniline, chloroacetamide, chloroacetanilide and isoxazoline. Herbicides within these families control annual grasses and small-seeded broadleaf weeds; generally, they control barnyardgrass, crabgrass species (Digitaria sp.), Panicum species, foxtail species (Setaria spp.), Amaranthus species and common lambsquarters (Chenopodium album L.) although the weed spectrum controlled is active ingredient specific [
The purpose of this study was a) to determine the benefit of adding a soil-applied grass herbicide to isoxaflutole + metribuzin and b) to develop an understanding of which soil-applied grass herbicides used in combination with isoxaflutole + metribuzin provided the best control of specific annual grass and broadleaf weeds in isoxaflutole-resistant soybean.
There were nine experiments conducted in 2017 (4 trials) and 2018 (5 trials) in south-western Ontario. The trial sites were located near Exeter, Ennotville, Cambridge and Ridgetown (two sites in 2018). Prior to seeding isoxaflutole-resistant soybean, the land was conventionally tilled. Soybean was planted to a depth of approximately 5 cm, in rows spaced 0.75 m apart at approximately 372,500 seeds per hectare. Soil characteristics, seeding dates, herbicide application dates and cumulative rainfall 0 to 7 and 0 to 14 days after treatment application (DAA) are presented in
Herbicide treatments were arranged in a randomized complete block design with four replications at each site. All plots measured 3 m wide (4 soybean rows) by 8 or 10 m long based on available space. Control treatments included an untreated weedy and weedfree plot in each replication. The weedfree control was maintained weedfree with imazethapyr (100 g a∙i∙ ha−1) plus metribuzin (400 g a∙i∙ ha−1) applied PRE followed by glyphosate (900 g a∙i∙ ha−1) applied postemergence (POST) and subsequent hand weeding if required. Herbicide treatments were applied using a CO2 pressurized backpack sprayer calibrated to deliver 200 L∙ha−1 at 240 kPa. The sprayer was equipped with a 1.5 m boom with four Hypro ULD 120-02 nozzles (Pentair, New Brighton, MN) spaced 50 cm apart resulting in a 2.0 m spray width. The treatments in this study were applied PRE and included the grass herbicides: pendimethalin (1000 g a∙i∙ ha−1), dimethenamid-P (544 g a∙i∙ ha−1), pethoxamid (840 g a∙i∙ ha−1), pyroxasulfone (125 g a∙i∙ ha−1) and S-metolachlor (1050 g a∙i∙ ha−1). Isoxaflutole + metribuzin was applied at three different rates: 52.5 + 210, 79 + 315 and 105 + 420 g a∙i∙ ha−1 hereafter referred to as low, medium and high rates, respectively and the grass herbicides applied with the low rate of isoxaflutole + metribuzin.
Soybean injury was evaluated 1, 2 and 4 weeks after soybean emergence (WAE) on a scale of 0 to 100, where 0 represented no injury and 100 was recorded when the soybean was dead. At 4, 8 and 12 weeks after application (WAA), visible control of naturally occurring weed species was evaluated on a scale of 0 to 100 with 0 being assigned when treatments provided no control relative to the
# | Location | Year | Soil Type | Sand | Silt | Clay | OM | pH | CEC | Planting Date | Application Date PRE | Rainfall 7 DAA | Rainfall 14 DAA |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
% | % | % | % | meq 100 g−1 | mm | mm | |||||||
1 | Ridgetown | 2018 | Clay Loam | 35 | 30 | 35 | 4.2 | 6.7 | 19 | May 25 | May 29 | 5.0 | 7.3 |
2 | Ridgetown | 2017 | Clay Loam | 41 | 28 | 31 | 4.0 | 7.1 | 14 | June 2 | June 7 | 2.7 | 24.8 |
3 | Ridgetown | 2018 | Clay Loam | 43 | 26 | 31 | 3.6 | 6.8 | 16 | May 31 | June 1 | 4.9 | 7.2 |
4 | Exeter | 2018 | Loam | 41 | 35 | 24 | 2.9 | 7.7 | 27 | May 18 | May 22 | 5.2 | 14.7 |
5 | Exeter | 2017 | Loam | 35 | 43 | 22 | 3.9 | 7.8 | 30 | June 3 | June 5 | 0.8 | 12.5 |
6 | Ennotville | 2017 | Silt Loam | 41 | 52 | 7 | 3.8 | 7.8 | 18 | May 31 | June 2 | 9.8 | 22.7 |
7 | Ennotville | 2018 | Silt Loam | 41 | 52 | 7 | 3.8 | 7.8 | 18 | May 25 | May 28 | 14.9 | 15.8 |
8 | Cambridge | 2017 | Sandy Loam | 68 | 26 | 6 | 2.2 | 7.2 | 9 | May 31 | June 2 | 5.9 | 7.3 |
9 | Cambridge | 2018 | Sandy Loam | 68 | 26 | 6 | 2.2 | 7.2 | 9 | May 25 | May 28 | 10.8 | 14.0 |
weedy control and 100 assigned when all weeds of the species evaluated were completely dead. At 8 WAA, weed density was determined for each species by counting the number of individual plants within two 0.5 m2 quadrats per plot. The weeds in the quadrats were cut at the soil surface and placed by species in paper bags, which were dried at 60˚C until constant moisture and then dry weight (biomass) was recorded. Soybean yield was measured at maturity by harvesting the centre two rowsof each plot with a small-plot research combine; yield was adjusted to 13% moisture.
Data were analyzed in SAS software (ver. 9.4., SAS Institute, Inc., Cary, NC) using the GLIMMIX procedure. When analyzing injury, weed control and yield sites were sorted into groups based on a Tukey-Kramer multiple means comparison test when there was a significant site by treatment interaction using a mixed model where the fixed effects were site, treatment and site by treatment and the random effects were replication within site. Site groupings from weed control at 4 WAA were kept constant throughout control ratings at 8 and 12 WAA, in addition to density and biomass for each species. If the site by treatment interaction was not significant all sites were pooled. When data were pooled across sites, the treatments were considered a fixed effect and the random effects include site, site by treatment and replication within site. An F-test was performed to test the significance of fixed effects and a Wald test was conducted to test the significance of random effects. Residual plots were used to confirm the assumptions that the variances were randomly distributed, independent and homogenous across treatments. Additionally, a Shapiro-Wilk test was performed to test the assumption of normally distributed residuals as described by Shapiro and Wilk in 1965 [
Although weed control was visually assessed at 4, 8 and 12 WAA, only the 12 WAA assessments are presented in Tables 3-10 to minimize the number of tables in the manuscript.
