Benefit of Adding Ammonium Sulfate or Additional Glyphosate to Glyphosate in Corn and Soybean

Nine field trials (4 in corn and 5 in soybean) were conducted over 2 years (2014 and 2015) to determine if there is greater benefit of adding ammonium sulfate (AMS) (2.5 L∙ha) or an equal dollar value of glyphosate (406 g∙ae∙ha) to glyphosate applied at 450, 675 or 900 g∙ae∙ha for weed control in corn and soybean. Glyphosate applied at 450 g∙ae∙ha controlled velvetleaf 90% to 98%, common ragweed 80% 97%, common lambsquarters 91% 99%, Eastern black nightshade 83% 100% and barnyardgrass 73% 97% in corn and common ragweed 37% 89%, common lambsquarters 39% 98%, barnyardgrass 90% 98% and green foxtail 91% 98% in soybean. The addition of AMS to glyphosate applied at 450, 675 or 900 g∙ae∙ha provided little to no added benefit for the control of velvetleaf, common ragweed, common lambsquarters, Eastern black nightshade, barnyardgrass and green foxtail in corn and soybean. There was a greater benefit in weed control efficacy by simply adding and equal dollar value of glyphosate (406 g∙ae∙ha) than AMS (2.5 L∙ha) to glyphosate. There was no difference in corn or soybean yield among the herbicide treatments evaluated. Based on these results, addition of AMS to glyphosate at rates evaluated had little benefit on weed control efficacy or yield of corn and soybean.


Introduction
The Ontario corn and soybean industry covers about 2,000,000 ha, and has annual farm-gate value of approximately $3.3 billion [1].Development of management strategies for the control of weeds in corn and soybean has been voted as the number one research priority by the Ontario Weed Committee (the voting members of the Ontario Weed Committee are representatives of the commodity groups such as corn, soybean, wheat and edible beans, industry personnel and public research scientists).Losses due to weeds include reduced yield, decreased quality, reduced field efficiency, and greater losses due to spoilage, increased dockage at the point of sale, and potential rejection of the crop.
Currently, more than 90% of the corn and 79% of soybean produced in Ontario are seeded to glyphosate-resistant hybrids/cultivars [2].Glyphosate-resistant corn and soybean provide Ontario crop producers cost-effective weed management options that are efficacious and environmental friendly [3].The net economic benefit to farmers is the main reason for wide adoption of this technology by crop producers in eastern Canada [4].Despite glyphosate broad spectrum weed control, inconsistencies in control of some weed species have been reported at some locations which have been associated with water carrier solution [5] [6].Nurse et al. [7] has shown benefit of adding AMS to glyphosate at low dose (225 g•ha −1 ) shortly after application (7 DAA) for control of velvetleaf (Abutilon theophrasti Medic.), however, at the same time there were no benefit for control of common lambsquarters (Chenopdium album L.), redroot pigweed (Amaranthus retroflexus L.), and annual grasses such as foxtails (Setaria spp.).It has been proposed that in the presence of AMS, the sulfate ion, SO − binds with Ca 2+ to form CaSO 4 and prevents formation of a Ca-salt of glyphosate, which is poorly absorbed by the leaves [6].Instead 4 NH + binds with the glyphosate molecule which re- sults in greater absorption of glyphosate into the leaves and therefore improves weed control [6].
Limited information exists on the effect of ammonium sulfate added to glyphosate in corn and soybean under Ontario environmental conditions which can lead to unnecessary expenditures.
The objectives of this research were to determine the benefit of adding ammonium sulfate or an equal dollar value of glyphosate (406 g•ae•ha −1 ) to glyphosate applied at 450, 675 or 900 g•ae•ha -1 for weed control in corn and soybean.to 83% sand, 5% to 36% silt, and 13% to 33% clay, with 2.6% to 5.6% organic matter content and pH 6.0 to 7.4.Seedbed preparation included fall mouldboard plowing followed by two passes with a field cultivator with rolling basket harrows in the spring.

