Selectivity of chemical weed control systems in conventional cotton

Seletividade de sistemas de controle químico de plantas daninhas em algodoeiro convencional

ARTIGOS

Selectivity of chemical weed control systems in conventional cotton

Seletividade de sistemas de controle químico de plantas daninhas em algodoeiro convencional

Arantes J.G.Z.^I; Constantin J.^II; Oliveira Jr. R.S.^II; Braz G.B.P.^II; Barbosa C.A.S.^III; Brugnera P.^III; Oliveira Neto A.M.^IV; Gemelli A.^II

^IUniversidade de Cuiabá (UNIC), Primavera do Leste-MT, Brazil, <[e-mail]>

^IIUniversidade Estadual de Maringá (UEM/NAPD), Maringá-PR, Brazil

^IIICírculo Verde Agronomic Consultancy & Research, Luís Eduardo Magalhães-BA, Brazil

^IVFaculdade Integrado, Campo Mourão-PR, Brazil

ABSTRACT

Cotton is highly susceptible to the interference imposed by weed community, being therefore essential to adopt control measures ensuring the crop yield. Herbicides are the primary method of weed control in large-scale areas of production, and usually more than one herbicide application is necessary due to the extensive crop cycle. This study aimed to evaluate the selectivity of different chemical weed control systems for conventional cotton. The experiment took place in the field in a randomized block design, with twenty nine treatments and four replications in a split plot layout (adjacent double check). Results showed that triple mixtures in pre-emergence increased the chance of observing reductions in the cotton yield. To avoid reductions in crop yield, users should proceed to a maximum mixture of two herbicides in pre-emergence, followed by S-metolachlor over the top, followed by one post-emergence mixture application of pyrithiobac-sodium + trifloxysulfuron-sodium.

Keywords: Gossypium hirsutum, application modalities, tank mixture.

RESUMO

O algodoeiro apresenta elevada sensibilidade à interferência imposta pela comunidade infestante, sendo fundamental a adoção de medidas de controle para assegurar a produtividade da cultura. O manejo químico consiste no principal método de controle de plantas daninhas utilizado em áreas de produção de larga escala, sendo por vezes necessária mais de uma aplicação de herbicidas, em razão do extenso ciclo que o algodoeiro possui. O objetivo deste trabalho foi avaliar a seletividade de diferentes sistemas de controle químico de plantas daninhas para o algodoeiro convencional. O experimento foi instalado em campo, em delineamento de blocos casualizados, adotando-se 29tratamentos e quatro repetições, em esquema de parcelas subdivididas. Os resultados demonstraram que a utilização de misturas triplas em pré-emergência aumenta a possibilidade de se verificar reduções na produtividade do algodoeiro. A fim de evitar reduções na produtividade da cultura, deve-se proceder à mistura de até dois herbicidas em pré-emergência, juntamente com o S-metolachlor em over the top, e à aplicação única em pós-emergência da mistura de pyrithiobac-sodium + trifloxysulfuron-sodium.

Palavras-chave: Gossypium hirsutum, modalidades de aplicação, mistura em tanque.

INTRODUCTION

Brazil currently detains a prominent place in cotton production and is ranked among the top five cotton producers in the world (ICAC, 2014). Two factors that are contributing to the variation of cotton planted area in Brazil are good economic returns that the marketing of the fiber production has brought to producers in recent seasons, as well as the possibility of a second crop cultivation in rotation with soybeans areas, which allowed its exploration in areas where the culture was not usually cultivated.

The high economic importance that the cotton represents for the agricultural sector creates a continuing need of cultural practices that are developed, in order to maintain and even increase the culture productivity levels, and this adopted strategy aims to minimize the risks affecting cotton plants throughout its growing cycle. Among the main factors responsible for losses in yield and the industrial quality of the fiber produced, weeds are the cause of huge losses to the cotton when not handled properly (Freitas et al., 2002; Keeling et al., 2011).