At 1 WAE, no soybean injury was visually evident from any of the herbicides applied (
At 4 WAE, the herbicides caused soybean leaf deformity and bleaching of the foliage at some locations. Leaf deformity occurred at sites 1, 4, 5, 6, 7, 8 and 9. Due to a significant site by treatment interaction (data not shown), site 6 was analyzed separately. At sites 1, 4, 5, 7, 8 and 9, pendimethalin, S-metolachlor and pendimethalin, dimethenamid-P and S-metolachlor with the addition of isoxaflutole + metribuzin caused 1% soybean injury (
1 WAE | 2 WAE | 4 WAE | |||||||||
---|---|---|---|---|---|---|---|---|---|---|---|
Treatment | Rate | All sites | Sites 2, 4, 5, 6, 7 | Sites 9 | Sites 1, 4, 5, 7, 8, 9 | Site 6 | Site 1, 2, 3, 6 | Site 8 | Site 5 | ||
g a∙i∙ ha−1 | % Leaf deformity injury | % Bleaching injury | |||||||||
Pendimethalin | 1000 | 0 | 1abc | 18fg | 1b | 16d | 0a | 0a | 0a | ||
Dimethenamid-P | 544 | 0 | 2abc | 5bcd | 0ab | 2ab | 0a | 0a | 0a | ||
Pethoxamid | 840 | 0 | 1abc | 7cde | 0ab | 3abc | 0a | 0a | 0a | ||
Pyroxasulfone | 125 | 0 | 1abc | 2b | 0ab | 4bc | 0a | 0a | 0a | ||
S-metolachlor | 1050 | 0 | 2bc | 11def | 1ab | 11cd | 0a | 0a | 0a | ||
Isoxaflutole + Metribuzin | 52.5 + 210 | 0 | 0a | 0a | 0a | 0a | 1bcd | 7abc | 16b | ||
Isoxaflutole + Metribuzin | 79 + 315 | 0 | 0a | 0a | 0a | 0a | 2cd | 10bc | 25c | ||
Isoxaflutole + Metribuzin | 105 + 420 | 0 | 0a | 0a | 0a | 0a | 4d | 13c | 30c | ||
Pendimethalin + Isoxaflutole + Metribuzin | 1000 + 52.5 + 210 | 0 | 2abc | 20f | 1b | 14d | 0abc | 7abc | 15b | ||
Dimethenamid-P + Isoxaflutole + Metribuzin | 544 + 52.5 + 210 | 0 | 2abc | 4bc | 1ab | 0a | 2cd | 5ab | 15b | ||
Pethoxamid + Isoxaflutole + Metribuzin | 840 + 52.5 + 210 | 0 | 1abc | 4bc | 0ab | 6bcd | 0ab | 9bc | 17b | ||
Pyroxasulfone + Isoxaflutole + Metribuzin | 125 + 52.5 + 210 | 0 | 1abc | 3bc | 0ab | 3abc | 1abcd | 4a | 15b | ||
S-metolachlor + Isoxaflutole + Metribuzin | 1050 + 52.5 + 210 | 0 | 3c | 13efg | 1ab | 11cd | 0abc | 5ab | 17b | ||
Note: Means followed by the same letter within a column are not statistically different according to the Tukey-Kramer multiple range test at p < 0.05.
isoxaflutole + metribuzin caused 4% to 9% bleaching injury, which was similar toisoxaflutole + metribuzin. At site 5, isoxaflutole + metribuzin at the low medium and high rate caused 16%, 25% and 30% soybean bleaching, respectively. The grass herbicides plus isoxaflutole + metribuzin caused 15% to 17% soybean bleaching, similar to isoxaflutole + metribuzin.
Soybean displayed the most sensitivity to pendimethalin and S-metolachlor. Rainfall after application appeared to influence the level of soybean leaf deformity at the various sites. Soybean at sites with more rainfall after application displayed more severe leaf deformity compared to sites receiving less rainfall. This was probably due to higher herbicide uptake in soybean with higher rainfall. Based on visible observations in the field, as soybean continued to grow, the leaf deformity injury occurred on the first 3 trifoliate leaves with no leaf deformity observed on new soybean growth after the third trifoliate.
Soybean leaf bleaching symptoms were observed at 4 WAE on the 3rd and 4th trifoliate leaves. This injury appeared to be influenced by rainfall received 14 to 21 DAA. Soybean injury (≤30%) was observed at sites 1, 2, 3, 5, 6 and 8 which received 12.3 to 43.5 mm of rain in the 21 days after herbicide application; in contrast sites 4, 7 and 9 received < 3 mm of rain in the 21 days after application and no soybean injury was observed. Rainfall during this period of time after herbicide application probably dissolved the herbicides into soil water solution, allowing for the absorption by the soybean, resulting in a higher herbicide concentration within the plant which the soybean could not metabolize quickly enough to avoid herbicide injury. Bleaching symptoms were evident one week later when injury was evaluated. As the 5th trifoliate leaves were emerging, no bleaching symptoms were present at any sites, as the soybean was probably able to metabolize isoxaflutole by that time.
