Tolerance of Maize ( Zea mays L . ) and Soybean [ Glycine max ( L . ) Merr . ] to Late Applications of Postemergence Herbicides

Seven maize (Zea mays L.) and three soybean [Glycine max (L.) Merr.] field experiments were conducted from 2006 to 2009 at various locations in southern Ontario, Canada to determine the tolerance of these crops to late applications of the maximum labeled herbicide dose. Single and sequential (simulating a spray overlap) applications were evaluated for visible injury, plant height, and crop yield in the absence of weed competition. Maize exhibited excellent tolerance to herbicides applied at the 9to 10-leaf growth stage as visible injury levels for almost all tested herbicides was similar to the untreated control 7 days after treatment (DAT). However, the sequential application of dicamba/diflufenzopyr or foramsulfuron caused 6 and 8% injury 7 DAT and 8 and 14% reduction in maize height 28 DAT, respectively. The observed injury and stunting were transient as there were no differences in yield at harvest. Soybean displayed good tolerance to most herbicides applied at the 7th trifoliate leaf growth stage as visible injury levels were similar to the untreated control. However, thifensulfuron-methyl was injurious regardless of application and imazethapyr was injurious with sequential applications. For example, single thifensulfuron-methyl, sequential thifensulfuron-methyl, and sequential imazethapyr application treatments caused 35, 48, and 25% injury 7 DAT, respectively. Sequential thifensulfuron-methyl treatments also caused a 28 and 17% reduction in soybean height 14 and 28 DAT, respectively. Visual injury continued to be detected up to 56 DAT for single thifensulfuron-methyl, sequential thifensulfuronmethyl, and sequential imazethapyr treatments. But, soybean yields were reduced by 10% for only sequential thifensulfuron-methyl application treatments. For all other herbicides tested, the yields at harvest were similar to the untreated control. This research demonstrated that maize had exceptional tolerance to all the herbicides used in this study whereas soybean was tolerant to most of the herbicides used in this study. Corresponding author. K. J. Mahoney et al.


Introduction
The critical weed-free period has provided Ontario growers with the knowledge of when to control the weeds that cause detrimental yield loss in maize [1] and soybean [2] for quite some time.Researchers have also recognized that the critical weed-free period can vary from year to year and location to location [3] [4], undermining the potential utility and implementation of this integrated weed management strategy [5].Yet, research continues to be conducted as a better understanding of some of the underlying physiological mechanisms that underpin the critical weed-free period have recently been published [6] [7].In general, crops need to be maintained under weed-free conditions from the start of the critical weed-free period until at least the 10-leaf stage in maize [1] [3] and the R1 (early flowing) stage in soybean [2] [3].
Ontario maize and soybean growers have numerous herbicide options [8] for managing weeds during the critical weed-free period.Unfortunately, growers can sometimes miss registered herbicide application windows due to adverse weather conditions or mechanical breakdowns which leave large, highly competitive weeds present in the crop at a point beyond the critical weed-free period when rapid yield loss occurs [1]- [3].At this time, growers would like to apply a high dose of postemergence (POST) herbicide to ensure effective control of these large weeds, but growers also are concerned that crop injury could negatively impact yield.Regrettably, the tolerance of maize and soybean to a high herbicide dose at a late POST application timing is largely unknown.The exception to this is maize, which can tolerate over two-fold of the maximum labeled dose of glyphosate applied at the 10-leaf stage with minimal injury and little to no yield loss [9].Furthermore, to the best of our knowledge, few studies have been conducted in the absence of confounding weed competition effects that examine both a range of herbicides comparing relative crop tolerance [10] and the tolerance of crops to a late POST herbicide application [11].Therefore, the objective of this research was to determine the tolerance of maize and soybean to a late application of select POST herbicides in the absence of weed interference.
In both the maize and soybean experiments, each treatment plot was 2 m wide by 8 to 10 m long.Glyphosateresistant maize hybrids (Table 1) were seeded 4 to 5 cm deep at a rate of approximately 75,000 seeds•ha −1 in rows spaced 0.75 m apart.Glyphosate-resistant soybean cultivars (Table 1) were seeded 2.5 to 3 cm deep at a rate of approximately 480,000 seeds•ha −1 in rows spaced 0.75 m apart.Herbicide treatments were applied to maize at the 9-to 10-leaf growth stage and to soybean at the 7 th trifoliate leaf growth stage, both application timings were later than recommended [8].At Exeter, herbicides were applied using a CO 2 pressurized backpack sprayer calibrated to deliver 200 L•ha −1 at 240 kPa through Tee-Jet 8002 VS nozzles (Spraying Systems Co., Glendale Heights, IL).At Harrow, a CO 2 pressurized backpack sprayer delivered 222 L•ha −1 at 210 kPa through Tee-Jet 11003 XR nozzles (Spraying Systems Co., Glendale Heights, IL).At Ridgetown, a CO 2 pressurized backpack sprayer delivered 200 L•ha −1 at 207 kPa through Ultra-Low Drift 120-02 nozzles (Hypro, New Brighton, MN).All plots were maintained weed-free for the entire growing season using preemergence herbicides and hand weeding as needed.