The critical period of weed control (CPWC) of weeds in cotton varies according to cultivars and adjustments used in seeding, but it usually starts around eight days after emergence and continues until the sixty-sixth day (Salgado et al., 2002). In more recent works, also evaluating CPWC in cotton in Brazilian "Cerrado" conditions, it was found that, for three different spacing of sowing times, the interference critical period was between 8 to 65 days for 0.90 m spacing, between 4 to 59 days for the 0.76m spacing and between 3 to 33 days for 0.45 m spacing (Raimondi et al., 2014). These data demonstrate that even when seeded densely, if on one hand there is the advantage of closing lines to occur earlier (reducing the period in which the crop is exposed to weed competition), on the other hand, there is the disadvantage of interference in the cotton productivity to start earlier.

The weed management in cotton is an important component of cultural practices, for if it is neglected or the control is done at the wrong time, the damage done by the competition between the crop and the weeds are high. Among the control methods employed in the cotton, the one with wider use is the chemical, for the rational use of herbicides enables an efficient control of weeds and provides an easy execution, ensuring a good profitability (Braz et al., 2013).

Although the chemical control is the main method of weed control in cotton, when proceeding to the choice of the herbicide, attention should be paid to its selectivity, for losses due to plant injury caused by the herbicide may be high. Among the factors that must be taken into consideration, when choosing an herbicide, is the application mode. In cotton, herbicides can be used in the management of pre-sowing, pre-plant incorporated, pre-emergence, post-emergence and post-emergence directed spray (Troxler et al., 2002).

Because the cotton plants show a longer cycle compared to other crops, it is usually required more than one application of herbicides in different modes, thus increasing the risk of phytotoxicity (Koger et al., 2007). Furthermore, a commonly herbicide mixture in tank is used to increase the weed control spectrum. Although there are several studies addressing the selectivity of herbicides for cotton, very few have focused on the overall assessment of herbicides application that are needed during the crop cycle. Therefore, the aim of this study was to evaluate the selectivity of weed management different systems in conventional cotton, taking into account the herbicide treatments effects related to growth, development, productivity and the quality of the fiber harvested.

MATERIALS AND METHODS

The experiment was conducted in 2010/2011 summer crop in the municipality of Luis Eduardo Magalhães, Bahia, in an experimental field, located at 12^º06'45.7"S and 45^º50'07.2" W, with an altitude of 748 meters.

Prior the installation of the experiment test facility, a collect soil was done in the experimental area for the physicochemical analysis realization, resulting in the following values: pH(CaCl₂) of 5.2; 1.6 cmol_c of H⁺ + Al⁺³ dm^-3 of soil; 2.3 cmol_c dm^-3 of Ca⁺²; 0.3 cmol_c dm^-3 of Mg⁺²; 129 mg dm^-3 de K^+; 46.7 mg dm^-3 of P; 1.2 dag kg^-1 of organic matter; 82% of sand; 5% silt; and 13% of clay (texture frank sand). Before the experiment, the soil was plowed twice with a disc harrow and a harrowing, seeking its unpacking and standardization, eliminating as well, existing weeds.

The sowing was done on December 18, 2010 with Delta Opal cultivar, distributing 11 seeds per meter, sown with 1 cm depth. The seeds treatment was carried out with the following insecticides: tiometoxan (210 g 100 kg^-1 seeds), abamectin (150 g 100 kg^-1 seeds) and fipronil (100 g 100 kg^-1 seeds), and fungicide formulated with the mixture of fludioxonil + metalaxil-M (7.5 + 3 g 100 kg^-1 seeds). A dose of 450 g 100 kg^-1 seeds of safener dietholate was added to the seed treatment, in order to confer protection to phytotoxicity caused by clomazone. The fertilizer used at sowing was 500 kg ha^-1 of commercial formula 0-21-0 (N-P-K), being complemented with 330 kg ha^-1 urea 20 days after emergence (DAE), 250 kg ha^-1 of potassium chloride 25 DAE and 2 kg ha^-1 boron divided in four applications at 40, 50, 60 and 70 DAE.

As per Köeppen classification, the climate in the experimental area was Aw, temperature averaging between 19 ^ºC and 28 ^ºC and less than 2,000 mm per year average rainfall. The experimental area was equipped with center pivot irrigation. The rainfall data during the beginning of the cycle are shown in Figure 1. When necessary, one irrigation depth of 22 mm was applied.