Common lambsquarters control was assessed at seven sites in this study. A significant treatment by site interaction occurred for common lambsquarters control (data not shown); therefore, results from sites 2 and 4 were combined, sites 3, 5, 8 and 9 were combined and site 7 was analyzed separately (
At 12 WAA, at sites 2 and 4, pendimethalin, dimethenamid-P, pethoxamid, pyroxasulfone and S-metolachlor controlled common lambsquarters 4% to 23% (
At 8 WAA, at all three site groupings, common lambsquarters density was reduced with application of pendimethalin, isoxaflutole + metribuzin at the low, medium or high rate, or any grass herbicide with the addition of isoxaflutole + metribuzin compared to the untreated control (
Control 12 WAA | Density | Biomass | |||||||||
---|---|---|---|---|---|---|---|---|---|---|---|
Treatment | Rate | Sites 2, 4 | Sites 3, 5, 8, 9 | Site 7 | Sites 2, 4 | Sites 3, 5, 8, 9 | Site 7 | Sites 2, 4 | Sites 3, 5, 8, 9 | Site 7 | |
g a∙i∙ ha−1 | % | #m−2 | g∙m−2 | ||||||||
Untreated Control | 32.5d | 21.5e | 2.5b | 31.4a | 16.5abc | 8.6bc | |||||
Pendimethalin | 1000 | 23abcd | 58ab | 91ab | 5.6abc | 4.5bcd | 0.2a | 3.5a | 4.2abc | 0.3ab | |
Dimethenamid-P | 544 | 6cd | 25b | 85b | 14.8cd | 15.0de | 0.7ab | 24.9a | 24.5c | 6.abc | |
Pethoxamid | 840 | 11bcd | 26b | 85b | 8.1bcd | 10.0de | 1.3ab | 22.7a | 15.7abc | 3.3abc | |
Pyroxasulfone | 125 | 7cd | 34b | 93ab | 14.2cd | 7.0cde | 0.4ab | 50.6a | 18.8bc | 1.3abc | |
S-metolachlor | 1050 | 4d | 24b | 67b | 12.1cd | 19.8de | 2.7b | 25.1a | 22.9c | 14.5c | |
Isoxaflutole + Metribuzin | 52.5 + 210 | 37abcd | 94a | 100a | 4.8abc | 1.4abc | 0.2a | 11.7a | 3.3abc | 0.1a | |
Isoxaflutole + Metribuzin | 79 + 315 | 68ab | 99a | 100a | 2.4ab | 0.6ab | 0.2a | 4.9a | 1.3abc | 0.8ab | |
Isoxaflutole + Metribuzin | 105 + 420 | 86a | 100a | 100a | 1.3a | 0.2a | 0.02a | 2.0a | 0.7a | 0.1a | |
Pendimethalin + Isoxaflutole + Metribuzin | 1000 +52.5 + 210 | 68ab | 99a | 100a | 2.0ab | 0.7ab | 0.02a | 2.3a | 0.8ab | 0.1a | |
Dimethenamid-P + Isoxaflutole + Metribuzin | 544 + 52.5 + 210 | 57abcd | 94a | 100a | 2.1ab | 1.4abc | 0.02a | 6.5a | 5.6abc | 0.1a | |
Pethoxamid + Isoxaflutole + Metribuzin | 840 + 52.5 + 210 | 60abc | 97a | 100a | 4.3abc | 1.1abc | 0.04a | 6.5a | 3.7abc | 0.1a | |
Pyroxasulfone + Isoxaflutole + Metribuzin | 125 + 52.5 + 210 | 57abcd | 93a | 100a | 4.2abc | 0.9ab | 0.02a | 10.0a | 2.9abc | 0.1a | |
S-metolachlor + Isoxaflutole + Metribuzin | 1050 + 52.5 + 210 | 41abcd | 94a | 100a | 3.6abc | 1.3abc | 0.02a | 9.9a | 2.1abc | 0.1a | |
aMeans followed by the same letter within a column are not statistically different according to the Tukey-Kramer multiple range test at p < 0.05.
common lambsquarters density compared to the untreated control. At site 2 and 4, the above-mentioned herbicide treatments reduced common lambsquarters density 83 to 96%. There was no difference in common lambsquarters density among pendimethalin, dimethenamid-P, pethoxamid, pyroxasulfone, S-metolachlor, the low rate of isoxaflutole + metribuzin or the combination of pethoxamid, pyroxasulfone or S-metolachlor with the addition of isoxaflutole + metribuzin. At sites 3, 5, 8 and 9, the above-mentioned herbicide treatments reduced common lambsquarters density 79% to 99%. Isoxaflutole + metribuzin reduced common lambsquarters density 20% more than pendimethalin. The addition of isoxaflutole + metribuzin to dimethenamid-P, pethoxamid, pyroxasulfone and S-metolachlor reduced common lambsquarters density 63%, 41%, 29% and 86% more than the grass herbicides applied alone, respectively. At site 7 there was a 92% to 99% reduction in common lambsquarters density with the above mentioned herbicide treatments compared to the untreated control. Pendimethalin, all three rates of isoxaflutole + metribuzin and the grass herbicides + isoxaflutole + metribuzin reduced density compared to the untreated control and S-metolachlor.
At 8 WAA, at sites 2 and 4 and sites 3, 5, 8 and 9, common lambsquarters biomass was not reduced significantly with any herbicide treatment compared to the untreated control (
In summary, common lambsquarters control was influenced by rainfall and weed density. Site 7, which received 14.9 mm of rainfall within 7 DAA and had the lowest common lambsquarters density and the highest level of common lambsquarters control. Sites 3, 5, 8 and 9, received 0.8 to 10.8 mm of rainfall 0 to 7 DAA; this probably was sufficient rainfall for the herbicide to be dissolved in soil water solution so that it could be taken up by weed seedlings. Site 5 received only 0.8 mm which would likely not be enough rain to activate the herbicide; it also may not be enough rain to allow for weeds to germinate. This site had delayed germination; therefore, after the rainfall 7 to 14 DAA, the herbicide was activated and controlled the late emerging weeds. The selectivity of each herbicide is highlighted in this group of sites; although, pendimethalin has very low water solubility (0.275 mg∙L−1), it still provided greater common lambsquarters control than the Group 15 herbicides. Chomas and Kells [
Redroot pigweed (Amaranthus retroflexus L.) and green pigweed (Amaranthus powellii S. Watson) were combined during evaluations. Pigweed spp. were assessed at 7 sites in this study and due to a significant treatment by site interaction, sites were separated (data not shown); sites 2 and 4 were combined and sites 3, 6, 7, 8 and 9 were combined for analysis (
At 12 WAA, at sites 2 and 4, pendimethalin, dimethenamid-P, pethoxamid, pyroxasulfone and S-metolachlor controlled pigweed spp. 4%, 10%, 13%, 32% and 6%, respectively (
Control 12 WAA | Density | Biomass | ||||||
---|---|---|---|---|---|---|---|---|
Treatment | Rate | Sites 2, 4 | Sites 3, 6, 7, 8, 9 | Sites 2, 4 | Sites 3, 6, 7, 8, 9 | Sites 2,4 | Sites 3, 6, 7, 8, 9 | |
g a∙i∙ ha−1 | % | #m−2 | g∙m−2 | |||||
Untreated Control | 33.7a | 23.2d | 183.8ab | 40.3e | ||||
Pendimethalin | 1000 | 4d | 63cd | 43.5a | 15.9cd | 205.1b | 29.6de | |
Dimethenamid-P | 544 | 10cd | 70cd | 25.5a | 3.9abc | 140.4ab | 10.5bcde | |
Pethoxamid | 840 | 13bcd | 52d | 33.1a | 7.9bcd | 123.2ab | 22.5de | |
Pyroxasulfone | 125 | 32abcd | 85bc | 32.3a | 3.4abc | 93.8ab | 4.7abcde | |
S-metolachlor | 1050 | 6d | 66cd | 33.2a | 8.7bcd | 130.4ab | 16.2cde | |
Isoxaflutole + Metribuzin | 52.5 + 210 | 27abcd | 93ab | 18.7a | 1.9ab | 55.9ab | 3.3abcd | |
Isoxaflutole + Metribuzin | 79 + 315 | 59abc | 95ab | 15.6a | 0.6a | 61.5ab | 1.1ab | |
Isoxaflutole + Metribuzin | 105 + 420 | 81a | 97ab | 7.3a | 0.4a | 25.4a | 1.2ab | |
Pendimethalin + Isoxaflutole + Metribuzin | 1000 + 52.5 + 210 | 65ab | 95ab | 13.1a | 0.8a | 51.8ab | 2.2abc | |
Dimethenamid-P + Isoxaflutole + Metribuzin | 544 + 52.5 + 210 | 54abcd | 98ab | 12.7a | 0.2a | 70.6ab | 0.4a | |
Pethoxamid + Isoxaflutole + Metribuzin | 840 + 52.5 + 210 | 48abcd | 97ab | 11.7a | 0.5a | 38.7ab | 1.2ab | |
Pyroxasulfone + Isoxaflutole + Metribuzin | 125 + 52.5 + 210 | 67ab | 99a | 14.3a | 0.5a | 54.7ab | 0.8ab | |
S-metolachlor + Isoxaflutole + Metribuzin | 1050 + 52.5 + 210 | 38abcd | 98ab | 16.5a | 0.7a | 72.3ab | 1.3ab | |
aMeans followed by the same letter within a column are not statistically different according to the Tukey-Kramer multiple range test at p < 0.05.