Data Collection and Analysis
In both the maize and soybean experiments, visible crop injury was rated 3, 7, 14, 21, 28, and 56 days after treatment (DAT) based on a scale of 0 (no injury) to 100% (complete plant death) relative to untreated, weed-free control plants.Average plant height was recorded 28 and 56 DAT in maize by measuring the height of the crop from the soil surface to the extended leaf height and 14 and 28 DAT in soybean by measuring the height of the crop from the soil surface to the growing point of the plant.Both crops were harvested at maturity using a small plot combine and crop moisture and weight were recorded; final yields were adjusted to 15.5 and 13% moisture content for maize and soybean, respectively.Data for crop injury, crop height, and crop yield were analyzed separately by crop using PROC MIXED (SAS Ver.9.2, SAS Institute Inc., Cary, NC).In the individual analysis of the maize and soybean experiments, variances were divided into fixed (herbicide treatment) and random effects [block; environment (i.e., year or location-year combinations); and the herbicide treatment × environment interaction].The significance of the fixed effect in the maize and soybean experiments was tested using an F-test and the significance of random effects was tested using a Z-test of the variance estimate.PROC UNIVARIATE in SAS was used to test data for normality and homogeneity of variance.The herbicide treatment × environment interactions in maize and soybean experiments were not significant and therefore the data for each set of experiments were pooled across environments within each crop.Means were separated using Fisher's Protected LSD at P < 0.05.

Maize Experiments
Maize exhibited excellent tolerance to the POST herbicides applied at the 9-to 10-leaf growth stage.At 3 DAT, only the sequential application of 2,4-D/atrazine caused significant visible injury of 9% (Table 2).At 7 DAT, the sequential application treatments of dicamba/diflufenzopyr, foramsulfuron, and 2,4-D/atrazine caused 6, 8, and 9% injury, respectively (Table 2).Sequential applications tended to cause at least two-fold greater injury than a single application.For example, 3% injury was observed for a single foramsulfuron application whereas the sequential application treatment caused 7% injury 14 DAT (Table 2).Furthermore, a single application of  dicamba/diflufenzopyr caused no injury, but the sequential application caused 6% injury 14 DAT (Table 2).Similar levels of injury to maize by foramsulfuron [12]- [15] and dicamba/diflufenzopyr [16]- [19] have been reported.Conversely, up to 16% injury 3 DAT has been found in maize treated with twice the labeled dose of foramsulfuron and 23% injury 7 DAT for maize treated with twice the dose of dicamba/diflufenzopyr [10].Maize tolerance to foramsulfuron can vary considerably by application timing and maize hybrid [20] [21].In the current study, the visible injury was transient with reduced injury observed at 21, 28, and 56 DAT.At 56 DAT, there were no differences in injury among the treated and non-treated maize (Table 2).Although little to no visible injury was detected for the foramsulfuron or dicamba/diflufenzopyr treatments 28 DAT (Table 2), maize plants at this time were shorter than the untreated control.For example, a 9% reduction in maize height was recorded for a single foramsulfuron application and the sequential application treatment caused a 14% reduction in height 28 DAT (Table 3).The height of maize plants treated with a single application of dicamba/diflufenzopyr was similar to the untreated control, whereas sequential applications caused an 8% reduction in height 28 DAT (Table 3).This is similar to other studies which demonstrated stunting or reduced growth in maize due to applications of foramsulfuron or dicamba/diflufenzopyr [10] [12]- [14].Nevertheless, the observed reductions in height in this study were transient as no differences were detected among the treated and non-treated maize by 56 DAT.Furthermore, the excellent tolerance to the herbicides used in this study was confirmed as the final maize yields were similar to the untreated control across all treatments (Table 3), consistent with other studies [10] [16] [19].

