Glyphosate Tolerant Soybean Response to Different Management Systems

The benefits of glyphosate tolerant crops technology are well-known, and its acceptance by farmers is undeniable. However, results of recent research indicate that, in some situations, glyphosate applied to herbicide-tolerant soybean crops may have phytotoxic effects affecting nutritional balance, photosynthesis and others biochemical process in plants. Despite the increasing information available on this subject, there are still scientific and technical issues that need to be clarified. Therefore, the present study aimed to assess the impact of applying different rates, management systems, and formulations of glyphosate to glyphosate-tolerant soybean trough different regions of Brazil in different environmental conditions. Two experiments were conducted over two crop seasons. A 2 × 2 × 5 (formulations × stage of application × doses) factorial design was used in each of them, for a total of 20 treatments with four replications. The study assessed a series of variables related to agronomic performance such as total chlorophyll and yield. The results suggest some problems associated with post-emergent use of glyphosate in tolerant soybean crop as 5% total yield reduction even without phytotoxicity symptoms dependent of season. There was not found any formulation interaction with yield decrease.


Introduction
Transgenic plants are being increasingly used in the development of products and services that have a significant impact on the lives of rural producers and worldwide consumers.Farmers, in particular, have embraced genetically modified organisms (GMOs) because of the advantages they offer (OECD-FAO, 2015).Before the appearance and expansion of herbicide-resistant crops, the major difficulty in managing weeds that interfere with commercial crops was to find a single product that ensured the effective control of all invasive plants, i.e., that exhibited a broad spectrum of weed control and was simultaneously selective and harmless to the crop, which happens to glyphosate (Ferreira et al., 2013).
With the expansion of area occupied by soybean crops in recent years in Brazil, there has been a significant increase in transgenic soybean (with tolerant to herbicides or resistant to insects, or both), covering over 96% of the soybean-cultivated area in the 2016-17 crop year (Céleres, 2017).these findings are a rule or if they depend on environment conditions, glyphosate formulation type or stage of plant development application.The findings of these studies will help to explain the actual impact of glyphosate on glyphosate tolerant-soybean crops under different field conditions and seasons.

Field Sites and Material Description
The experiments were conducted in the cities of Assis Chateaubriand (24°16′10.49″S,53°39′40.06″W),referred hereafter as Exp. 1, and Marialva (23°22′4.01″S,51°39′40.06″W),referred hereafter as Exp. 2, both repeated in 2011-12 and 2012-13 seasons.The region in which the experimental areas were located was Paraná, a Brazilian state with great potential for soybean production.Exp. 1 area is located at an altitude of 406 m, and the soil classified as a typical eutroferric red latosol.Exp. 2 area is located at an altitude of 612 m in a latosolic eutroferric red nitosol soil.Fertilization procedures, crop establishment, and phytosanitary measures followed the methods recommended by Empresa Brasileira de Pesquisa Agropecuária (EMBRAPA, 2011).The experimental areas were kept free of weed plants throughout the study via hand weeding.Data on rainfall and maximum and minimum temperatures were collected daily during the experiments.
The soybean cultivar used in both locations was NK 7059 RR (V-max RR, Syngenta Crop Protection, Santo Amaro, Brazil) the most cultivated over the last three crop years in Paraná.For Exp. 1, the dates of sowing and harvest, respectively, were September 29, 2011, and February 1, 2012, in the 2011-12 crop year and September 27, 2012, and February 3, 2013 in the 2012-13 crop year.For Exp. 2, these dates were October 20, 2011, and February 20, 2012, in the 2011-12 crop year and November 3, 2012, and March 2, 2013, in the 2012-13 crop year, respectively.
Two different commercial formulations of glyphosate, registered for post-emergence treatment of RR soybean crops in Brazil, were selected for the applications.One of the two glyphosate formulations applied was isopropylamine salt defined hereafter as IS (Roundup Ready ® , 480 g ai L -1 , Monsanto São José dos Campos, Brazil), and the other one, the potassium salt, defined hereafter as PS (Zapp QI ® , 620 g ai L -1 , Syngenta Crop Protection Santo Amaro, Brazil).