higher pigweed spp. control than pendimethalin, dimethenamid-P, pethoxamid and S-metolachlor. Pendimethalin, dimethenamid-P, pethoxamid, pyroxasulfone and S-metolachlor with the addition of isoxaflutole + metribuzin controlled pigweed spp. 38% to 67%; there was no difference in control among the grass herbicides. The addition of isoxaflutole + metribuzin to pendimethalin increased pigweed spp. control 61% compared to pendimethalin applied alone. At sites 2 and 4, there was no increase in pigweed spp. control when a grass herbicide was added to isoxaflutole + metribuzin. This was expected as these herbicides generally do not control broadleaved weeds such as pigweed species. At sites 3, 6, 7, 8 and 9, pendimethalin, dimethenamid-P, pethoxamid, pyroxasulfone and S-metolachlor controlled pigweed spp. 63%, 70%, 52%, 85% and 66%, respectively. Pyroxasulfone provided greater control than pethoxamid; all other grass herbicides provided similar pigweed spp. control. Isoxaflutole + metribuzin at the varying rates provided 93% to 97% control and did not differ among rates. Isoxaflutole + metribuzin provided greater pigweed spp. control than the grass herbicides with the exception ofpyroxasulfone. Pendimethalin, dimethenamid-P, pethoxamid, pyroxasulfone, or S-metolachlor applied in a tank-mix with isoxaflutole + metribuzin controlled pigweed spp. 95% to 99%. The addition of isoxaflutole + metribuzin to the grass herbicides increased control compared to the respective grass herbicide applied alone. There was no improvement in pigweed spp. control with the addition of a grass herbicide to isoxaflutole + metribuzin.
At 8 WAA, at sites 2 and 4, no herbicide treatment reduced pigweed spp. density compared to the untreated control and there was no difference in pigweed spp. density among the herbicide treatments evaluated (
At 8 WAA, at sites 2 and 4, no treatment reduced pigweed spp. biomass compared to the untreated control (
In summary, pigweed spp. control was influenced by rainfall after application. Pigweed spp. control was lower at sites 2 and 4 which received 2.7 and 0.8 mm of rainfall 0 to 7 DAA, respectively. In contrast, pigweed spp. control was greater at sites 3, 6, 7, 8 and 9 which received higher rainfall of 4.9 to 14.9 mm 0 to 7 DAA. Of the grass herbicides evaluated, pyroxasulfone provided the highest pigweed spp. control across sites with differing levels of rainfall. Redroot pigweed is very sensitive to pyroxasulfone; rates as low as 93 g a∙i∙ ha−1 controlled pigweed 90% [
Common ragweed (Ambrosia artemisiifolia L.) populations were present at sites 1, 3 and 5 in this study (
At 12 WAA, at site 1, pendimethalin, dimethenamid-P, pethoxamid, pyroxasulfone and S-metolachlor controlled common ragweed 28% to 62%; there was no difference in control among the grass herbicides (
At 8 WAA, at site 1, the herbicide treatments evaluated did not reduce common ragweed density compared to the untreated control (
At 8 WAA, at site 1, none of the herbicide treatments evaluated reduced common ragweed biomass compared to the untreated control, additionally, there were no treatment differences (
Treatment | Rate | Site 1 | Site 3 | Site 5 | Site 1 | Site 3 | Site 5 | Site 1 | Site 3 | Site 5 | ||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
g a∙i∙ ha−1 | % | #m−2 | g∙m−2 | |||||||||||
Untreated Control | 5.7ab | 64.8b | 74.2c | 0.5a | 14.6bcd | 65.6b | ||||||||
Pendimethalin | 1000 | 55abc | 20b | 0b | 3.5ab | 71.7b | 58.9c | 0.3a | 31.3d | 98.7b | ||||
Dimethenamid-P | 544 | 28c | 46b | 0b | 11.0b | 50.6b | 43.1bc | 5.8a | 19.8cd | 66.9b | ||||
Pethoxamid | 840 | 31c | 54b | 1b | 4.3ab | 34.4b | 38.4bc | 0.8a | 9.9abcd | 46.6b | ||||
Pyroxasulfone | 125 | 62abc | 45b | 0b | 1.1ab | 28.4b | 20.1b | 1.0a | 17.1cd | 45.6b | ||||
S-metolachlor | 1050 | 43bc | 40b | 0b | 10.8b | 71.2b | 79.8c | 3.7a | 33.4d | 82.4b | ||||
Isoxaflutole + Metribuzin | 52.5 +210 | 82abc | 97a | 100a | 0.4ab | 0.6a | 0.01a | 1.1a | 0.5a | 0.02a | ||||
Isoxaflutole + Metribuzin | 79 + 315 | 90abc | 99a | 100a | 0.4ab | 0.2a | 0.01a | 0.2a | 0.8ab | 0.02a | ||||
Isoxaflutole + Metribuzin | 105 + 420 | 98ab | 100a | 100a | 0.4ab | 0.02a | 0.01a | 0.3a | 0.1a | 0.02a | ||||
Pendimethalin + Isoxaflutole + Metribuzin | 1000 + 52.5 + 210 | 66abc | 99a | 99a | 0.6ab | 0.6a | 0.2a | 0.8a | 2.2abc | 0.1a | ||||
Dimethenamid-P + Isoxaflutole + Metribuzin | 544 + 52.5 + 210 | 100a | 98a | 100a | 0.1a | 0.02a | 0.01a | 0.1a | 0.1a | 0.02a | ||||
Pethoxamid + Isoxaflutole + Metribuzin | 840 + 52.5 + 210 | 90abc | 97a | 100a | 0.7ab | 0.7a | 0.01a | 1.3a | 2.0abc | 0.02a | ||||
Pyroxasulfone + Isoxaflutole + Metribuzin | 125 + 52.5 + 210 | 95ab | 100a | 100a | 1.0ab | 0.7a | 0.01a | 3.2a | 1.9abc | 0.02a | ||||
S-metolachlor + Isoxaflutole + Metribuzin | 1050 + 52.5 + 210 | 98ab | 98a | 98a | 1.2ab | 0.2a | 0.3a | 0.9a | 1.3abc | 0.2a | ||||
aMeans followed by the same letter within a column are not statistically different according to the Tukey-Kramer multiple range test at p < 0.05.
