Precipitation Influences Pre-and Post-Emergence Herbicide Efficacy in Corn

Selecting a preemergence (PRE) and postemergence (POST) herbicide program that has the greatest efficacy can be difficult for corn producers and is highly dependent on weed spectrum. Weather conditions before and after herbicide application can further complicate decisions because they influence herbicide efficacy. Eleven field trials were conducted at three locations in Southwestern Ontario from 2003 to 2006, to determine the most effective PRE and POST corn herbicides for weed control. The most abundant weed species across all locations were redroot pigweed (Amaranthus retroflexus L.), common ragweed (Ambrosia artemisiifolia L.), common lambsquarters (Chenopodium album L.), and green foxtail (Setaria viridis (L.) Beauv.). Nine PRE herbicide treatments and eleven POST (applied at the 3 4 leaf stage of corn) herbicide treatments were tested. Results from this study suggest that the timing and amount of precipitation influence herbicide efficacy. For example, precipitation levels 0 17 mm within seven days after herbicide application (PRE or POST) provided unacceptable weed control in treatments that included atrazine, dimethenamid-p, isoxaflutole/atrazine or S-metolachlor/benoxacor. Cumulative precipitation during the 14 days after PRE application that exceeded the monthly average (by at least 64%) reduced Setaria viridis control with pendimethalin. This study demonstrates that a better understanding of how environmental conditions, especially precipitation affect herbicide efficacy, need to be considered by growers when selecting a corn herbicide program to reduce the possibility of weed control failure.


