Phytosociological and floristic survey of weeds in Western Paraná

Weeds can reduce yield, which justify the need for their efficient management. The stage that precedes the management consists of identifying the species present in the area, and then making the decision of the best method of control. Thus, this study aimed to perform a phytosociological survey, and to analyze the composition and floristic similarity of weeds between the end of the corn crop cycle and in the off-season period. The authors analyzed nine areas representing the second crop corn and six in the off-season period, distributed in four municipalities: Francisco Alves, Marechal Cândido Rondon, Palotina and Terra Roxa. For estimation of weed species, the square inventory technique of 1 m 2 was used. The sampling design consisted of three collection ranges, the first starting at 5 m from the edge, followed by 25 and 50 m inside the crop. In the second crop corn, 29 species grouped into 17 families, in the off-season period, and 35 species grouped into 18 families were recorded. Parietaria spp. and Conyza spp. stood out as main species in areas occupied by second crop corn and Parietaria spp. and Gnaphalium spicatum Lam. in areas of the off-season period and inside sampling range of this period. The similarity index evidenced a high similarity between the periods sampled.


INTRODUCTION
Soybean (Glycine max), corn (Zea mays) and wheat (Triticum spp.) are the main crops in western Paraná.Soybean production is estimated at 14,786,987 tons, corn at 15,779,557 tons and wheat at 3,994,903 tons (IPARDES, 2014).The excellent yield results can be mainly attributed to the favorable climate, topography, soil fertility and structure.In this region, the cultivation of these important crops takes up major market share, thus causing a large development of agribusiness (Belusso and Sierra, 2006).Reflection of that excellent yield results according to Conab (2016) Paraná State is expected to achieve yield of 3944 kg ha -1 for the season 15/16, 0.6% higher than the yield of the crop in 14/15.
One of the crops that deserve attention is the corn *Corresponding author.E-mail: viniciuscanepp@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 grown in the second growing season, known as second crop corn, which occupies most areas of the region from February to August.According to Conab (2015), second crop corn production in Brazil reached 5254 kgha -1 , and with this, the production of second crop corn was consolidated in 53.99 million tons in 2015, providing an increase of 5.3% over the previous harvest.In the same period, the area planted with first crop corn was 6109.3 thousand hectares; and with second crop corn, 9599.7 thousand hectares.However, according to Galon et al. (2008), this productivity is lower than those obtained in high tech farming.Among many factors that are responsible for low productivity, one of the most important is the interference by weeds.
A study conducted by Rossi et al. (1996) on the interference by weeds with seven corn cultivars showed reduced first ear height, length and circumference of ears, weight of ears and grains and crop yield, regardless of the cultivar.In agreement with Kozlowski (2002), weed interference throughout the crop cycle can reduce production by 87%.
The off-season is characterized by a period when the cultivation areas remain fallow, soon after second crop corn harvesting until the new sowing of soybean, and may comprise the months of August and September.This rest favors the emergence of invasive species in agricultural areas, which, if not managed efficiently, can become problem at sowing and early development of soybeans.
One of the first steps for a proper weed management in growing areas involves the identification of invasive species and also species with the highest importance.Identification of such species enables a better management decision-making, whether cultural, mechanical, physical, biological or chemical.It also allows establishing an order of priority among the species present to set an effective control program (Oliveira and Freitas, 2008;Kuva et al., 2007;Yanagizawa and Maimoni-Rodella, 1999).
Due to the great importance of identifying weed species, several floristic composition works have been carried out in diverse cultures, such as Albertino et al. (2004) with guarana culture in the Amazon; Marques et al. (2010) with the cultivation of cowpea; Galvão et al. (2011) in pasture.The same is observed with phytosociological surveys in irrigated rice (Erasmo et al., 2004); in sugarcane production (Oliveira and Freitas, 2008); in green sugarcane (Kuva et al., 2007); in sunflower crops (Adegas et al., 2010) and in soybean (Pereira et al., 2000).There are no studies on corn, especially with second crop corn.
Given the importance of second crop corn for the country and regional economy and the losses caused by weed infestation and also the need for drying the areas before planting soybean, an efficient control program becomes essential.In this way, weed identification works are extremely important for the studied regions and crops that may be infested.
In this context, the present study aimed to perform a phytosociological survey, and to analyze the composition and floristic similarity of weeds in the second crop corn during the off-season period in Western Paraná State.