metribuzin reduced biomass 96% to 99% compared to the untreated control. At site 5, the grass herbicides applied alone did not reduce common ragweed biomass relative to the untreated control. Isoxaflutole + metribuzin and the tank-mixtures of a grass herbicide plus isoxaflutole + metribuzin reduced common ragweed biomass 99%.
In summary, common ragweed control was influenced by rainfall after application and weed density. Sites 1, 3 and 5 had 5.0, 4.9 and 0.8 mm of rain within 7 DAA, respectively. Although the low rain at all three sites was probably inadequate to sufficiently activate the soil-applied grass herbicides; the grass herbicides would have provided minimal control of ragweed. In contrast, isoxaflutole + metribuzin was activated sufficiently and controlled common ragweed 82% to 100% 12 WAA. By 14 DAA, sites 1, 3 and 5 received 7.3, 7.2 and 12.5 mm of rainfall. At 4 WAA the grass herbicides provided 0% to 14% common ragweed control which is similar to a study by Soltani et al. [
Velvetleaf (Abutilon theophrasti Medik.) was assessed at 3 sites in this study (
At 12 WAA, at sites 1 and 2, pendimethalin, dimethenamid-P, pethoxamid, pyroxasulfone and S-metolachlor controlled velvetleaf 58, 46, 43, 52 and 33%; they did not differ statistically (
At 8 WAA, at sites 1 and 2, no treatment reduced velvetleaf density compared to the untreated control (
At 8 WAA, at sites 1 and 2, there was no reduction in velvetleaf biomass with any of the herbicides treatments compared to the untreated control (
In summary, generally, the grass herbicides evaluated provided poor control of velvetleaf. At site 3, dimethenamid-P and pyroxasulfone provided suppression
Control 12 WAA | Density | Biomass | ||||||||
---|---|---|---|---|---|---|---|---|---|---|
Treatment | Rate | Sites 1, 2 | Sites 3 | Sites 1, 2 | Sites 3 | Sites 1, 2 | Sites 3 | |||
g a∙i∙ ha−1 | % | #m−2 | g∙m−2 | |||||||
Untreated Control | 2.7abcd | 4.6a | 1.6a | 8.0ab | ||||||
Pendimethalin | 1000 | 58bcd | 56ab | 5.4d | 1.8a | 2.0a | 2.5ab | |||
Dimethenamid-P | 544 | 46cd | 61ab | 4.3cd | 3.7a | 2.0a | 4.6ab | |||
Pethoxamid | 840 | 43cd | 36b | 4.0bcd | 3.9a | 1.8a | 12.0b | |||
Pyroxasulfone | 125 | 52bcd | 72ab | 4.6bcd | 0.9a | 5.4a | 0.7ab | |||
S-metolachlor | 1050 | 33d | 33b | 3.2cd | 2.0a | 1.9a | 10.9ab | |||
Isoxaflutole + Metribuzin | 52.5 + 210 | 91abc | 99a | 0.3abcd | 0.5a | 0.6a | 0.3a | |||
Isoxaflutole + Metribuzin | 79 + 315 | 96ab | 99a | 0.2ab | 0.1a | 0.4a | 0.1a | |||
Isoxaflutole + Metribuzin | 105 + 420 | 100a | 100a | 0.2a | 2.2a | 0.4a | 2.1ab | |||
Pendimethalin + Isoxaflutole + Metribuzin | 1000 + 52.5 + 210 | 99a | 98a | 0.2a | 0.1a | 0.1a | 0.1a | |||
Dimethenamid-P + Isoxaflutole + Metribuzin | 544 + 52.5 + 210 | 99a | 100a | 0.3a | 0.1a | 0.3a | 0.1a | |||
Pethoxamid + Isoxaflutole + Metribuzin | 840 + 52.5 + 210 | 89abc | 99a | 1.0abcd | 0.3a | 1.6a | 0.5ab | |||
Pyroxasulfone + Isoxaflutole + Metribuzin | 125 + 52.5 + 210 | 99a | 98a | 0.5abc | 0.3a | 1.2a | 0.8ab | |||
S-metolachlor + Isoxaflutole + Metribuzin | 1050 + 52.5 + 210 | 93abc | 99a | 0.3ab | 0.1a | 0.4a | 0.1a | |||
a Means followed by the same letter within a column are not statistically different according to the Tukey-Kramer multiple range test at p < 0.05.