Introduction
Weather conditions, specifically precipitation amount and timing, can significantly impact the efficacy of individual preemergence (PRE) and postemergence (POST) herbicides [1][2][3][4][5].When tank-mixed, the conditions underwhich each herbicide has the greatest efficacy can differ [1].However, this also means that tank-mixtures have the potential to provide more consistent weed control over a broader range of climatic conditions [6].Tankmixing herbicides is also an effective way to combine several herbicide modes of action and control a broad spectrum of weeds, while also being cost effective by reducing the requirement for multiple applications [6][7][8][9].Some growers attempt to implement a PRE only program for weed control [10], which can be problematic if the PRE herbicide fails to control all weeds.However, the grower always has the opportunity to apply a POST herbicide should a failure occur [11,12].The benefit of including a POST component is that it allows the grower to make herbicide choices based on the weed spectrum in each individual field [13].However, timing POST herbicide application with weed size is critical [8]; if weeds become too large, control can be reduced [11,14].Weather conditions at the optimum weed size prevents accurate timing of POST application allowing weeds the opportunity to become too large for effective control [11,15,16].
For most PRE herbicides precipitation is required within 7 -14 days after application to dissolve the herbicide in soil water solution so that it can be taken up by the emerging weeds after germination [10,[17][18][19][20]. Inadequate or delayed precipitation can reduce herbicide effectiveness and decrease weed control [1,4,11,[21][22][23]. Depending on soil type, high amounts of precipitation (i.e.greater than 25 mm), especially immediately after application, can cause herbicides to leach through the soil profile and consequently reduce efficacy [6,24,25].Most POST herbicides require that there is no precipitation for several hours after application to ensure that movement across the leaf membrane can occur.
It is widely known that PRE herbicides such as S-metolachlor/benoxacor/atrazine, isoxaflutole + atrazine, and dimethenamid-p + atrazine require precipitation within 7-10 days after application for proper movement into the active zone of weed seed germination [26][27][28][29].However, when several modes of action are being tank-mixed, the differential sensitivity of each herbicide to precipitation can become complicated.For example, S-metolachlor/ benoxacor/atrazine, an acetanilide/s-triazine mixture that controls a broad-spectrum of annual grasses and broadleaved weeds [6,12,17] requires precipitation within 7 -10 days for proper activation [1].However, precipitation greater than 45 mm, over 2 days, beginning within 12 hours of application may result in leaching of the atrazine component away from the active zone of weed seed germination [3].It is possible that if S-metolachlor/benoxacor/atrazine is tank-mixed with an additional mode of action such as mesotrione that there would be no decrease in broadleaved weed control because mesotrione may respond differently to precipitation after application.Pendimethalin is an example of an herbicide that is more persistent in the soil under dry conditions and can affect rotational crops, but is easily leached when soil conditions are wet [27,30,31].Furthermore, pendimethalin's weed spectrum is reduced, especially the control of annual grasses, when soil conditions are dry up to 3 weeks after application [2].
Glyphosate is a non-selective, broad-spectrum herbicide used to control many annual grasses and broadleaved weeds post emergently [32].This herbicide lacks residual control which allows weeds to emerge after application and escape control [33][34][35]; however, sequential in-crop applications of glyphosate are effective at controlling late-emerging weeds [36,37].Glyphosate efficacy can be reduced by precipitation occurring 15 min to 6 hr after application depending on formulation [24,25,27].Thus application may have to be delayed and weed size at the time of application may no longer be optimal.
Field-specific data from diverse environments that describe the performance of herbicide mixtures registered for corn are needed.These data will help to identify weed management strategies that provide season-long weed control without reducing crop yields under variable precipitation.Therefore, the specific objectives of this research were to 1) determine the most effective PRE and POST herbicide mixtures for corn when precipitation varies within 14 days of application; 2) determine if a glyphosate only or conventional management strategy is better in unfavourable environmental conditions.2. Procedures at all sites were the same unless otherwise noted.The soil was moldboard plowed in the fall and the seedbed was prepared with two passes with an s-tine cultivator with rolling basket harrows the following spring of each year.A total of twenty treatments were tested in two separate trials (Trial 1 = PRE and Trial 2 = POST) at each location.The first trial consisted of nine PRE treatments: a non-treated weedy control, S-metolachlor/benoxacor/atrazine, dimethenamid-p + atrazine, isoxaflutole + atrazine, S-metolachlor/beno-xacor/atrazine + dicamba, dimethenamid-p + dicamba/atrazine, S-metolachlor/benoxacor/atrazine + mesotrione, pendimethalin + dicamba/atrazine, pendimethalin + atrazine and rimsulfuron + S-metolachlor/benoxacor + dicamba.Rates for PRE herbicide are listed in Table 3.The second trial consisted of eleven POST treatments that were applied at the 3 -4 leaf stage of the corn unless otherwise stated: a nontreated weedy control, glyphosate (3 -4 leaf stage), glyphosate (3 -4 leaf stage) followed by glyphosate (7 -8 leaf stage), rimsulfuron + S-metolachlor/benoxacor + dicamba + a non-ionic surfactant, nicosulfuron/rimsulfuron + dicamba/diflufenzopyr + a non-ionic surfactant + urea ammonium nitrate (UAN), nicosulfuron/rimsulfuron + mesotrione + atrazine + a non-ionic surfactant, nicosulfuron + dicamba/diflufenzopyr + a non-ionic surfactant + UAN, nicosulfuron + prosulfuron + dicamba + a non-ionic surfactant, nicosulfuron + mesotrione + atrazine + a non-ionic surfactant, nicosulfuron + pendimethalin + dicamba + a non-ionic surfactant, nicosulfuron + primisulfuron/dicamba + a non-ionic surfactant, and foramsulfuron + dicamba/diflufenzopyr + UAN + a MSO.Rates for POST herbicides are listed in Table 4.