Materials and Methods
Studies were established as a randomized complete block design with four replica-tions.Treatments for corn and soybean trials were the same and are listed in Tables 1-9.Weed-free control was maintained weed-free by hand hoeing as needed.Each plot was 3 m wide and 8 or 10 m long and consisted of four rows of glyphosate resistant corn/soybean spaced 0.75 m apart.Corn was planted at a rate of approximately 75,000 seeds•ha −1 in May of each year.Soybean was planted at a rate of approximately 370,000 seeds•ha −1 in May to early June of each year.
Herbicide treatments were applied using a CO 2 -pressurized backpack sprayer calibrated to deliver 200 L•ha −1 aqueous solution at 240 kPa.The boom was 1.5 m wide with four ULD120-02 nozzles (Hypro, New Brighton, MN, USA) spaced 0.5 m apart.
The herbicides were applied postemergence when weeds were approximately 10 cm in height which corresponded to the V2-4 stage for corn and V2-3 stage for soybean.
Weed control was visually estimated on a scale of 0 (no control) to 100% (complete control) at 1, 2, 3, 4 and 8 weeks after herbicide application (WAT).Weed density (soybean only) and dry weight (soybean only) by species were evaluated 4 WAT by counting and cutting plants at the soil surface form 1 square meter in each plot.Plants were dried at 60 C to constant moisture and then weighed.Corn or soybean was harvested at maturity with a small plot combine, weight and moisture recorded and yields were adjusted to 15.5% and 13% seed moisture content for corn and soybean, respectively.
Data were analyzed using PROC GLIMMIX in Statistical Analysis Systems Software         (SAS) [8].Herbicide treatment was considered a fixed effect, while random effects included environment (year-location combinations), environment by herbicide treatment interaction, and replicate nested within environment.Significance of the fixed effect was tested using the F-test and random effects were tested using a likelihood ratio.Residual plots were used to check that variances were randomly distributed, independent and homogeneous.The Laplace method was used for estimation method; weed control and yield were analyzed using a normal distribution and identity link and weed density and dry weight were analyzed using a lognormal distribution and identity link.Data were square root or arcsine square root transformed prior to analysis when necessary.
Treatment comparisons were made using Fisher's Protected LSD at P < 0.05.Data compared on the transformed scale were converted back to the original scale for presentation of results.

Results and Discussion
Data were pooled and averaged over years and locations when there was no statistically significant interaction between year, location and treatments (Tables 1-9).Weeds were included when present in 3 or more trials.

Corn Trials
There was no observable corn injury for any of the treatments tested (data not shown).
For corn trials the predominant weed species included velvetleaf, common ragweed, common lambsquarters, Eastern black nightshade and barnyardgrass.
There were two exceptions, at 2 WAT, velvetleaf control was improved with the addition of glyphosate (406 g•ae•ha −1 ) to glyphosate at 450 and 675 g•ae•ha −1 (Table 1).
Results are similar to other studies that have shown no improvement in control of velvetleaf, redroot pigweed, common lambsquarters and annual grasses when AMS was added to glyphosate [9].However, Pratt et al. [5] reported greater control of velvetleaf when AMS was added to glyphosate compared to glyphosate alone.Nurse et al. (2008) and Young et al. [10] have reported that velvetleaf control is enhanced with the addition of AMS to glyphosate.Nurse et al. [7] and Young et al. [10] found that the control of other weed species such as common lambsquarters does not always improve with the addition of AMS to glyphosate.It should be noted that all of these studies only reported a benefit of the addition of AMS to glyphosate at glyphosate doses <450 g•ae•ha −1 , which is half the labeled dose in Ontario.

Soybean Trials
There was no observable soybean injury for any of the treatments tested (data not shown).For soybean trials the predominant weed species included common ragweed, common lambsquarters, barnyardgrass and green foxtail.
Reduced weed interference with glyphosate applied at 450, 675 and 900 g•ae•ha −1 resulted in soybean yields that were equivalent to the weed-free check (Table 9).The addition of AMS (2.5 L•ha −1 ) or additional glyphosate (406 g•ae•ha −1 , same dollar value as AMS) to glyphosate at 450, 675 and 900 g•ae•ha −1 did not result in increased soybean yield (Table 9).In other studies, Nurse et al. [7] and Soltani et al. [9] also reported no difference in yield when AMS was added to a full labeled rate of glyphosate (900 g•ae•ha −1 ).

Conclusion
The addition of AMS or an equal dollar value of glyphosate to glyphosate applied at 450, 675 or 900 g•ae•ha −1 provided no consistent benefit for the control of velvetleaf, common ragweed, common lambsquarters, Eastern black nightshade and barnyardgrass in corn and common ragweed, common lambsquarters, barnyardgrass and green foxtail in soybean.There was no difference in corn or soybean yield among the herbicide treatments evaluated.The similar levels of weed control provided by the herbicide treatments evaluated provide a plausible reason for there being no effect of adding AMS or additional glyphosate to glyphosate on corn or soybean yield.Based on these results, the addition of AMS to glyphosate at the full label rate had little benefit on weed control efficacy and corn and soybean yield under Ontario environmental conditions.

Four field trials ( 1 at
Harrow in 2014 and 3 at Ridgetown in 2015) in corn and 5 field trials in soybean (1 at Harrow in 2014, 1 at Exeter in 2014 and 3 at Ridgetown in 2015) were conducted at the Agriculture and Agri-Food Canada Research Station, Harrow, Ontario, Huron Research Station, Exeter, Ontario and University of Guelph, Ridgetown Campus, Ridgetown, Ontario.The soil ranged from sandy loam to clay loam with 30%

Table 1 .
Percent visible control of velvetleaf 1, 2, 3, 4 and 8 WAT in corn with various rates of glyphosate alone or with AMS or additional glyphosate equivalent to the cost of the AMS at Ridgetown ON (2014-2015) a .
a Abbreviations: AMS, ammonium sulfate; WAT, weeks after herbicide application.Means followed by the same letter within a column are not significantly different according to Fisher's Protected LSD at P < 0.05.