of velvetleaf. Among the grass herbicides, pyroxasulfone provided the greatest control of velvetleaf, however, at 12 WAA, control only reached 72%. In contrast, other studies have reported that pyroxasulfone (125 g a∙i∙ ha−1) controlled velvetleaf 90% [
Green foxtail and giant foxtail were combined during evaluations in this study (
At 12 WAA, at sites 1, 2 and 4, pendimethalin, dimethenamid-P, pethoxamid, pyroxasulfone and S-metolachlor controlled foxtail spp. 24% to 38%, there was no difference in foxtail spp. control among the five soil-applied grass herbicides (
Control 12 WAA | Density | Biomass | |||||||
---|---|---|---|---|---|---|---|---|---|
Treatment | Rate | Sites 1, 2, 4 | Sites 3, 5, 7, 9 | Sites 1, 2, 4 | Sites 3, 5, 7, 9 | Sites 1, 2, 4 | Sites 3, 5, 7, 9 | ||
g a∙i∙ ha−1 | % | #m−2 | g∙m−2 | ||||||
Untreated Control | 64.5a | 59.2b | 44.4b | ||||||
Pendimethalin | 1000 | 34cde | 70bc | 30.7a | 13.4a | 21.4ab | 6.5a | ||
Dimethenamid-P | 544 | 26e | 84abc | 25.1a | 7.5a | 35.9ab | 6.6a | ||
Pethoxamid | 840 | 24e | 59c | 26.8a | 18.2ab | 35.1ab | 11.6ab | ||
Pyroxasulfone | 125 | 38bcde | 83abc | 29.8a | 19.2ab | 29.0ab | 8.8ab | ||
S-metolachlor | 1050 | 31de | 86abc | 32.7a | 6.8a | 38.8ab | 3.0a | ||
Isoxaflutole + Metribuzin | 52.5 +210 | 50abcde | 84abc | 27.5a | 6.8a | 31.4ab | 6.0a | ||
Isoxaflutole + Metribuzin | 79 + 315 | 58abcde | 90abc | 21.9a | 8.5a | 20.2ab | 6.3a | ||
Isoxaflutole + Metribuzin | 105 + 420 | 78a | 96a | 17.4a | 6.8a | 14.8ab | 3.3a | ||
Pendimethalin + Isoxaflutole + Metribuzin | 1000 + 52.5 + 210 | 81abc | 87abc | 16.1a | 5.2a | 10.2a | 3.3a | ||
Dimethenamid-P + Isoxaflutole + Metribuzin | 544 + 52.5 + 210 | 84ab | 96ab | 19.8a | 2.7a | 16.5ab | 1.67a | ||
Pethoxamid + Isoxaflutole + Metribuzin | 840 + 52.5 + 210 | 53abcde | 88abc | 29.3a | 7.9a | 30.3ab | 5.0a | ||
Pyroxasulfone + Isoxaflutole + Metribuzin | 125 + 52.5 + 210 | 65abcd | 93ab | 18.8a | 8.1a | 17.0ab | 4.4a | ||
S-metolachlor + Isoxaflutole + Metribuzin | 1050 + 52.5 + 210 | 59abcde | 96a | 18.3a | 2.7a | 19.2ab | 2.3a | ||
aMeans followed by the same letter within a column are not statistically different according to the Tukey-Kramer multiple range test at p < 0.05.
foxtail spp. 50%, 58% and 78%, respectively. Control among the rates of isoxaflutole + metribuzin did not differ, additionally the low and medium rate did not differ compared to the grass herbicides; however, the high rate provided 40% to 54% greater foxtail spp. control compared to the grass herbicides. The tank-mixtures of a grass herbicides plus isoxaflutole + metribuzin controlled foxtail spp. 53% to 84%, there was no difference in control with these five herbicide treatments. Dimethenamid-P was the only grass herbicide which benefited from the addition of isoxaflutole + metribuzin where control increased 58%. The grass herbicides plus isoxaflutole + metribuzin did not differ from the varying rates of isoxaflutole + metribuzin. At sites 3, 5, 7 and 9, pendimethalin, dimethenamid-P, pethoxamid, pyroxasulfone and S-metolachlor controlled foxtail spp. 59% to 86%; there were no differences in foxtail spp.control with the grass herbicides. Isoxaflutole + metribuzin at the low, medium and high rate controlled foxtail spp. 84%, 90% and 96%, respectively; there was no difference in control among the three rates evaluated. The low and medium rate did not differ from the grass herbicides. The high rate provided 26% and 37% greater control than pendimethalin and pethoxamid, respectively, but did not differ from dimethenamid-P, pyroxasulfone and S-metolachlor. The grass herbicides plus isoxaflutole + metribuzin controlled foxtail spp. 87% to 96% and did not differ among each other or with isoxaflutole + metribuzin at the low, medium or high rate. There was no difference in foxtail spp. control with the grass herbicides applied alone or in a tank-mixture with isoxaflutole + metribuzin. S-metolachlor + isoxaflutole + metribuzin controlled foxtail spp. 26% and 37% more than pendimethalin and pethoxamid, respectively. Additionally, dimethenamid-P or pyroxa-s ulfone plus isoxaflutole + metribuzin controlled foxtail spp. 34% to 37% more than pethoxamid.
At 8 WAA, at sites 1, 2 and 4, there was no decrease in foxtail spp. density with the herbicide treatments evaluated (
At 8 WAA, at sites 1, 2 and 4, pendimethalin + isoxaflutole + metribuzin was the only treatment that reduced foxtail spp. biomass compared to the untreated control, it reduced biomass 83% (
In summary, at sites 1, 2 and 4, there was lower weed control than at sites 3, 5, 7 and 9. Generally, more rainfall was received at sites 3, 5, 7 and 9, by 28 DAA, compared to sites 1, 2 and 4 which may partially explain the reduced foxtail spp. control at sites 1, 2 and 4; however, site 3 received a lower amount of rain during this time period than site 2. Generally, at sites 1, 2 and 4, pyroxasulfone was the grass herbicide that provided the best control of foxtail spp.; in contrast, at sites 3, 5, 7 and 9, S-metolachlor provided the best control. At both site groups the grass herbicides plus isoxaflutole + metribuzin provided higher foxtail spp. control than isoxaflutole + metribuzin at the low rate.
Yellow foxtail (Setaria pumila Poir. Roem. & Schult.) was evaluated at sites 7 and 9 in this study (
At 12 WAA, the grass herbicides controlled yellow foxtail 55% to 90%, there was no difference in control among the five herbicides (
Control 12 WAA | Density | Biomass | ||
---|---|---|---|---|
Treatment | Rate | Sites 7 9 | Sites 7, 9 | Sites 7, 9 |
g a∙i∙ ha−1 | % | #m−2 | g∙m−2 | |
Untreated Control | 11.4a | 10.1a | ||
Pendimethalin | 1000 | 79ab | 3.8a | 3.1a |
Dimethenamid-P | 544 | 75ab | 3.2a | 2.3a |
Pethoxamid | 840 | 55b | 3.5a | 2.2a |
Pyroxasulfone | 125 | 80ab | 6.7a | 6.3a |
S-metolachlor | 1050 | 90ab | 2.2a | 1.7a |
Isoxaflutole + Metribuzin | 52.5 + 210 | 81ab | 2.9a | 2.9a |
Isoxaflutole + Metribuzin | 79 + 315 | 90ab | 4.3a | 2.8a |
Isoxaflutole + Metribuzin | 105 + 420 | 92ab | 2.2a | 1.5a |
Pendimethalin + Isoxaflutole + Metribuzin | 1000 + 52.5 + 210 | 94a | 2.5a | 2.6a |
Dimethenamid-P + Isoxaflutole + Metribuzin | 544 + 52.5 + 210 | 94a | 1.2a | 0.6a |
Pethoxamid + Isoxaflutole + Metribuzin | 840 + 52.5 + 210 | 87ab | 3.8a | 2.8a |
Pyroxasulfone + Isoxaflutole + Metribuzin | 125 + 52.5 + 210 | 90ab | 4.5a | 2.3a |
S-metolachlor + Isoxaflutole + Metribuzin | 1050 + 52.5 + 210 | 97a | 0.8a | 0.8a |
aMeans followed by the same letter within a column are not statistically different according to the Tukey-Kramer multiple range test at p < 0.05.
