Influence of Uroclhoa brizantha cv . Marandu phytomass in the control of Bidens subalternans under dystrophic yellow latossol

1 Departament of Agriculture, Campus Professora Cinobelina Elvas, UFPI, 64900-000, Bom Jesus, PI, Brazil. 2 Departament of Agriculture, Campus Universitário de Rondonópolis, UFMT, 78735-901, Rondonópolis, MT, Brazil. 3 Departament of Biology, Campus Professora Cinobelina Elvas, UFPI, 64900-000, Bom Jesus, PI, Brazil. 4 Departament of Agriculture, Campus Universitário, UFLA, 37200-000, Lavras, MG, Brazil.


INTRODUCTION
Bidens subalternans, commonly known as beggar-ticks, occurs in several agricultural regions in Brazil (Gazziero et al., 1998).Until recently, its existence was little mentioned, probably due to the fact of the species being particularly similar to Bidens pilosa, being basically differentiated by the number of beards in the achenes (Kissmann and Groth, 1993).However, in numerous areas its occurrence has significantly increased and, in many farm properties within the Brazilian States of Mato Grosso and the northern region of Mato Grosso do Sul, the infestation is greater than the one observed for B. pilosa (Sanches and Zandonade, 1997).Furthermore, its resistance to ALS inhibitors was reported by Monqueiro et al. (2000).*Corresponding author.E-mail: adanielssagro@gmail.com.
Author(s) agree that this article remain permanently open access under the terms of the Creative Commons Attribution License 4.0 International License Among the species infesting corn plantations, the genus Bidens sp. is to be highlighted.Species that fit within this genus influence plant growth, development and productivity of crops, due to the competition for light, nutrients and water, as well as increasing the operational expenses for harvesting, drying and processing of grains (Fleck et al., 2002).Moreover, in hot regions, the species B. subalternans may produce three to four generations within the period of one year.In addition, this species is a plant native to South America, with an annual cycle and very similar to Bidens pilosa.In soybean plantations where the species is found in high level of infestation, the production may have an average reduction of 30% (Kissmann and Groth, 1993).
Non-tillage system (DPS), as a sustainable alternative of agricultural production (Pacheco et al., 2008), offers organic matter to the soil.However, in the conditions of the Cerrado's region, with an area under DPS exceeding twenty-six millions of hectares (FEBRAPD, 2010), the system has exhibited difficulties to achieve two basic requirements: crop rotation and straw's formation/conservation (Ceretta et al., 2002).Moreover, the use of cover plants leads to a reduction in expenses and the use of fertilizers, being able to return considerable amounts of nutrients to the crops, once these plants assimilate nutrients from sub-superficial layers and release them later to the surface layer by the decomposition of their residues (Torres et al., 2008).Allelopathy is characterized primarily by a negative interaction between plants through chemical signaling (El-khawas and Shehata, 2005).
According to Golisz et al. (2008), allelopathy consists in the plant ability to synthetize metabolites that inhibit or stimulate the growth and development of other neighboring plants, with a competition for limited resources as light, water and nutrients.The allelochemicals found in soil cover plants are soluble in water, and are released in the environment by volatilization, radicular exudation, lixiviation and plant tissue breakdown.These released substances cause physiological and/or morphological modifications, influencing processes such as germination, growth, flowering, fructification, senescence and abscission in sensitive species (Correia et al., 2005).
The interference of one plant on another may occur in direct or indirect way.When direct, the allelochemical molecules links to the membranes of the receptor plant or penetrates into cells, directly interfering in metabolism, and due to the transformation of the allelochemicals in the soil and/or by the activity of microorganisms.Allelopathic substances may induce the rise of abnormal plantlets, with the most common symptom being radicular necrosis (Ferreira and Borghetti, 2004).
With a concern about agricultural practices, the need for the utilization of sustainable techniques that will preserve the environment avoiding contamination becomes evident, thus adequate production models that will reduce costs come to be significant.Therefore, the present study had the objective to evaluate Uroclhoa brizantha cv.Marandu used under different phytomass' quantities and management methods to control beggarticks (B.subalternans).

