Abstracts

INTRODUCTION

Herbicide saflufenacil is from the pyrimidinedione chemical group, inhibits protoporphyrinozen IX oxidase (PPO) enzyme, is rapidly absorbed by the roots and leaves of the plants and is translocated mainly by the xylem, with mobility limited by the phloem (Grossmann et al., 2011GROSSMANN, K. et al. Saflufenacil (Kixor(tm)): Biokinetic Properties and Mechanism of Selectivity of a New Protoporphyrinogen IX Oxidase Inhibiting Herbicide. Weed Sci., v. 59, n. 3, p. 290-298, 2011.). In Brazil, this herbicide was recorded for applications in pre-sowing soybean, maize and wheat; in direct spray or as a defoliant in cotton crop desiccation (potatoes, beans), etc. (Agrofit, 2014AGROFIT. Sistema de agrotóxicos fitossanitários. Disponível em: <http://extranet.agricultura.gov.br/agrofit_cons/principal_agrofit_cons>. Acesso em: 22 set. 2014.
http://extranet.agricultura.gov.br/agrof... ). In the US, saflufenacil was also recorded for sorghum grain, in applications in pre-sowing or post-sowing, in preemergence of the plants (Sharpen, 2014SHARPEN. Sharpen powered by Kixor herbicide. Disponível em: <http://www.agproducts.basf.us/products/research-library/kixor-sharpen-technical-information-bulletin.pdf>. Acesso em: 8 mar. 2014.
http://www.agproducts.basf.us/products/r... ).

Among the types of sorghum (Sorghum bicolor), there is sweet sorghum, which can be used to produce ethanol from sugars directly fermentable in the culm. Research has already indicated the viability of sweet sorghum use in sugarcane harvests in Brazil, allowing the anticipation and the expansion of the crushing period by the companies plants (Teixeira et al., 1997TEIXEIRA, C. G.; JARDINE, J. G.; BEISMAN, D. A Utilização do sorgo sacarino como matéria-prima complementar à cana-de-açúcar para obtenção de etanol em microdestilaria. Ci. Tecnol. Alimentos, v. 17, n. 3, p. 221-229, 1997.). On the other hand, there are some doubts about the sweet sorghum yield system that need to be clarified, including the chemical control of weeds. One of the main barriers to the use of herbicides in this crop is the shortage of registered products, being only atrazine and 2.4-D (Rodrigues & Almeida, 2011RODRIGUES, B. N.; ALMEIDA, F. S. Guia de herbicidas. 6.ed. Londrina: 2011. 697 p.). This fact awakens the need for studies on herbicide selectivity for this crop and, through them, to stimulate the agrochemical companies for registration with the competent bodies.

Selectivity is the characteristic of the herbicides that enables its application to control weeds without damaging the crops. In general, selectivity is the result of differences in the response of a given plant species to a certain herbicide (Alterman & Jones, 2003ALTERMAN, M. K.; JONES, A. P Herbicidas fundamentos fisiológicos y bioquímicos del modo de acción. Santiago do Chile: Universidad Católica de Chile, 2003. 333 p.). However, selectivity depends on a number of other interrelated factors and can not be attributed solely to the herbicide, but to the applied dosage and the development stage of the plants. Soil, climate and the use of adjuvants can also change the degree of selectivity. Also in some cases selectivity varies as a function of the genetic material used (cultivars, hybrids, clones, etc.) (Alterman & Jones, 2003ALTERMAN, M. K.; JONES, A. P Herbicidas fundamentos fisiológicos y bioquímicos del modo de acción. Santiago do Chile: Universidad Católica de Chile, 2003. 333 p.).