Soybean Experiments
Soybean displayed good tolerance to most of the POST herbicides applied in a single application at the 7 th trifoliate leaf growth stage as visible injury levels 3 and 7 DAT for almost all of these treatments were similar to the untreated control.The exception to this trend was thifensulfuron-methyl, which caused 27% injury 3 DAT and 35% injury 7 DAT (Table 4), which concurs with other studies [22] [23].Injury levels observed 7 DAT for se-  quential chlorimuron-ethyl, sequential imazethapyr, and sequential thifensulfuron-methyl treatments were greater than the untreated control with 19, 25, and 48% injury, respectively (Table 4).This is similar to other studies which demonstrated 15 to 20% injury from chlorimuron-ethyl [24] [25], 16 to 30% injury from imazethapyr [22] [23] [26]- [28], and up to 44% injury from thifensulfuron-methyl [29].In this study, soybean injury decreased over time across all treatments.However, soybean injury continued to be detected up to 56 DAT for sequential imazethapyr, single thifensulfuron-methyl, and sequential thifensulfuron-methyl treatments with 13, 20, and 29% injury, respectively (Table 4).For sequential thifensulfuron-methyl treatments, visual injury symptoms were also coupled with decreased plant height.For example, sequential thifensulfuron-methyl treatments caused a 28 and 17% reduction in plant height 14 and 28 DAT, respectively (Table 5), similar to previous research [26] [29].In the current study, persistent observations of injury and reduced height resulted in a 10% reduction in soybean yield for sequential thifensulfuron-methyl treatments (Table 5), consistent with other studies [22] [26] [27] [29].For all other treatments used in this study, soybean exhibited good tolerance to these herbicides as the yields at harvest were similar to the untreated control (Table 5).

Conclusions
For maize and soybean growers concerned about crop injury when a high dose of herbicide is applied later than recommended [8], this research demonstrated that maize had exceptional tolerance to all the herbicides applied at the 9-to 10-leaf growth stage.This study expands upon related work with glyphosate in maize [9].The most injurious treatments in this study, sequential foramsulfuron and sequential 2,4-D/atrazine treatments, caused only 8 and 9% injury 7 DAT, respectively.Yet, by harvest, these and the other herbicides tested had no effect on yield when compared to the untreated weed-free control.Conversely, soybean growers needing a late POST application using a high dose should exercise some caution during herbicide selection as soybean was tolerant to most of the herbicides used in this study.Thifensulfuron-methyl was injurious regardless of application and imazethapyr was injurious with sequential applications as significant soybean injury was detected from 3 to 56 DAT for these herbicides.However, soybean yield were reduced by 10% for only sequential thifensulfuron-methyl treatments.For the remaining herbicides, soybean yields at harvest were similar to the untreated control, indicative of good tolerance.
DAT, days after treatment; fb, followed immediately by.b Means followed by the same letter within a column are not significantly different according to Fisher's Protected LSD (P < 0.05).c Included Agral 90 at 0.2% v/v.d Included 28% UAN at 2.5 L•ha −1 .e Included Agral 90 at 0.25% v/v and 28% UAN at 1.25% v/v.

Table 1 .
Seeding and emergence dates of maize hybrids and soybean cultivars and the spray date for postemergence herbicides used in ten field experiments established from 2006 to 2009 at various locations in Ontario, Canada to examine the tolerance of maize and soybean to late applications.

Table 2 .
Visible injury of maize after late applications of postemergence herbicides at three locations (Exeter, Harrow, and Ridgetown) in Ontario, Canada in 2006 and 2007.ab

Table 3 .
Maize height and yield after late applications of postemergence herbicides at three locations (Exeter, Harrow, and Ridgetown) inOntario, Canada in 2006 and 2007.ab a Abbreviations: DAT, days after treatment; fb, followed immediately by.b Means followed by the same letter within a column are not significantly different according to Fisher's Protected LSD (P < 0.05).c Included Agral 90 at 0.2% v/v.d Included 28% UAN at 2.5 L•ha −1 .e Included Agral 90 at 0.25% v/v and 28% UAN at 1.25% v/v.