Materials and Methods
Glyphosate-resistant corn was seeded in rows spaced 76 cm apart at densities of at least 74,000 seeds ha -1 at each location.Plots were arranged in a randomized complete block design with four replications and plots were 2 m wide × 8 to 11 m long.
Percent weed control and crop yield were measured at    all sites.Percent weed control was visually assessed 28 and 56 days after treatment (DAT) using a scale of 0 to 100 where a rating of 0 was defined as no visible weed control and a rating of 100 was defined as complete control.Only data from 28 DAT are presented in this manuscript.Corn was mechanically harvested at physiological maturity using a plot combine at all sites.Corn yields were adjusted to a 15.5% moisture level.All data were subjected to analysis of variance (ANOVA) and analyzed using the PROC MIXED pro-cedure in SAS statistical software [38].Variances were partitioned into the fixed effect of herbicide treatment and into the random effects of year and location, the interaction of year and location by the fixed effect, and blocks nested within year and location establishing the environment groupings presented in each of the data tables.The assumptions of the variance analysis were tested by ensuring that the residuals were random, homogeneous, with a normal distribution about a mean of zero using residual plots and the Shapiro-Wilk normality test.When the interaction between year, location and treatment was not significant, data were pooled by environment.Percent weed control data required an acrsine square-root transformation.Crop yield data did not require transformation.Transformed data were back-transformed for presentation in the tables.Treatment means were separated at the 5% level of significance using a Fisher's Protected LSD test.

Chenopodium album
A significant treatment by year by location interaction resulted in two distinct environments for Chenopodium album control with PRE herbicides (Table 3) and two distinct environments with POST herbicides (Table 4).Preemergence control of Chenopodium album was near perfect (>99%) in environment 1 for all treatments (Table 3); however, control was variable in environment 2. S-metolachlor/benoxacor/atrazine, dimethenamid-p + atrazine, and isoxaflutole + atrazine had lower control of Chenopodium album in environment 2 compared to all other treatments (Table 3).S-metolachlor/benoxacor/ atrazine provided only 67% control of Chenopodium album in environment 2, and dimethenamid-p + atrazine or isoxaflutole + atrazine provided less than 80% control.Because PRE herbicides require precipitation to move into the zone of active seed germination, a decrease in precipitation for May 2005 of up to 59% of the 30-yr average may explain the variability among treatments (Smetolachlor/benoxacor/atrazine, isoxaflutole + atrazine, and dimethenamid-p + atrazine) in environment 2 [4,17,26,29,39].Additionally, Environment 2 (Exeter 2005) received only 3 mm of precipitation within 14 DAT (Table 2).The reduced control with isoxaflutole and S-metolachlor is supported by Chomas and Kells [10] who found that when precipitation was limited to 2 mm 14 DAT, isoxaflutole + atrazine and S-metolachlor + atrazine only controlled Chenopodium album 33% and 75%, respectively, in comparison to a weed-free control in corn.In the same study; however, pendimethalin plus atrazine provided 91% control of Chenopodium album; which is comparable to our observations of 94% averaged over both environments.
Postemergence control of Chenopodium album in environment 1 was 94% to 100% (Table 4).Sequential applications of glyphosate increased Chenopodium album control by 7% in environment 2, compared to a single application of glyphosate.Chenopodium album control among the other herbicide treatments in environment 2 only varied by 2%.The efficacy of glyphosate can be reduced if precipitation occurs 15 min to 6 hr after application depending on formulation [24,25,27].The formulation of glyphosate used in this study has a rain fast time of 15 min.Therefore, precipitation on the day of glyphosate application (Ridgetown, 2003 and 2006) (Table 2) is unlikely to have contributed to reduced Chenopodium album control.