Chemicals, called antidotes, safeners or protectors, may also be used in order to provide selectivity to certain herbicides, aiming increase the tolerance of cultivated species without affecting the sensitivity of weeds (Alterman & Jones, 2003ALTERMAN, M. K.; JONES, A. P Herbicidas fundamentos fisiológicos y bioquímicos del modo de acción. Santiago do Chile: Universidad Católica de Chile, 2003. 333 p.). In this regard, some herbicides are used as safeners of other herbicides, providing reduction of injuries to crops and increasing the application time (Moran et al., 2011MORAN, M. et al. Sodium safens saflufenacil applied postemergence to corn (Zea mays). Weed Sci., v. 59, n. 1, p. 4-13, 2011.). As examples, there are benoxacor for metolachlor in maize (Foy & Witt, 1991FOY, C. L.; WITT, H. L Effects of safener CGA-154281 on metolachlor/atrazine injury to corn (Zea mays) . Weed Technol., v. 5, n. 2, p. 325-330, 1991.; Viger et al., 1991VIGER, P. R.; EBERLEIN, C. V.; FUERST, E. P Influence of available soil water content, temperature, and CGA-154281 on metolachlor injury to corn. Weed Sci., v. 39, n. 2, p. 227-231, 1991.), cyprosulfamide for isoxaflutole in maize (Nelson & Penner, 2006NELSON, E. A.; PENNER, D. Reduction of isoxaflutole injury to corn (Zea mays) with herbicide safeners and water-repellent adjuvants. Weed Technol., v. 20, n. 4, p. 999-1003, 2006.), Na-bentazon for saflufenacil in maize (Moran et al., 2011MORAN, M. et al. Sodium safens saflufenacil applied postemergence to corn (Zea mays). Weed Sci., v. 59, n. 1, p. 4-13, 2011.) and tritosulfuron in wheat (Weinberg et al., 2007WEINBERG, T.et al. Basis for antagonism by sodium bentazon of tritosulfuron toxicity to white bean (Phaseolus vulgaris L) .J. Agric. Food Chemistry, v. 55, n. 6, p. 2268-2275, 2007.).

The protective effect of Na-bentazon was attributed to reduced absorption and translocation of the herbicide by plants (Weinberg et al., 2007WEINBERG, T.et al. Basis for antagonism by sodium bentazon of tritosulfuron toxicity to white bean (Phaseolus vulgaris L) .J. Agric. Food Chemistry, v. 55, n. 6, p. 2268-2275, 2007.; Moran et al., 2011MORAN, M. et al. Sodium safens saflufenacil applied postemergence to corn (Zea mays). Weed Sci., v. 59, n. 1, p. 4-13, 2011.). Interference in the foliar uptake was attributed to antagonism of ATPase by bentazon (Couderchet & Retzlaff, 1991COUDERCHET, M.; RETZLAFF, G. The role of the plasma membrane ATPase in bentazone-sethoxydim antagonism. Pest. Sci., v. 32, n. 3, p. 295-306, 1991.). However, other bentazon formulations, such as H-bentazon and NH₃-bentazon, did not cause the same protective effect (Wanamarta et al., 1989WANAMARTA, G.; PENNER, D.; KELLS, J. J The basis of bentazon antagonism on sethoxydim absorption and activity. Weed Sci., v. 37, n. 3, p. 400-404, 1989.; Weinberg et al., 2007WEINBERG, T.et al. Basis for antagonism by sodium bentazon of tritosulfuron toxicity to white bean (Phaseolus vulgaris L) .J. Agric. Food Chemistry, v. 55, n. 6, p. 2268-2275, 2007.).

This study was conducted with the hypothesis that the saflufenacil herbicide is selective for sweet sorghum, since sprayed in preemergence, in smaller doses, or in postemergence, when a safener is added (Na-bentazon). The objective was to study the selectivity of saflufenacil herbicide for sweet sorghum hybrids, when sprayed in preemergence and postemergence, besides the use of Na-bentazon as a safener for saflufenacil.

MATERIALS AND METHODS

Three experiments were carried out in the period from 01/09 to 05/24/2013, at Departamento de Fitossanidade at UNESP, Jaboticabal campus, SP.

In each experiment, the experimental design was completely randomized with four replications. In the first (2 x 6 factorial), two hybrids of sorghum were studied (CVSW 80007 and CVSW 80147) and six doses (0; 35; 52.5; 70; 87.5; and 105 g ha^-1) of saflufenacil were applied in preemergence of the plants. In the second (2 x 5 factorial), the same hybrids of sweet sorghum sprayed in postemergence with saflufenacil were assessed (0; 35; 52.5; 70; and 87.5 g ha^-1). In the third (4 x 5 factorial), the combination of Na-bentazon (0, 240, 480 and 720 g ha^-1) to saflufenacil (0, 35, 52.5, 70, and 87.5 g h^-1) was studied when sprayed in postemergence of the sweet sorghum plants (CVSW 80007).