The experimental design was a randomized block design in split plots (2 x 29) with four replications. The herbicide treatment factor (29 treatments) was allocated to the plots and the herbicides absence and presence factors (two treatments), in the subplots. Therefore, each plot consisted of a subplot that received the herbicide treatment, and an absolute control, which received no herbicide treatment. The adoption of this experimental arrangement allows the comparison (split) of herbicide treatments with the control that has been installed within the same plot, which consequently efficiently minimizes the area variability and the experimental error, which is critical for selectivity experiments that evaluate a large number of treatments.

The experimental unit (subplot) comprised four planting rows spaced 0.76 meters, with six meters in length, comprising a total area of 18.24 m². The useful floor area corresponded to the two central lines with a surround of 0.5 meters at each end. The herbicide treatments were applied with a pre-crop emergence mixture over the top (cotyledon stage of culture) and/or in post- emergence crop (Table 1).

Pre-emergence applications took place on the same day of sowing, being in full pre-emergence (crop and weed). The application conditions were: moist soil, average temperature of 28 ^ºC, air relative humidity average of 76%, wind speed of 0.5 km h^-1 and clear skies with few clouds. The over the top application was carried out when the cotton plants had two fully expanded cotyledons at 11 days after sowing (DAS). The conditions for this type of application were: moist soil, average temperature of 28 ^ºC, air relative humidity average of 65%, wind speed of 1.5 km h^-1 and clear skies without clouds.

The first post-emergence application was performed at 25 DAS, and the conditions of the application were: moist soil, average temperature of 29 ^ºC, air relative humidity average of 60%, wind speed of 2.5 km h^-1 and partly cloudy skies. The second post-emergence application was held at 40 DAS. The application conditions for the experiment were: moist soil, average temperature of 26 ^ºC, air humidity average of 69%; wind speed of 0.5 km h^-1 and cloudy sky. All applications were made with a backpack sprayer constant pressure CO₂ based (35 lb pol^-2), equipped with six spray nozzles type XR 110.02, and spaced 0.5 m, giving a syrup volume equivalent to 200 L ha^-1.

Regardless the herbicide treatment used, all plots were kept free of weeds presence throughout all cycle, preventing weeds to interfere with the selectivity results. During the crop cycle, four weeding were carried out, the last one at 60 DAE. All cultural practices necessary for the farming conduct, as pest and diseases control, use of plant growth regulator, defoliant, among others, were equally used on all plots, so that the herbicide treatment was the only variable tested.

In plant injury visual assessments, notes were assigned in each experimental unit on the scale (1-9, where 1 corresponds to no symptoms and 9 complete plant death) proposed by EWRC (1964), at 11, 25, 31 and 60 DAS. Two reviews of cotton plants population were held at 11 and 144 DAS, counting the number of emerged plants in 4 linear feet of two tree rows (8 linear feet) within each plot. The average plant height was determined by the evaluation of ten plants per plot, at 25 and 144 DAS. The plants were measured from the cervix to the insertion of the youngest fully expanded leaf.

The number of bolls per plant was valued in ten plants per plot, adding up all plant open bolls and grenades apples. At 144 DAS, the intersections of branches (nodes) of the plant stem were also quantified, evaluating ten plants per experimental unit.

At harvest, 30 bolls per plot were randomly selected, 10 were collected at each plant position (lower, middle and upper). For the analysis purposes, the average mass of seed cotton per boll was considered. After the bolls mass appraisal, the plume mass contained in 30 bolls was evaluated. The cotton was ginned with an electric ginner to evaluate the mass of the plume. For the statistical analysis, an average value of thirty bolls was taken. The cotton harvest was performed manually in all the useful floor area of the plot, and the seed cotton production per kg per plot was quantified; this result was later extrapolated to kg per hectare.

Plumes samples of 30 bolls of each plot were sent to the ABAPA Fiber Analysis Center - Bahia Association of Cotton Producers in Luís Eduardo Magalhães-BA. These samples were used to determine the fibers technological quality through the HVI test (High Volume Instrument) for the following characteristics: average fiber length (AFL), length uniformity index (IU), short fiber content (SFC), resistance (RES), micronaire index (MIC) and maturity (MAT) (Ng et al., 2013).