Ambrosia artemisiifolia
A significant treatment by environment interaction resulted in two distinct environments for Ambrosia artemisiifolia control with PRE herbicides (Table 5) and two distinct environments with POST herbicides (Table 6).Ambrosia artemisiifolia control with PRE herbicides was excellent (>97%) in environment 1, but varied among treatments in environment 2 (Table 5).S-metolachlor/benoxacor/atrazine had the lowest control of Ambrosia artemisiifolia compared to any other treatment.Pendimethalin + atrazine had 7% lower control compared to the dimethenamid + dicamba/atrazine and pendimethalin + dicamba/atrazine treatments, respectively, in environment 2. Poor control with S-metolachlor/benoxacor/atrazine and pendimethalin + atrazine alone was expected as neither herbicide provides adequate control of this species [27,40].The addition of dicamba to S-metolachlor/benoxacor/atrazine or pendimethalin + atrazine increased the Ambrosia artemisiifolia control already being provided by the atrazine.Rimsulfuron + S-metolachlor/benoxacor + dicamba was a tank-mix that provided excellent (100%) control of Ambrosia artemisiifolia except in environment 2. Exeter and Ridgetown in 2005 (environment 2) received 66% less precipitation in May and June compared to a 30-yr average (data not shown).It is likely that these exceptionally dry conditions contributed to the reduced Ambrosia artemisiifolia control with dicamba because of reduced uptake of the herbicide from the soil.
Postemergence herbicide treatments provided greater than 92% control of Ambrosia artemisiifolia in both environments (Table 6).

Amaranthus retroflexus
A significant treatment by environment interaction resulted in two distinct environments for Amaranthus retroflexus control with PRE herbicides (Table 7), and two distinct environments for control with POST herbicides (Table 8).Amaranthus retroflexus control did not differ among treatments in environment 1.In environment 2, S-metolachlor/benoxacor/atrazine and dimethenamid-p + atrazine treatments had lower control compared to all other treatments except pendimethalin + atrazine.Limited precipitation at Ridgetown in 2005 (Table 2) after herbicide application may have contributed to reduced control of this species.Both of these treatments require precipitation to move into the zone of active seed germination [17,26,39].Control of Amaranthus retroflexus did not differ among POST treatments in environment 1. Sequential applications of glyphosate provided an increase in Amaranthus retroflexus control by up to 7% compared to a single application in environment 2. Amaranthus retroflexus control varied by up to 7% among treatments in environment 2, except for treatment 1 where control was 9% lower.Control of Amaranthus retroflexus was reduced when nicosulfuron + mesotrione + atrazine was applied compared to nicosulfuron/rimsulfuron + dicamba/diflufenzopyr, nicosulfuron + dicamba/diflufenzopyr and foramsulfuron + dicamba/diflufenzopyr.Mesotrione plus atrazine (POST) can be antagonistic when tank-mixed with sulfonylureas such as nicosulfuron, but to-date this has only been shown to affect control of annual grass species [7,41,42].

Setaria viridis
A significant treatment by environment interaction resulted in three distinct environments for Setaria viridis control with PRE herbicides (Table 9) and two distinct environments for POST herbicides (Table 10).For PRE herbicides, there was little variability among treatments in environment 3.In environment 1, Setaria viridis control was the lowest with pendimethalin + dicamba/ atrazine (Table 9).This result can be attributed to the high amount of precipitation received.Exeter received 68 mm of precipitation in 2003 and Ridgetown received 49 mm in 2006 during the 14 DAT (Table 2), representing 87 and 64% of the 30-year norm for the entire month of May, respectively (data not shown).Pendimethalin is more persistent under dry conditions [31] and can leach with 20 mm of precipitation in turfgrass [30].Atrazine also has the potential to leach with relatively low levels of precipitation [25].In environment 2, isoxaflutole + atrazine provided 46% control of Setaria viridis, while all other treatments had 76% or better control.Low levels of precipitation 7 DAT at Exeter in 2004 contributed to reduced control, because isoxaflutole + atrazine requires precipitation for activation [29].
With the exception of sequential applications of glyphosate, all treatments in environment 2 provided 90% or less control of Setaria viridis.Precipitation at Exeter in May and June 2005 was 41% and 55% of the monthly norm, respectively (data not shown).Dry conditions at Exeter in 2005 may have lead to reduced control through decreased herbicide translocation and uptake.Morton and Harvey [43] found similar results with primisulfuron applied POST on quackgrass (Elytrigia repens L. Nevski.) in dry conditions (no moisture 6 -8 days before application) and attributed the result to reduced translocation.Bailey et al. [21] attributed reduced control of yellow nutsedge (Cyperus esculentus L.) with POST applied metribuzin + rimsulfuron to abnormally low precipitation amounts prior to POST application.Additionally, 59 mm of precipitation was received 14DAT, which may have promoted late emergence of Setaria viridis.Nicosulfuron, foramsulfuron and glyphosate do not have residual con-trol, therefore would not have controlled late emerging weeds [27,34].Sequential applications of glyphosate had 18% greater control of Setaria viridis than a single application of glyphosate in environment 2 (Table 10).