Each experimental unit consisted of a plastic pot with a capacity of eight liters of soil. The pots were kept at ambient condition (second and third experiments) and inside a greenhouse (first experiment). As substrates, sand and organic compound were used to mix the soil at a ratio of 3:1:1, respectively. The substrate presented pH 5.7; organic matter of 33 g dm^-3; P of 38 mg dm^-3; and K, Ca, MG and CTC of 4.2, 37, 22 and 89 mmol_c dm^-3, respectively, having been classified as clayey, with 404, 441 and 155 g kg^-1 of sand, silt and clay, respectively.

Ten sweet sorghum seeds were homogeneously distributed in the pot and incorporated up to 2 cm deep in the soil surface. Subsequently, in the postemergence experiments (second and third) thinning was done, remaining four plants per pot.

The products were applied in preemergence (soon after sowing) or in postemergence, with plants in stage V5. A backpack sprayer was used, at a constant pressure (maintained by compressed CO₂) of 2.0 kgf cm^-2, equipped with a bar with two flat fan nozzles TT 11002, spaced 0.5 m, with liquid consumption equivalent to 200 L ha^-1. Weather conditions at the time of application, besides date and time, are shown in Table 1.

In postemergence applications the non-ionic adjuvant (p.c. Dash^(r)) was added at 0.5% v/v to the saflufenacil spray (p.c. Heat^(r)), associated or not to Na-bentazon (p.c. Basagran 600^(r)).

At 42 days after application (DAA) in the first and second experiments, and at 49 DAA in the third, plant height, stem diameter and the dry matter of the stem, leaves and panicle (only in the third) were determined. In the first experiment, at 42 DAA the number of plants per pot was also assessed.

The data were submitted to the F test of analysis of variance. The effects of sorghum hybrids and dosing of saflufenacil (first and second experiments) or their interaction, when significant, were compared by Tukey test at 5% probability or by polynomial fit of the data. The effects of doses of saflufenacil and bentazon (third experiment) or the interaction thereof, when significant, were compared by polynomial fit of the data.

RESULTS AND DISCUSSION

In experiment 1, in which the application of saflufenacil was studied in preemergence of the sweet sorghum plants, the hybrids of sorghum differed in number of plants, stem diameter, and dry matter of leaves (Table 2). The plants of hybrid CVSW 80147 had larger diameter stem and dry matter of the leaves than plants of hybrid CVSW 80007. However, these results were not dependent on the dosage of saflufenacil. It is then natural differences between the genetic materials studied.

There was a significant effect of the saflufenacil dosages and of the hybrid versus saflufenacil interaction only for number of plants. Therefore, the greatest damage caused by the herbicide to the sweet sorghum was in the emergence of plants, since the surviving plants were not growth inhibited by the herbicide. In another study, higher doses of saflufenacil, sprayed in preemergence, reduced the population of plants, in some cases below acceptable levels, with a loss in grain yield with dosage over 40 g ha^-1 (Keeling et al., 2013KEELING, J. W. et al. Grain sorghum response to saflufenacil applied preemergence. Crop Protec., v. 46, n. 1, p. 1-6, 2013.).

Hybrid CVSW 80147 was not affected by doses of saflufenacil, unlike CVSW 80007, whose emergence decreased linearly with increasing dosage, with mortality of 52% of the plants with the dosage of 105 g ha^-1 of saflufenacil (Figure 1). The difference in sensitivity of sorghum hybrids to saflufenacil was also reported by Keeling et al. (2013)KEELING, J. W. et al. Grain sorghum response to saflufenacil applied preemergence. Crop Protec., v. 46, n. 1, p. 1-6, 2013., in which hybrid Pioneer 85G01 was more sensitive than Dekalb 44-20. Miller et al. (2012)MILLER, R. T. et al. Soybean (Glycine max) cultivar tolerance to saflufenacil. Can. J. Plant Sci., v. 92, n. 7, p. 1319-1328, 2012. have reported that soybean cultivar OAC Hanover was more sensitive to saflufenacil compared to the other 11 cultivars tested. These studies reinforce that the selectivity of herbicides for crops is also the result of the tested genetic material.