In summary, the addition of a grass herbicide to isoxaflutole + metribuzin numerically increased yellow foxtail control, although differences were not statistically significant. S-metolachlor with and without isoxaflutole + metribuzin had the highest level of control at 12 WAA and the largest reduction in density and biomass compared to the other grass herbicides.
Barnyardgrass control was assessed at five sites in this study (
At 12 WAA, at sites 1 and 2, there were no treatment differences, all herbicides controlled barnyardgrass 32 to 81% (
Control 12 WAA | Density | Biomass | |||||||
---|---|---|---|---|---|---|---|---|---|
Treatment | Rate | Sites 1, 2 | Sites 5, 6, 9 | Sites 1, 2 | Sites 5, 6, 9 | Sites 1, 2 | Sites 5, 6, 9 | ||
g a∙i∙ ha−1 | % | #m−2 | g∙m−2 | ||||||
Untreated Control | 13.1a | 5.8b | 11.6a | 5.3ab | |||||
Pendimethalin | 1000 | 53a | 48c | 7.0a | 2.9ab | 3.7a | 6.7b | ||
Dimethenamid-P | 544 | 45a | 96ab | 7.8a | 0.9ab | 7.0a | 0.3a | ||
Pethoxamid | 840 | 56a | 72bc | 7.7a | 2.6ab | 5.3a | 3.3ab | ||
Pyroxasulfone | 125 | 47a | 95ab | 7.4a | 1.4ab | 4.9a | 1.4ab | ||
S-metolachlor | 1050 | 32a | 98ab | 6.3a | 0.7ab | 7.8a | 0.7ab | ||
Isoxaflutole + Metribuzin | 52.5 + 210 | 61a | 92ab | 4.9a | 0.8ab | 9.1a | 0.8ab | ||
Isoxaflutole + Metribuzin | 79 + 315 | 59a | 95ab | 9.1a | 1.0ab | 9.3a | 1.9ab | ||
Isoxaflutole + Metribuzin | 105 + 420 | 69a | 97ab | 10.1a | 0.6a | 12.2a | 0.3a | ||
Pendimethalin + Isoxaflutole + Metribuzin | 1000 + 52.5 + 210 | 68a | 91ab | 5.6a | 0.7ab | 3.7a | 1.6ab | ||
Dimethenamid-P + Isoxaflutole + Metribuzin | 544 + 52.5 + 210 | 81a | 100a | 4.2a | 0.4a | 1.5a | 0.2a | ||
Pethoxamid + Isoxaflutole + Metribuzin | 840 + 52.5 + 210 | 70a | 93ab | 5.6a | 0.6ab | 3.7a | 0.9ab | ||
Pyroxasulfone + Isoxaflutole + Metribuzin | 125 + 52.5 + 210 | 73a | 99ab | 4.6a | 0.5a | 3.0a | 0.5a | ||
S-metolachlor +Isoxaflutole + Metribuzin | 1050 + 52.5 + 210 | 77a | 99ab | 8.5a | 0.3a | 4.3a | 0.4a | ||
aMeans followed by the same letter within a column are not statistically different according to the Tukey-Kramer multiple range test at p < 0.05.
Dimethenamid-P + isoxaflutole + metribuzin provided 52% and 28% better control than pendimethalin and pethoxamid, respectively.
At 8 WAA, at sites 1 and 2, there was no decrease in barnyardgrass density with the herbicide treatments evaluated compared to the untreated control (
At 8 WWA, at sites 1 and 2, the herbicide treatments evaluated did not reduce barnyardgrass biomass compared to the untreated control (
In summary, barnyardgrass control was influenced by amount of rainfall 0 to 7 DAA and 0 to 14 DAA. Sites 1 and 2 received 5 and 2.7 mm of rain 0 to 7 DAA, respectively and had poorer weed control than sites 5, 6 and 9 which received 0.8, 9.8 and 10.8 mm of rain, respectively. Although site 5, had less rainfall than sites 1 and 2, it received more rain by 14 DAA, which allowed for activation of the herbicides. At sites 5, 6 and 9, S-metolachlor provided 50% higher control than pendimethalin 12 WAA. However, opposite results were found by Janak and Grichar [
Witchgrass populations occurred at sites 6 and 7 in this study (
At 8 WAA, dimethenamid-P, pyroxasulfone, S-metolachlor, isoxaflutole + metribuzin at the three rates and the grass herbicides plus isoxaflutole + metribuzin reduced witchgrass density 96% to 99% compared to the untreated control (
At 8 WAA, all treatments reduced witchgrass biomass 98% to 99% compared to the untreated control with the exception of pendimethalin and pethoxamid which did not differ from the untreated control or other herbicide treatments (
In summary, the addition of a grass herbicide to isoxaflutole + metribuzin did not improve witchgrass control compared to isoxaflutole + metribuzin at the low, medium or high rate, this may have been due to the high level of control provided by isoxaflutole + metribuzin.
Soybean yield had a significant site by treatment interaction (data not shown), therefore sites 1, 2, 3, 4, 5 and 8 were combined, site 6 and 7 were combined and site 9 was analyzed independently (
Control 12 WAA | Density | Biomass | ||
---|---|---|---|---|
Treatment | Rate | Site 6, 7 | Site 6, 7 | Site 6, 7 |
g a∙i∙ ha−1 | % | #m−2 | g∙m−2 | |
Untreated Control | 25.3b | 19.8b | ||
Pendimethalin | 1000 | 83a | 2.4ab | 2.7ab |
Dimethenamid-P | 544 | 96a | 0.8a | 0.3a |
Pethoxamid | 840 | 83a | 3.9ab | 2.6ab |
Pyroxasulfone | 125 | 98a | 0.7a | 0.3a |
S-metolachlor | 1050 | 91a | 1.0a | 0.3a |
Isoxaflutole + Metribuzin | 52.5 + 210 | 94a | 0.8a | 0.4a |
Isoxaflutole + Metribuzin | 79 + 315 | 99a | 0.6a | 0.2a |
Isoxaflutole + Metribuzin | 105 + 420 | 99a | 0.4a | 0.2a |
Pendimethalin + Isoxaflutole + Metribuzin | 1000 + 52.5 + 210 | 100a | 0.2a | 0.1a |
Dimethenamid-P + Isoxaflutole + Metribuzin | 544 + 52.5 + 210 | 99a | 0.2a | 0.1a |
Pethoxamid + Isoxaflutole + Metribuzin | 840 + 52.5 + 210 | 98a | 0.4a | 0.1a |
Pyroxasulfone + Isoxaflutole + Metribuzin | 125 + 52.5 + 210 | 99a | 0.2a | 0.1a |
S-metolachlor + Isoxaflutole + Metribuzin | 1050 + 52.5 + 210 | 100a | 0.1a | 0.1a |
aMeans followed by the same letter within a column are not statistically different according to the Tukey-Kramer multiple range test at p < 0.05.