The results obtained in the experiment were subjected to the variance analysis; subsequently, the comparison of the unfolding factor of the herbicide treatment absence and presence was taken by the F test at 5% probability (p < 0.05). The partial unfolding of the interaction is justified by the experiment objective, which is evaluating the herbicides treatments selectivity, since a treatment may be considered selective, when the yield and quality of the product are similar to treatments without herbicide.

RESULTS AND DISCUSSION

At 11 DAS, the plants were given the pre-emergence treatment only, slight injuries being observed in all those receiving this herbicide application mode, with banknotes ranging from 2.25 (slightly yellowing or deformation in some plants cotyledons in the plot) and 4 (light yellowing or deformation in the cotyledons of various plants in the plot, without, however, presenting necrosis) (Table 2). In general, when it comes to the use of pre-emergence herbicides, it is common to observe herbicide symptoms, which not necessarily reflect the low selectivity of the product, being just off effects, from which plants tend to recover with the issuance of new leaves (Dan et al., 2011).

The low toxic symptoms observed on the treated plants with clomazone in pre-emergence possibly occurred due to the use of safener dietholate, which increases the tolerance to such herbicides in cotton. This behavior occurs because the dietholate insecticide inhibits the cytochrome P-450 activity in treated plants, making them more tolerant to clomazone (Yun et al., 2005).

In the assessment at 25 DAS (14 days after the over the top application), it is noted that, in general, treatments with PRE triple mixtures had higher injuries than in the previous evaluation, a higher intoxication in treatments receiving the clomazone + trifluralin + prometryn mixture being observed; in two treatments (T11 and T15), necrosis was observed at some leaves edges. It is also noticed that there was no increased toxicity on treatments that received pre-emergence herbicides compared to, apart this one, those that also received S-metolachlor over the top.

Treatments that received over the top applications only after 25 DAS (T27, T28 T29), without the pre-emergence herbicides use, it showed very few or no herbicide symptoms, which demonstrates the S-metolachlor selectivity in this application modality. Previous results with S-metolachlor application experiments in higher doses, or even in plants treated with pre-emergence clomazone, also showed no symptoms of intoxication on cotton plants (Freitas et al., 2006a; Dan et al., 2011).

At 31 DAS (four days after the first post-emergence application), treatments with no post-emergence application (treatments 1-8 and 29) had low intoxication levels, being 2.25 the highest grade obtained, characterizing a mild chlorosis or leaf deformation in a few plants. In contrast, treatments that received the post-emergence application showed high levels of injuries, with notes ranging between 5.00 and 5.25 (necrosis at some points of the leaf), which indicates that the application of herbicide pyrithiobac-sodium + trifloxysulfuron- sodium association in post-emergence was responsible for the observed symptoms.

At 60 DAS, 19 days after the second post-emergence application, no symptoms of intoxication were observed on young leaves, indicating the culture recovery of the plant injury caused by herbicides applied in different ways. The cotton lower sensitivity to pyrithiobac-sodium + trifloxysulfuron-sodium sequential applications may be related to a more advanced stage of the culture at the time of the second application. In general, plants at more developed formation stages have alterations in morphological and physiological characteristics that confer increased tolerance to herbicides.

In species such as cotton, soybean and corn, the higher cuticle deposition on the leaf surface and the larger herbicides degradation capacity are factors that have been used to explain the increased herbicide tolerance in more advanced development stages (Braz et al., 2012). Regarding trifloxysulfuron-sodium it has been proven that injuries' levels vary according to the dose applied and specifically the culture development stage at the time of application (Freitas et al., 2006b). As the second post-emergence application was made after 41 DAS only, plants possibly had become more tolerant to herbicides, because of their higher development, when compared with the first post-emergence application.

For the population of cotton plants, at 11 DAS, treatments 4 and 7 were the only ones reducing the culture population (Table 3) compared to the respective ones without herbicides treatment.