In preemergence, selectivity of saflufenacil for crops may be attributed to herbicide retention in the soil, which affects its availability in the soil solution and possible absorption by plants (Liebl et al., 2008LIEBL, R. et al. BAS 800H: a new herbicide for preplant burndown and preemergence dicot weed control. In: WEED SCIENCE SOCIETY OF AMERICA CONFERENCE, 48., 2008, Lawrence Abstract... .Lawrence: Weed Science Society of America, 2008. p. 120.). Thus, the texture and content of organic matter in the soil, besides rainfall or irrigation after application, directly interfere with the bioactivity of saflufenacil in the soil (Keeling et al., 2013KEELING, J. W. et al. Grain sorghum response to saflufenacil applied preemergence. Crop Protec., v. 46, n. 1, p. 1-6, 2013.). In the US, saflufenacil was recorded for sorghum grain at dosages up to 45 g ha^-1 for preemergence applications.

For other crops, saflufenacil at 50 and 70 g ha^-1, applied in preemergence, has caused no visible damage to potato plants or tubers decreased yield (Boydston et al., 2012BOYDSTON, R. A.; FELIX, J.; AL-KHATIB, K. Preemergence herbicides for potential use in potato production. Weed Technol., v. 26, n. 4, p. 731-739, 2012.). The same has occurred in maize plants at dosages up to 200 g ha^-1 for preemergence application. Maize has a natural tolerance to saflufenacil, due to its rapid metabolism for non-phytotoxic metabolites in shoot and root tissues, and less translocation from the site of absorption (roots) until the shoot tissues (Soltani et al., 2009SOLTANI, N.; SHROPSHIRE, C.; SIKKEMA, P. H Response of corn to preemergence and postemergence applications of saflufenacil. Weed Technol., v. 23, n. 3, p. 331-334, 2009.).

In experiment 2, when saflufenacil was sprayed in postemergence of sweet sorghum plants, there were significant effects of sorghum hybrids in plant height, diameter and dry matter of the stem (Table 3). The saflufenacil dosages and the hybrid versus saflufenacil interaction have significantly influenced plant height, dry matter of stem and leaves.

Hybrid CVSW 80147 was more tolerant to saflufenacil than CVSW 80007, therefore, regardless of the dose sprayed, the plant height and dry matter of the stem and leaves were not inhibited by the herbicide (Figure 2). Nonetheless, for CVSW 80007, growth and mass accumulation by plants decreased linearly with increasing herbicide dosage. For each 10 g of saflufenacil there was a decrease of 3.1 cm in the height of the plants and of 1.1 g and 0.4 g, respectively, in the dry matter of stem and leaves.

The application of saflufenacil did not affect the stem diameter of the plants. On the other hand, hybrid CVSW 80147 showed greater stem diameter than CVSW 80007, regardless of the sprayed herbicide dosage. These results relate to the natural differences between the genotypes studied.

Soltani et al. (2009)SOLTANI, N.; SHROPSHIRE, C.; SIKKEMA, P. H Response of corn to preemergence and postemergence applications of saflufenacil. Weed Technol., v. 23, n. 3, p. 331-334, 2009. have found that maize had an acceptable tolerance to saflufenacil at dosages of 100 g ha^-1 when sprayed in postemergence. However, when added as an adjuvant to the spray liquid, the herbicide has caused severe injuries to the maize plants with reflection on grain yield (Soltani et al., 2009SOLTANI, N.; SHROPSHIRE, C.; SIKKEMA, P. H Response of corn to preemergence and postemergence applications of saflufenacil. Weed Technol., v. 23, n. 3, p. 331-334, 2009.). In this study, to all herbicide sprays (experiments 2 and 3) was added a non-ionic adjuvant at 0.5% v/v, as recommended by the manufacturer for use in postemergence. This may have contributed to an increased uptake of the herbicide by plants and thus greater phytotoxic effect, especially in hybrid CVSW 80007. Failure in adding an adjuvant to the sprays of saflufenacil may contribute to the selectivity of the herbicide for crops, but may reflect in an ineffective weed control.