Soybean seed yield | ||||
---|---|---|---|---|
Treatment | Rate | Sites 6, 7 | Sites 1, 2, 3, 4, 5, 8 | Sites 9 |
g a∙i∙ ha−1 | T∙ha−1 | |||
Untreated Control | 4.3a | 3.1e | 0.9g | |
Weed Free | 5.8a | 4.7a | 3.8a | |
Pendimethalin | 1000 | 5.0a | 3.5bcde | 1.9bcdef |
Dimethenamid-P | 544 | 5.2a | 3.4cde | 1.5defg |
Pethoxamid | 840 | 5.1a | 3.3ed | 1.5efg |
Pyroxasulfone | 125 | 4.9a | 3.5bcde | 1.6cdefg |
S-metolachlor | 1050 | 4.9a | 3.3ed | 1.1fg |
Isoxaflutole + Metribuzin | 52.5 + 210 | 4.8a | 3.8bcd | 0.3bcde |
Isoxaflutole + Metribuzin | 79 + 315 | 5.2a | 4.0b | 2.6abcd |
Isoxaflutole + Metribuzin | 105 + 420 | 5.1a | 4.1ab | 2.9ab |
Pendimethalin + Isoxaflutole + Metribuzin | 1000 + 52.5 + 210 | 4.9a | 4.0b | 3.1ab |
Dimethenamid-P + Isoxaflutole + Metribuzin | 544 + 52.5 + 210 | 5.2a | 4.1b | 2.7abc |
Pethoxamid + Isoxaflutole + Metribuzin | 840 + 52.5 + 210 | 4.7a | 3.9bc | 2.9ab |
Pyroxasulfone + Isoxaflutole + Metribuzin | 125 + 52.5 + 210 | 5.3a | 4.1ab | 2.9ab |
S-metolachlor + Isoxaflutole + Metribuzin | 1050 + 52.5 + 210 | 4.8a | 4.0b | 2.7abcd |
aMeans followed by the same letter within a column are not statistically different according to the Tukey-Kramer multiple range test at p < 0.05.
from the untreated control. Reduced weed interference with the application of pendimethalin and isoxaflutole + metribuzin at the low rate resulted in increased soybean yield of 1.0 and 1.4 T∙ha−1 compared to the untreated control; however, were not equivalent to the weed-free control. The yield potential was lower at site 9 due to low levels of rainfall throughout the growing season.
General trends suggest the addition of a grass herbicide to isoxaflutole + metribuzin at the low rate increases control of pigweed spp., green and giant foxtail and yellow foxtail regardless of site or assessment timing. Control of other species usually increased with the addition of a grass herbicide to isoxaflutole + metribuzin at the low rate although this was not consistent across all grass herbicides, especially when the low rate of isoxaflutole + metribuzin provided a high level of control. Generally, isoxaflutole + metribuzin at the medium or high rate provided equivalent or better control of most species evaluated than the grass herbicides applied alone or with isoxaflutole + metribuzin at the low rate. The addition of pendimethalin, dimethenamid-P, pethoxamid, pyroxasulfone or S-metolachlor to isoxaflutole + metribuzin may provide an additional effective mode of action which will reduce the selection intensity for the evolution of herbicide-resistant weed biotypes. Weed control varied by species. The grass herbicides, as the name suggests, controlled the grass weed species the best. However, when sites received > 4.9 mm of rainfall within 7 DAA, control of the pigweed spp. and common lambsquarters with pendimethalin, dimethenamid-P and pyroxasulfone was 85% to 93% and 62% to 85%, respectively. The grass herbicides controlled ragweed and velvetleaf < 65% and < 72%, respectively. Generally, across all sites, pendimethalin and pyroxasulfone provided greater broadleaf weed control than the other grass herbicides. The grass herbicides provided lower control of grass species at sites 1, 2 and 4 than sites 3, 5, 6, 7, 8 and 9. This may be due to lack of activating rainfall; general trends occur where sites 1, 2 and 4 received 2.7 to 5.2 mm of rain 0 to 7 DAA and sites 3, 5, 6, 7, 8 and 9 received 0.8 to 14.9 mm of rain 0 to 7 DAA. Site 5 received 0.8 mm of rain 0 to 7 DAA, but there was delayed weed emergence at this location due to the lack of moisture. By 14 DAA, site 5 had received 12.5 mm of rain which provided moisture to activate the herbicides and control weeds prior to emergence. In general, S-metolachlor provided the most consistent grass control in this study. There was a numeric improvement in the control of all weed species with increasing rates of isoxaflutole + metribuzin in this study. The medium and high rates typically provided higher numeric control of all the species than the grass herbicides applied alone, while the low rate rarely provided similar control to the best grass herbicide for each species.
Due to natural environmental variability, weed species composition, and unforeseen outcomes within these studies, there are limitations on the conclusions obtained. Interspecies weed competition may have affected weed control otherwise accounted for by the herbicides in this study due to the variation in weed species and populations at the 10 sites. Some weed species are more competitive in nature, which would potentially suppress other species. Additionally, the competitiveness of differing species at each site may have altered the impact of weed interference on soybean yield. However, in real field situations, it is highly unlikely that two fields will have the exact same weed populations and species, therefore, results in these studies give a general trend to the efficacy of the herbicides.
Funding for this project was provided in part by the Ontario Centre of Excellence (OCE) and Bayer CropScience Canada Inc.
The authors declare no conflicts of interest regarding the publication of this paper.
Smith, A., Soltani, N., Hooker, D.C., Robinson, D.E., Kaastra, A.C. and Sikkema, P.H. (2019) Activity of Isoxaflutole plus Metribuzin Tankmixes in Isoxaflutole-Resistant Soybean. American Journal of Plant Sciences, 10, 1350-1373. https://doi.org/10.4236/ajps.2019.108097