Corn Yield
A significant treatment by environment interaction resulted in two distinct environments for corn yield with PRE herbicides (Table 11).Corn yield did not differ among herbicide treatments in environment 1. Corn yield was reduced with S-metolachlor/benoxacor/atrazine, dimethenamid-p + atrazine and pendimethalin + atrazine compared to isoxaflutole + atrazine and S-metolachlor/ benoxacor/atrazine + mesotrione in environment 2. The above results are not linked to reduced weed control.Therefore, the reason for the decrease in yield is unclear.
In environment 2, S-metolachlor/benoxacor/atrazine and S-metolachlor/benoxacor/atrazine + mesotrione had the lowest amount of corn yield compared to all other herbicide treatments.Dimethenamid-p + atrazine and pendimethalin + atrazine treatments also had reduced corn yield and this reduced corn yield coincided with reduced weed control at each location for these treatments.
A significant treatment by environment interaction resulted in two distinct environments for corn yield with POST herbicides (Table 12).There was no variation among herbicide treatments in environment 2 and only a slight variation among treatments in environment 1. Corn yield with nicosulfuron + prosulfuron + dicamba was reduced by 0.5 -0.8 T•ha -1 compared to six other treatments, which corresponds to 3% and 6% reduction in Chenopodium album and Setaria viridis control, respecttively (Table 12).

Conclusion
Results demonstrate that the efficacy of PRE and POST herbicides are affected by the timing and amount of precipitation.Limited precipitation 7 days before or after herbicide application can reduce efficacy of PRE herbicides that require precipitation for optimal activity and excessive precipitation may delay POST herbicide application allowing weeds to grow beyond an optimal size.Additionally, precipitation on the day of PRE herbicide application or precipitation 14 DAT that is greater than 50% of the monthly average can cause some herbicides to leach (i.e.atrazine and pendimethalin), resulting in reduced efficacy.Therefore, precipitation amount and timing should be considered when selecting the most appropriate weed management strategy in corn.A further objective of this study was to determine if a glyphosate only program would be provide more sustainable weed control than a mixed mode of action program under variable precipitation.These data suggest that two sequential applications of glyphosate was the most consistent weed management program for postemergence control of weeds.However, with careful monitoring of precipitation and other climatic conditions the authors recommend the incorporation of other modes of action to ensure proper herbicide resistance management.

a
Data were pooled by environment (location and year) when the interaction between environment and treatment was non-significant.Means are presented on the back-transformed scale.Means followed by the same letter within a column are not significantly different according to Fisher's Protected LSD (P < 0.05); b Environment 1: Exeter 2003, Ridgetown 2006; Environment 2: Exeter 2004/2005, Ridgetown 2005; Environment 3: Exeter 2006, Harrow 2005/2006, Ridgetown 2004.
Eleven field trials were conducted from 2003 to 2006 at the Huron Research Station, Exeter, Ontario, and the University of Guelph, Ridgetown Campus, Ridgetown, Ontario and from 2004 to 2006 at the Greenhouse and Processing Crops Research Centre, Agriculture and Agri-Food Canada, Harrow, Ontario.Soil characteristics are presented in Table 1 and precipitation in Table

Table 2 . Mean cumulative precipitation (mm) for day of application and 7 and 14 days before and after PRE and POST her- bicide application at Exeter, Harrow, and Ridgetown, Ontario between 2003 and 2006. a
a Abbreviations: PRE, preemergence; POST, postemergence; DBA, Days before application; DAA, Days after application.