Evaluating the cotton plants final population, held at 144 DAS in addition to the treatments 4 and 7, the treatment 23 showed inferior stand than its corresponding control (Table 3). Treatment 13 (clomazone + prometryn/2 applications of pyrithiobac-sodium-sodium + trifloxysulfuron) was far away from normality, presenting upper stand than its comparison.

All treatments showing differences in crop stand consisted of pre-emergence triple mixtures, with clomazone + trifluralin + diuron or prometryn + clomazone + trifluralin, which were responsible for the cotton stand reduction, since from the first evaluation when the culture received pre-emergence treatments only, these differences were already observed. Dan et al. (2011), on work conducted on a higher clay content soil (39%), apart from certifying the double mixtures selectivity (clomazone + clomazone + prometryn or diuron) to cotton, also found no stand differences between triple mixtures (clomazone + trifluralin + diuron or clomazone + trifluralin + prometryn). This finding indicates that on soils with higher sand content, damages are more intense, the soil texture playing therefore a key role in the herbicides selectivity with residual activity in the soil.

In evaluations related to the cotton plant height (Table 4), at 25 DAS, all treatments with pre-emergence clomazone + trifluralin + diuron, reconciled or not with S-metolachlor over the top treatment, showed lower plant height.

Another triple mix, clomazone + trifluralin + prometryn, also showed a size reduction, in both cases without the over the top treatment addition. The unique double mix that was down-sized at 25 DAS was treatment 10 (clomazone + diuron) applied in pre-emergence.

As the plant height at 144 DAT treatments 27, 28 and 29 showed no difference compared to the respective untreated control. For treatments that received pre-emergence double mixtures, when applied in that modality only or combined with an alternative application (over the top or post-emergence), there was no difference in height, except for treatment 10 (clomazone + diuron pre-emergence/pyrithiobac-sodium-sodium + trifloxysulfuron post-emergence), which had already height reduction since the beginning of the crop cycle. However, when the double mixture was used in conjunction with other application methods (over the top plus one or two post-emergence applications) reduction of the culture size was observed in all cases. Considering the pre-emergence triple mixtures, the clomazone + trifluralin + diuron mixture remained inferior to the respective controls, regardless the complement with other application modalities. For treatments with clomazone + trifluralin + prometryn, the height reduction was observed when associated with one or two post-emergence applications only (T11 and T15), or when associated with over the top application plus a post-emergence application (T19) (Table 4).

Treatments with pre-emergence triple mixtures were generally responsible for the plant growth reduction, indicating that the three herbicides interaction (clomazone + trifluralin + diuron or prometryn) may affect the plant growth (Table 4). Results demonstrated that, despite the cotton high tolerance to an application modality, when considering the addition of more than one method, the plant's metabolic system apparently metabolizing these herbicides may imply a lower allocation of resources on growth.

In the plant number of bolls evaluation, no rated treatment significantly differed from those without herbicides (Table 5). These results corroborate others reported in literature, where the authors also found no influence of herbicides on this variable (Brambilla, 2007; Arantes, 2008).

The differences in the number of nodes per plant were observed when were assessed the pre-emergence triple mixtures only, the plants receiving these treatments showing fewer nodes than their respective controls (Table 5). The mixture clomazone + trifluralin + prometryn showed reduction of the nodes number when reconciled to S-metolachlor in over the top (treatment 7) or one or two post-emergence applications (treatments 11 and 15). As for clomazone + trifluralin + diuron mixture, differences in the nodes number when associated to two post-emergence applications (treatment 16) or associated to S-metolachlor in over the top plus one or two post-emergence applications (treatments 20 and 24). Regarding the seed cotton mass (Table 6), there was no significant difference between treatments and their respective controls for this variable.

In the boll plume mass evaluation (Table 6), no herbicide treatment showed statistically inferior mass than its adjacent control, although treatment 17 has shown plume mass above its control.