In experiment 3, in which the application of saflufenacil alone and mixed with bentazon was assessed, the single factors and the interaction of these were significant for all characteristics assessed, except for stem diameter, in which there was no significant interaction effect (Table 4). For this variable, the values increased linearly with increasing dosages of bentazon, yielding up to 11% increase (Figure 3), unlike saflufenacill, which gave rise to a linear reduction in diameter of the stem by increasing the dose, with 11% decrease at the highest dosage (87.5 g ha^-1).

When unfolding the saflufenacil versus bentazon interaction, there was an option for the analysis of bentazon dosages within each dosage of saflufenacil. Without the addition of saflufenacil to the sprays, increasing bentazon dosages (up to 720 g ha^-1) did not affect the height and dry matter of stem, leaves and panicles of sorghum plants (Figure 4). In turn, the association of bentazon to the dosages of 35, 52.5 and 70 g ha^-1 of saflufenacil contributes to the protection of plants from the negative effect of the herbicide, because, due to to increase of the dosage of bentazon, the plant height and the dry matter of stem, leaves and panicles increased. The same was observed for height and dry matter of the panicle with the application of 87.5 g ha^-1. However, for dry matter of stem and leaves, in the highest dosage of saflufenacil there was no significant difference between the dosages of bentazon with the application of 87.5 g ha^-1. As the yield of ethanol by sweet sorghum is performed directly from fermentable sugars in the culm, the stem dry matter has great importance in defining the sensitivity levels of the plants to certain herbicides.

In another study, saflufenacil (25 and 50 g ha^-1), applied in postemergence, has reduced by 33 and 34% the dry matter of wheat plants; however, with the addition of bentazon (560 g ha^-1) to the sprays this negative effect was not observed (Frihauf et al., 2010FRIHAUF, J. C.; STAHLMAN, P. W.; GEIER, P. W Winter wheat and weed response to postemergence saflufenacil alone and in mixtures. Weed Technol., v. 24, n. 3, p. 262-268, 2010.). Moran et al. (2011)MORAN, M. et al. Sodium safens saflufenacil applied postemergence to corn (Zea mays). Weed Sci., v. 59, n. 1, p. 4-13, 2011. have reported that the combination of Na-bentazon to saflufenacil has reduced noticeable injuries and increased the height, dry matter of plants and the yield of maize, compared to the isolated application of saflufenacil.

There are two main mechanisms by which the antidotes reduce the damage caused by herbicides on crops, both related to the increase in metabolism and detoxification of herbicides (Alterman & Jones, 2003ALTERMAN, M. K.; JONES, A. P Herbicidas fundamentos fisiológicos y bioquímicos del modo de acción. Santiago do Chile: Universidad Católica de Chile, 2003. 333 p.). The first corresponds to an increase in glutathione levels and/or activity of GSH-S-transferase enzyme. The second is related to the increase in the activity or the amount of cytochrome P450 mono-oxygenase. These enzymes can reduce the activity of the herbicides by means of oxidation reactions, which often precede reactions of conjugation with glucose in the presence of glucosyltransferases enzymes (Alterman & Jones, 2003ALTERMAN, M. K.; JONES, A. P Herbicidas fundamentos fisiológicos y bioquímicos del modo de acción. Santiago do Chile: Universidad Católica de Chile, 2003. 333 p.).

The results have indicated that hybrid CVSW 80147 was more tolerant to herbicide saflufenacil than CVSW 80007 in preemergence or postemergence applications. The variables that best assessed the sensitivity of sweet sorghum to saflufenacil were the number of emerged plants, for preemergence applications, and dry matter of the stem, in postemergence. Na-bentazon has shown promise for use as a safener for applications in postemergence of saflufenacil on sweet sorghum (hybrid CVSW 80007) for dosages up to 70 g ha^-1.

LITERATURE CITED

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