Table 2 .
Percent visible control of common ragweed 1, 2, 3, 4 and 8 WAT in corn with various rates of glyphosate alone or with AMS or additional glyphosate equivalent to the cost of the AMS at Ridgetown ON (2014-2015) a .
a Abbreviations: AMS, ammonium sulfate; WAT, weeks after herbicide application.Means followed by the same letter within a column are not significantly different according to Fisher's Protected LSD at P < 0.05.

Table 3
. Percent visible control of lambsquarters 1, 2, 3, 4 and 8 WAT in corn with various rates of glyphosate alone or with AMS or additional glyphosate equivalent to the cost of the AMS at Ridgetown and Harrow, ON (2014-2015) a .a Abbreviations: AMS, ammonium sulfate; WAT, weeks after herbicide application.Means followed by the same letter within a column are not significantly different according to Fisher's Protected LSD at P < 0.05.

Table 4 .
Percent visible control of Eastern black nightshade 1, 2, 3, 4 and 8 WAT in corn with various rates of glyphosate alone or with AMS or additional glyphosate equivalent to the cost of the AMS at Ridgetown and Harrow, ON (2014-2015) a .
a Abbreviations: AMS, ammonium sulfate; WAT, weeks after herbicide application.Means followed by the same letter within a column are not significantly different according to Fisher's Protected LSD at P < 0.05.

Table 5 .
Corn yield and percent visible control of barnyardgrass 1, 2, 3, 4 and 8 WAT in corn with various rates of glyphosate alone or with AMS or additional glyphosate equivalent to the cost of the AMS at Ridgetown and Harrow, ON (2014-2015) a .
a Abbreviations: AMS, ammonium sulfate; WAT, weeks after herbicide application.Means followed by the same letter within a column are not significantly different according to Fisher's Protected LSD at P < 0.05.

Table 6 .
Percent visible control of common ragweed 1, 2, 3, 4 and 8 WAT in soybean with various rates of glyphosate alone or with AMS or additional glyphosate equivalent to the cost of the AMS at Ridgetown, Exeter and Harrow ON (2014-2015) a .
a Abbreviations: AMS, ammonium sulfate; WAT, weeks after herbicide application.Means followed by the same letter within a column are not significantly different according to Fisher's Protected LSD at P < 0.05.

Table 7 .
Percent visible control of lambsquarters 1, 2, 3, 4 and 8 WAT in soybean with various rates of glyphosate alone or with AMS or additional glyphosate equivalent to the cost of the AMS at Ridgetown, Exeter and Harrow ON (2014-2015) a .
a Abbreviations: AMS, ammonium sulfate; WAT, weeks after herbicide application.Means followed by the same letter within a column are not significantly different according to Fisher's Protected LSD at P < 0.05.

Table 8 .
Percent visible control of barnyardgrass 1, 2, 3, 4 and 8 WAT in soybean with various rates of glyphosate alone or with AMS or additional glyphosate equivalent to the cost of the AMS at Ridgetown, Exeter and Harrow ON (2014-2015) a .
a Abbreviations: AMS, ammonium sulfate; WAT, weeks after herbicide application.Means followed by the same letter within a column are not significantly different according to Fisher's Protected LSD at P < 0.05.

Table 9 .
Corn yield and percent visible control of green foxtail 1, 2, 3, 4 and 8 WAT in soybean with various rates of glyphosate alone or with AMS or additional glyphosate equivalent to the cost of the AMS at Ridgetown and Exeter ON (2014-2015) a .
a Abbreviations: AMS, ammonium sulfate; WAT, weeks after herbicide application.Means followed by the same letter within a column are not significantly different according to Fisher's Protected LSD at P < 0.05.

Table 7 )
. The addition of AMS (2.5 L•ha −1 ) to glyphosate (at 450, 675 and 900 g•ae•ha −1 ) did not improve control of common lambsquarters at 1, 2, 3, 4 and 8 WAT.At 1 and 3 WAT the addition of glyphosate (406 g•ae•ha −1 ) to glyphosate at 450 g•ae•ha −1 resulted in improved control of lambsquarters.Lambsquarters control was greater with glyphosate at 1181 g•ae•ha−1 glyphosate (406 g•ae•ha −1 , same dollar value as AMS) to glyphosate 675 g•ae•ha −1 improved the control of barnyardgrass; and at 3 WAT, the addition of additional glyphosate (406 g•ae•ha −1 , same dollar value as AMS) to glyphosate 900 g•ae•ha −1 improved the control of barnyardgrass but there was no decrease in barnyardgrass density or dry weight (Table