MATERIALS AND METHODS
The experiment was performed in greenhouse from August to October 2014, at the campus of the Federal University of Piauí (UFPI/CPCE), located within the county of Bom Jesus (Latitude 9° 16' 78"S, Longitude 44° 44' 25"W and Altitude of 300 m), in the State of Piauí, Brazil.
The experimental was designed under random blocks, with four replicates in a factorial scheme (3 × 4) + 1, with Factor A constituted by three methods of management (incorporated, incorporated+surface and surface) and Factor B constituted by five straw levels (3; 6; 9 and 12 t ha -1 ), plus one treatment without the use of any cover plant (control treatment), with a total of 52 experimental units.
The composition of each experimental unit was distributed in pots with 8 dm 3 soil capacity, and 35 cm diameter.The substrate used was constituted by soil samples obtained from 40 to 60 cm layers of a dystrophic yellow latossol (oxissol).This sampling depth was chosen in order to avoid seeds from weed pants existing predominantly at soil superficial layers.
Seeds of B. subalternans used in the experiment were collected in August 2014, from a central pivot area at the Agricultural School from Bom Jesus (CABJ).Fifteen seeds were randomly sowed and covered with an approximately 1.0 cm soil layer.The fresh vegetal cover was incorporated, preserved in the surface and part ( 1 /2) incorporated + other part ( 1 /2) preserved in the soil surface, in quantities equivalent to the different treatments (3, 6, 9 and 12 t ha - 1 ) in dry mass.The vegetal material was collected and fractionated the same day of the experiment installation, with the aim of avoid potential allelochemical losses.
In order to obtain enough phytomass, seeds of Urochloa brizantha were sowed manually, cultivated in beds of 5 m² and the aerial parts were collected 60 days after sowing.Vegetal residues were segmented in sections of approximately 2 to 3 cm, weighted and corrected by the reference of a dry base, after that, plant samples remained in an oven at 65°C for 72 h and/or until reaching constant weight.The humid material was set according to the desired dry mass per hectare, where latterly was homogenized and preserved in the soil surface, incorporated to the soil and one part ( 1 /2) incorporated + another part ( 1 /2) in the surface, according with the treatments.Irrigation was performed daily according to the plants requirements.
Leaf area (LF) was determined when the weeds in average, reached the pre-flowering stage, with the aid of a LI-3100 Area meter equipment (LI-COR, Inc.Lincoln, NE, EUA).Leaves were detached from the plant stalk to perform the evaluation, expressed as cm 2 pot -1 .Besides that, roots were separated from the plant, washed with tap water and removed from the soil, and then the volume of roots (VR) was determined and expressed in terms of cm 3 pot -1 utilizing the test-tube method (Basso, 1999).The aerial part as much as the radicular portion were dried in an oven at a  Means followed by the same letter within the column do not differ statistically by Tukey's test at 5% probability; CVcoefficient of variation.
temperature of 65°C until reached constant weight, in order to obtain their dry phytomass.Data was submitted to variance analysis (ANOVA) and when relevant for qualitative data, Tukey's test 5% was used to compare mean values, with the aid of the software SISVAR 4.2.Quantitative data was set in equations, with the aid of the software SIGMA PLOT 10.1.