Table 7 shows the results of the seed cotton productivity. When the crop received two pyrithiobac-sodium + trifloxysulfuron-sodium post-emergence applications (treatment 26), the yield was lower than its control. Analyzing the double mixtures selectivity, we noted that the clomazone + prometryn mixture was selective in any combination, and clomazone + diuron damage productivity compared to controls and when associated to two post-emergence applications (treatment 14) or over the top application plus two post-emergence applications. Brambilla (2007), when evaluating two cotton cultivars (Delta Opal and FMT 701), also did not observe reduced productivity by mixing clomazone + diuron (900 + 900 g ha^-1). Similarly, using the same combination of these herbicides at higher doses, clomazone + diuron (1.000 + 1.500 g ha^-1), Dan et al. (2011) observed no difference in productivity between the control one and the mentioned treatment applied in pre-emergence.

The diuron + trifluralin + clomazone triple mixture applied in pre-emergence caused yield reduction when applied alone (treatment 4), when associated to a post-emergence application (treatment 12), or when associated to S-metolachlor over the top plus one or two post-emergence applications (treatments 20 and 24).

The clomazone + trifluralin + prometryn mixture applied in pre-emergence, combined with one or two post-emergence applications (treatments 11 and 15) or without addition with another application (treatment 3) showed a productivity reduction. These treatments also showed reduced plant height in at least one assessment. Similar results were observed in experiments on soils with 39% clay and Nu Opal cultivar where clomazone + trifluralin + diuron or prometryn triple mixtures (1.250 + 1.800 + 1.500 or 1.500 g ha^-1) showed lower cotton productivity in pits than their respective controls (Dan et al., 2011).

Pre-emergence clomazone + prometryn, S-metolachlor applied over the top or a single post-emergence application of pyrithiobac-sodium + trifloxysulfuron-sodium showed no differences in productivity relative to its adjacent controls, being as selective. Previous studies, which corroborate the results of this work showed that there is no negative effect on cotton yield caused by post-emergence trifloxysulfuron-sodium single application at doses up to 7,875 g ha^-1 (Freitas et al., 2006a, b).

These results indicate that pre-emergence triple mixtures or two applications of pyrithiobac-sodium + trifloxysulfuron-sodium are responsible for yield losses in treatments hereby studied. Therefore, a safer recommendation would be to use mixtures of two herbicides in pre-emergence, plus S-metolachlor over de top and a single post-emergence pyrithiobac-sodium + trifloxysulfuron-sodium application.

The fiber formation process is divided into four different phases: initiation, elongation, secondary wall formation and maturation. During the fiber growth and development, a strong interference from the middle factors occurs, specifically temperature, solar radiation, relative humidity and plants mineral nutrition, among others, mainly affecting thickness, maturity, strength and fiber length. From the intrinsic point of view, the final fiber quality depends on genes, environment, harvesting form and other external factors, such as the herbicides use (Snipes Mueler & 1994). Results regarding the cotton fiber technological characteristics evaluation are presented in Table 8.

Concerning the fiber length assessment (AFL), only treatment 17 (pre-emergence clomazone + prometryn associated to S-metolachlor over the top and a post-emergency pyrithiobac-sodium + trifloxysulfuron-sodium) application, negatively affected the variable factor. For the variable fiber length uniformity index response (IU), apart from treatment 17, which showed shorter fiber length, treatment 9 also showed lower IU, indicating that pre-emergence clomazone + prometryn application associated to post-emergence pyrithiobac-sodium + trifloxysulfuron-sodium may be responsible for the negative evaluation in that response. The short fibers index variable (SFC), resistance (RES), micronaire (MIC) and fiber maturity (MAT) showed no differences between treatments and their respective adjacent controls, signaling that herbicides had no influence on these variables (no data shown).

The triple mixtures use of herbicides applied in pre-emergence increases the possibility of reductions' occurrence in cotton yield.

In order to avoid crop yield reductions, one must proceed to two herbicides mixture in pre-emergence, together with S-metolachlor over on top, plus a post-emergence single application of pyrithiobac-sodium and trifloxysulfuron-sodium association.

LITERATURE CITED

Recebido para publicação em 2.4.2014

Aprovado em 22.7.2014

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Publication Dates

• Publication in this collection
  22 Jan 2015
• Date of issue
  Dec 2014

History

• Received
  02 Apr 2014
• Accepted
  22 July 2014