Table 3 . Mean percent control of Chenopodium album in response to preemergence (PRE) herbicides 28 days after treatment (DAT) at Exeter and Ridgetown, Ontario from 2003 to 2006 and Harrow, Ontario from 2005-2006. a
a Data were pooled by environment (location and year) when the interaction between environment and treatment was non-significant.Means are presented on the back-transformed scale.Means followed by the same letter within a column are not significantly different according to Fisher's Protected LSD (P < 0.05); b Environment 1: Exeter 2003/2004/2006, Harrow 2005/2006, Ridgetown 2003/2004/2006; Environment 2: Exeter 2005; Ridgetown 2005

Table 4 . Mean percent control of Chenopodium album in response to POST herbicides 28 days after treatment (DAT) at Exe- ter and Ridgetown, Ontario from 2003 to 2006 and Harrow, Ontario from 2005-2006. a
a Data were pooled by environment (location and year) when the interaction between environment and treatment was non-significant; Means are presented on the back-transformed scale.Means followed by the same letter within a column are not significantly different according to Fisher's Protected LSD (P < 0.05).Abbreviations: POST, postemergence; fb, followed by; b All treatments other than glyphosate and glyphosate fb glyphosate had a non-ionic surfactant added at 0.2 % v/v; c Environment 1: Exeter 2003, Harrow 2005; Environment 2: Exeter 2004/2005/2006, Harrow 2006, Ridgetown 2003/2004/2005/2006; d UAN 28% was added to nicosulfuron/rimsulfuron + dicamba/diflufenzopyr; nicosulfuron + dicamba/diflufenzopyr; and foramsulfuron + dicamba/diflufenzopyr treatments at 1.25 L•ha -1 ; e An MSO at 1.75 L•ha -1 was added to this treatment.

Table 5 . Mean percent control of Ambrosia artemisiifolia in response to preemergence (PRE) herbicides 28 days after treat- ment (DAT) at Exeter, Ontario from 2003 to 2006, Ridgetown, Ontario in 2003 and 2006 and Harrow, Ontario in 2004 and 2006. a
a Data were pooled by environment (location and year) when the interaction between environment and treatment was non-significant.Means are presented on the back-transformed scale.Means followed by the same letter within a column are not significantly different according to Fisher's Protected LSD (P < 0.05); b Environment 1: Exeter 2003/2004/2006, Harrow 2004/2006, Ridgetown 2003/2006; Environment 2: Exeter 2005; Ridgetown 2005.

Table 6 . Mean percent control of Ambrosia artemisiifolia in response to POST herbicides 28 days after treatment (DAT) at Exeter and Ridgetown, Ontario from 2003 to 2006 and Harrow, Ontario from 2005 and 2006. a
a Data were pooled by environment (location and year) when the interaction between environment and treatment was non-significant.Means are presented on the back-transformed scale.Means followed by the same letter within a column are not significantly different according to Fisher's Protected LSD (P < 0.05).Abbreviations: POST, postemergence; fb, followed by; b All treatments other than glyphosate and glyphosate fb glyphosate had a non-ionic surfactant added at 0.2 % v/v; c Environment 1: Exeter 2003, Harrow 2005, Ridgetown 2005/2006; Environment 2: Exeter 2004/2005/2006, Harrow 2004/2006, Ridgetown 2003/2004; d UAN 28% was added to nicosulfuron/rimsulfuron + dicamba/diflufenzopyr; nicosulfuron + dicamba/diflufenzopyr; and foramsulfuron + dicamba/ diflufenzopyr treatments at 1.25 L•ha -1 ; e An MSO at 1.75 L•ha -1 was added to this treatment.