RESULTS AND DISCUSSION
For variables of total number of emerged plants (NEP), emergence velocity index (EVI), leaf area (LF), dry phytomass of the aerial plant portion (DPAPP), volume of roots (VR) and dry phytomass of roots (DPR), a positive interaction was observed (P<0.01) between the factors: management method (MM) and straw level (SL) of Urochloa brizantha (Table 1).
All management methods of U. brizantha tested promoted a reduction in NEP and EVI, with an emphasis for surface and surface+incorporated management, which showed the greatest reduction within all straw levels studied (Table 2).These results may be explained by the physical control promoted by the soil cover (Severino and Christofoletti, 2001), by diminishing the luminosity required for germination of Bidens subalterna (Theisen et al., 2000), as well as by constituting a physical obstacle able to produce the exhaustion of the reserve material from plantlets during the early developmental stage (Pacheco et al., 2013).These results also confirm the observations mentioned by Ferreira et al. (2007), when extracts of Eucalyptus citriodora significantly reduced the germination velocity index (GVI) in beggar-ticks.
Management methods of U. brizantha showed exponential decreasing behavior for variables NEP and EVI as a function of straw levels (Figure 1).These results demonstrated that treatments with soil surface management, had the most significant effects in the reduction of these variables at the initial straw levels, with 3 t ha -1 of straw reducing NEP and EVI in 73.91 and 89.21%, respectively, when compared to the control (0 t ha -1 ).In a previous study performed by Pacheco et  (2013) with cover plants, the authors reported that 4 t ha -1 of Urochloa ruziziensis in soil surface reduced NEP and GVI of Bidens pilosa in 61.63 and 75.91%, respectively.
The management of U. Brizantha by the method incorporated + surface, even when it resulted in lower means for variables NEP and GVI and compared to the surface management method, still promoted a reduction of such variables in 56.52 and 59.09%, respectively, when using 3 t ha -1 of straw, compared to the control treatment (0 t ha -1 ) (Figure 1).The incorporated management method showed an expressive reduction of such variables at levels higher than 3 t ha -1 of straw.The lowest mean values for variables LF and DPAPP of Bidens subaltern, were observed when sowed in pots with surface management and incorporated + surface management treatments (Table 3).These results may be explained by the exponential reduction in the number of emerged plants (Figure 1), as well as by the delay of plantlets' emergence.Promissory results were observed by Moraes et al. (2011), with a significant reduction of LF and DPAPP of Bidens pilosa, when using a phytomass of 4 t ha -1 of Italian ryegrass (Lolium multiflorium), preserved in the soil surface.
Concerning variables LF and DPAPP, management methods of U. brizantha showed an exponential decreasing behavior (Figure 2).Means followed by the same letter within the column did not differ statistically by the Tukey's test at 5% probability; CVcoefficient of variation.
Straw levels ( t ha -1 ) Leaf area (cm Surface management expressively reduced variables of LF and DPAPP even at the initial quantities of straw, with 3 t ha -1 of straw being enough to reduce these variables in 78.25 and 74.49%, respectively, compared with the control treatment (0 t ha -1 ) (Figure 2).Gimenes et al. (2011) demonstrated that 10 t ha -1 of phytomass of B. decumbens after 60 days of emergence, were enough to reduce more than 80% of the leaf area in Digitaria horizontalis and C. echinatus.Pacheco et al. (2013) also reported efficient results with 4 t ha -1 of U. ruziziensis in soil surface, resulting in a reduction of LF and DPAPP of B. pilosa in 71.24 and 76.66%,respectively.Management methods of U. brizantha also promoted reductions in variables of DPR and VR, with emphasis on surface management with the higher reduction in the radicular system of B. subalterna (Table 4).These results may be explained by the decrease in the number of germinated plants, as well as by the delay in plantlets' Dry phytomass of roots (g pot emergence.In such way, the lower development of the radicular system may result in reduction of the competitive capacity of the infesting plants, by the reduction of the nutrient and water absorption capability, especially under conditions of water stress (Pacheco et al., 2013).Management methods of U. Brizantha showed a decreasing exponential behavior for variables of DPR and VR, with the exception for the method of Incorporated + Surface, with a linear reduction (Figure 3).These results reveal that when using 3 t ha -1 of U. Brizantha straw in the soils surface, a reduction of 83.14% and 88.68% in DPR and VR respectively, is expected, when compared to the control treatment (0 t ha -1 ).In a similar study performed by Moraes et al. 2011), utilizing rapeseed (Brassica napus), radish (Raphanus sativus), arrowleaf clover (Trifolium vesiculosum) and Italian ryegrass (Lolium multiflorum), (the authors demonstrated that the radicular system of B. pilosa plants was reduced with the increment of phytomass levels of these species in the soil surface.
From the results, it is possible to see that the surface management method of U. Brizantha shows the highest efficiency to control B. Subalterna.A quantity of 3 t ha -1 of U. Brizantha in surface management is enough to promote a significant reduction in the emergence and growth of B. Subalterna.
The incorporated + surface management method of U. Brizantha results in a reduction of more than 50% of the germination and growth, when applied in quantities of more than 6 t ha -1 of straw.Incorporated management of U. Brizantha shows efficiency reducing 50% of B. Subalterna emergence when used at only 6 t ha -1 of straw.

Figure 1 .
Figure 1.Number of plants emerged and index of germination velocity of Bidens subaltern as a function of straw levels of Urochloa brizantha.** and * significant at 1 and 5%, respectively.

Figure 2 .
Figure 2. Leaf area and dry phytomass of the aerial portion of Bidens subalterna as a function of management method and straw levels of Urochloa brizantha.** and * significant at 1 and 5%, respectively.

Figure 3 .
Figure 3. Dry phytomass of roots and volume of roots of Bidens subaltern as a function of management method and straw levels of Urochloa brizantha.** and * significant at 1 and 5%, respectively.

Table 1 .
Summary of the variance analysis (F values) of the total number of emerged plants (NEP), emergence velocity index (EVI), leaf area (LA), dry phytomass of the aerial plant portion (DPAPP), dry phytomass of roots (DPR) and volume of roots (VR) of Bidens subaltern.

Table 2 .
Number of plants emerged and index of emergence velocity of Bidens subalterna as a result of management method and straw levels of Urochloa brizantha.

Table 3 .
Leaf area and dry phytomass of the aerial plant portion of Bidens subaltern, as a function of the management method and straw levels of Urochloa brizantha.

Table 4 .
Dry phytomass of roots and root's volume of Bidens subalterna as a function of management and straw levels of Urochloa brizantha.