Experimental Design
Both experiments were conducted using a randomized block design with four replicates, being treatments combined in a 2 × 2 × 5 (formulations × stage of plant development at application × doses) factorial arrangement, for a total of 20 treatments.Management system one defined hereafter as M1 consisted of a single application of glyphosate at the V4 soybean stage (four unfolded trifoliate leaves).Management system two, M2, consisted of glyphosate sequential applications, the first applied at the V4 soybean stage and the second between V5 and V6 soybean stages (10 days after the first application) (Fehr et al., 1971).In M2, the glyphosate dose was divided into the two applications (Table 1).
Application of glyphosate was performed with a CO 2 -pressurized backpack sprayer equipped with a bar and six flat tips (Jacto ® 11002, Jacto São Paulo, Brazil) at a constant pressure of 29 psi and an output of 0.65 L min -1 .Spraying was performed at a distance of 50 cm from the target in a velocity of 1 m s -1 .The treated area was 50 cm wide, and the spray volume applied was 200 L ha -1 .All applications were performed under adequate environmental conditions.The plots consisted of six 5 m long rows with spacing of 0.45 m.A work area of 5.4 m 2 was used for assessment, which included only the four central rows and discarded the 1 m borders on each end of the rows.
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Effects
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Effects
In The mean yield for this year was similar to the historical regional mean (around 3,000 kg ha -1 ) and different to that obtained in the first year (1,843 kg ha -1 ).There was a clear reduction in yield with increasing rates of glyphosate.Figure 11 shows a regression-curve fitting for formulation IS in M2 displaying a linear decreasing response, with a 0.120 kg reduction in yield grain for every glyphosate g ia ha -1 increase.In this experiment, the number of pods was not affected.Therefore, the variable that correlated with decreasing yield was seed weight.
With the results observed in the two seasons, for Exp. 1 location, it noticeable that the stress caused by environmental conditions can be more harmful to soybean productivity than glyphosate (Carvalho et al., 2002).The yield reduction in these location reached 40% caused by water stress, while glyphosate had no effect.In addition, when there exists favorable conditions for plants development, glyphosate can cause yield losses even without symptoms on soybean, in this case, more pronounceable with potassium salt in a sequential application.
The differences between formulations were already observed (Santos et al., 2007a), which observed differences in the translocation of 14 C-glyphosate, for application of IS and PS products, especially in the nodules of soybean plants.
In Exp. 2 location, during crop year 2011-12, there were no differences in yield.The results obtained from both locations in this crop year were similar; some previous considerations are valid for this experiment.Even not statistical significant, there was a reduction in yield with increasing glyphosate PS rates in management M1.
There was a 0.0397 kg reduction in grain yield for every g ia ha -1 increase in glyphosate and, therefore, a reduction in seed weight.
It was not possible to fit a model of yield response to this Exp. 2 location during 2012-13 crop year (Table 3).However, the environmental conditions in this location and crop year did produce visually detectable phytotoxicity symptoms that had, until that point, remained latent.This observation was not associated with a reduction in yield, but rather, with a negative effect on other traits, such as plant height and photosynthetic systems.This finding indicates that a positively relation between traits and final yield are not always present, and the same, for initial phytotoxicity symptoms.Xenobiotic stress caused by glyphosate or its degradation products on RR soybean plants is often unnoticed by the producer and even by expert technicians.This is because, in most cases, there is no visual phytotoxic effect on the plants, even at the highest doses, as was demonstrated here in Exp. 1, second season.Even without symptoms, there is a yield reduction on crop that can reach almost 5% of total production.It is, however, possible to infer from the data and the tests of interaction that increasing glyphosate rates caused damage to the plants and, specifically, the crop's yield components and that this is not related to visual crop observations.This can be related to the development of new generation tolerant-soybeans.
The lack of significant visual effects on plants from the application of glyphosate is an important phenomenon that can prevent a producer from understanding the actual damage caused by a high rate of glyphosate to the crop.The widespread notion that RR soybean plants are totally resistant to glyphosate is an obstacle to the awareness of this phenomenon associated with the secondary effects of glyphosate (Zobiole et al., 2010b).
The putative deleterious effects of glyphosate on crop characteristics of agronomic interest have been directly discussed by several authors.These impacts are probably associated with a result of injury from the adverse action of glyphosate and its metabolites (Zobiole et al., 2010b(Zobiole et al., , 2010d;;Albrecht et al., 2012).Changes to other physiological mechanisms, such as photosynthesis and other biosynthetic processes leading to biomass accumulation result from glyphosate use and directly affect soybean yield components (Zobiole et al., 2010d.The presence of any symptom is clear related to the glyphosate application, as GMOs proved to have the same compositional variability as conventional soybean in different regions in Brazil (Zhou et al., 2011).

Conclusion
The results obtained in the present study demonstrate that, in general, any of the investigated formulations and management systems of glyphosate can be used.However, the glyphosate rate is the main limiting factor because it significantly affects crop performance and, therefore, crop yield depending on location or environmental conditions and this often occurs in the absence of visual phytotoxic symptoms.Despite the large number of experiments and assessments within the study, these results are not final.Further research is needed, especially because Brazil has numerous soybean cultivars adapted to different environmental conditions and a large land extension.

Figure
Figure 9. P

Table 1 .
Treatments performed with each glyphosate formulation (IS and PS) in 2011/12 and 2012/13 crop years

Table 3 .
Soybean yield after the application of different rates of two glyphosate formulations (IS and PS) under two management systems (M1 and M2) during 2012-13 crop year in Exp. 2 location