Table 7 . Mean percent control of Amaranthus retroflexus in response to preemergence (PRE) herbicides 28 days after treat- ment (DAT) at Exeter, Ontario from 2003 to 2006, Ridgetown, Ontario from 2004 to 2005 and Harrow, Ontario in 2006. a
a Data were pooled by environment (location and year) when the interaction between environment and treatment was non-significant.Means are presented on the back-transformed scale.Means followed by the same letter within a column are not significantly different according to Fisher's Protected LSD (P < 0.05); b Environment 1: Exeter 2003/2004/2005/2006, Harrow 2006, Ridgetown 2004; Environment 2: Ridgetown 2005.

Table 8 . Mean percent control of Amaranthus retroflexus in response to POST herbicides 28 days after treatment (DAT) at Exeter, Ontario from 2003 to 2006, Ridgetown, Ontario in 2003, 2004 and 2006 and Harrow, Ontario in 2006. a
Data were pooled by environment (location and year) when the interaction between environment and treatment was non-significant.Means are presented on the back-transformed scale.Means followed by the same letter within a column are not significantly different according to Fisher's Protected LSD (P < 0.05).Abbreviations: POST, postemergence; fb, followed by; b All treatments other than glyphosate and glyphosate fb glyphosate had a non-ionic surfactant added at a

Table 10 . Mean percent control of Setaria viridis in response to POST herbicides 28 days after treatment (DAT) at Exeter, Ontario from 2003 to 2006 and Ridgetown, Ontario from 2003 to 2005. a Rate Percent Weed Control kg•ai•ha −1 Environment 1 c Environment 2 Treatment b MT ha −1
Data were pooled by environment (location and year) when the interaction between environment and treatment was non-significant.Means are presented on the back-transformed scale.Means followed by the same letter within a column are not significantly different according to Fisher's Protected LSD (P < 0.05).Abbreviations: POST, postemergence; fb, followed by; b All treatments other than glyphosate and glyphosate fb glyphosate had a non-ionic surfactant added at 0.2 % v/v; c Environment 1: Exeter 2003/2004/2006, Ridgetown 2003/2005; Environment 2: Exeter 2005, Ridgetown 2004; d UAN 28% was added to nicosulfuron/rimsulfuron + dicamba/diflufenzopyr; nicosulfuron + dicamba/diflufenzopyr; and foramsulfuron + dicamba/diflufenzopyr treatments at 1.25 L•ha -1 ; e An MSO at 1.75 L•ha -1 was added to this treatment. a

Table 11 . Mean corn yield in the untreated check and in response to application of preemergence (PRE) herbicides at Exeter and Ridgetown, Ontario from 2003 to 2006 and Harrow, Ontario from 2004 to 2006. a Rate Yield kg•ai•ha −1 Environment 1 b Environment 2 Treatment MT ha −1
a Data were pooled by environment (location and year) when the interaction between environment and treatment was non-significant.Means followed by the same letter within a column are not significantly different according to Fisher's Protected LSD (P < 0.05); b Environment 1: Exeter 2003/2004/2006, Ridgetown 2003/2005/2006, Harrow 2004/2005/2006, Ridgetown 2004; Environment 2: Exeter 2005.

Table 12 . Mean corn yield in the untreated check and in response to application of POST herbicides at Exeter and Ridgetown, Ontario from 2003 to 2006 and Harrow, Ontario from 2004 to 2006. a
Data were pooled by environment (location and year) when the interaction between environment and treatment was non-significant.Means followed by the same letter within a column are not significantly different according to Fisher's Protected LSD (P < 0.05).Abbreviations: POST, postemergence; fb, followed by; b All treatments other than glyphosate and glyphosate fb glyphosate had a non-ionic surfactant added at 0.2 % v/v; c Environment 1: Exeter 2003/2004/2005, Ridgetown 2005; Environment 2: Exeter 2006, Harrow 2004/2005/2006, Ridgetown 2003/2004/2006; d UAN 28% was added to nicosulfuron/rimsulfuron + dicamba/diflufenzopyr; nicosulfuron + dicamba/diflufenzopyr; and foramsulfuron + dicamba/diflufenzopyr treatments at 1.25 L•ha -1 ; e An MSO at 1.75 L•ha -1 was added to this treatment. a