MATERIALS AND METHODS
For this study, two surveys of weeds were carried out, one in the late corn cultivation, and another in the off-season period, considered a short period of about 1 month.In the off-season period, six areas under no-till system with crop succession: Soybean (first crop) -Corn (second crop) were sampled.The survey was conducted in the municipalities of Palotina (areas 1 and 2), Terra Roxa (3), Francisco Alves (4 and 5) and Marechal Cândido Rondon (6), with the same description of soil and climate mentioned above.
For weed survey, the square inventory technique (Braun-Blanquet 1979) was used, with a square of 1.0 x 1.0 m, totaling 1 m 2 .The sampling design consisted of three collection range, the first starting at 5 m from the edge, followed by 25 and 50 m (locations 1, 2 and 3, respectively).Five sampling sites of 1 m 2 were set in each collection range, totaling 15 samples (15 m 2 ) per area.With the sample model, it was possible to measure the weeds by area and the occurrence over the edge or at the time it distanced itself from the same.
Importance value index (IVI) is calculated based on the sum of the values of relative frequency, relative density and relative abundance.For similarity analysis (estimated similarity in species composition) between botanical populations, the Sorensen similarity index (SI) was calculated as follows: SI = (2a/b + c) x 100, where a = number of species common to both areas; b and c = total number of species in both areas compared.SI range from 0 to 100, being maximum when all species are common to the two areas, and minimum when there is no species in common.
After the identification and counting, data were analyzed for density, relative density, frequency, relative frequency, abundance, relative abundance and importance value index, as proposed by Mueller-Dombois and Ellemberg (1974).

RESULTS AND DISCUSSION
In the nine areas of the second crop corn, 28,516 thousand plants were sampled, they divided into 29 species, including seven monocot species (representing 23% total species), 22 dicot species (representing 77% total species).The results confirm the works of Albertino et al. (2004), in guarana culture, which found 80.5% dicots, 15% monocots and 4.5% pteridophytes, Marques et al. (2010), found 74.51% of dicots in cowpea crop.
In accordance with Pitelli (1987), weeds selected usually have botanical characteristics very similar to the culture, which was not verified herein, because the greatest number of species found was represented by dicots, a fact that can be because the survey has been made at the end of the crop growth cycle, or because the previous crop was soybeans.
The 29 species found are grouped into 17 families, the most representative are Asteraceae and Poaceae, with 6 species each.These families are also the major ones in other crops, as in sunflower (Adegas et al., 2010) and in sugarcane cultivations (Oliveira and Freitas, 2008).Brassicaceae and Euphorbiaceae had 2 species each.The other families had only one species each, as listed in Table 1.
In the six areas of off-season, 13,697 thousand plants were sampled, they divided into 35 species, of which 8 are considered monocots (representing 22.8% of the species), dicots were represented by 27 species (representing 77.2% of the species).The 35 species are grouped into 18 families, standing out Asteraceae and Poaceae with 8 and 7 species, respectively (Table 1).
It was observed that the predominance of some species showed a high value of importance in the phytosociological context because of their great phenotypic adaptability.In second crop corn, some species stood out in relation to their importance value: Parietaria spp.( 150), Conyza spp.(32.2),Stachys arvensis L. ( 18) and Gnaphalium spicatum (17) (Figure 1).Parietaria spp.presented the highest value of importance value over the other species.
Parietaria spp. is the species with the greatest potential to damage the next crop, or that should receive special attention in the post-harvest management, in order to reduce the seed bank in the soil and negatively affect the dynamics of weed populations in the agro-ecosystem.This is a species little known in Brazilian environments, thus, it is believed that it deserves more attention in future studies.Another weed that deserves attention is Conyza spp., which showed IVI of 32.2 (Figure 1).This species is highly competitive with crops and rapidly proliferates, which facilitates its spread across the cultivated areas.In an area without soil management, Conyza canadensis, at a density of 10 plants m 2 , can produce about 200 thousand seeds per plant (Bhowmik and Bekech, 1993).In turn, Conyza bonariensis can produce an average of 110 thousand seeds per plant (Wu and Walker, 2004).Another characteristic that provides high capacity for multiplication is the seed dispersal ability of Conyza spp.For dispersal by wind, there were adaptations in achenes, which are formed by structures called papus (Andersen 1993), this characteristic gives the achenes the possibility of being carried on the wind for long periods (Regehr and Bazzaz, 1979).For instance, C. canadensis can disperse its seeds in the wind for distances greater than 100 m (Dauer et al., 2006).On the other hand, in a corn crop, distribution of C. canadensis seeds was 14,950 m 2 at a distance of up to 6 m from the mother plant; at a distance of 122 m, there was, on average, 149 seeds m 2 (Loux et al., 2006).
Besides the high competition with grown plants, the high multiplication and dispersal rates, Conyza spp.may have biotypes resistant to one or more active ingredients, such as the case of Conyza sumatrensis, which exhibited biotypes resistant to glyphosate and chlorimuron-ethyl in the state of Paraná (Santos et al., 2012).This resistance makes its control even more difficult, allowing the permanence of this weed species in the field, and the growth of descendants more frequently, if effective control measures are not taken.
In the off-season period, as illustrated in Figure 2, Parietaria spp.again stood out from the other species, with IVI of 164.60, followed by Gnaphalium spicatum (35.13),Richardia brasiliensis G. (13.08) and Conyza spp.(11.78).These species are noteworthy in the off-season period, as they will be dried out for planting soybeans, therefore, by knowing the weeds prior to drying, it is possible to choose the best method for control of weeds.Moreover, combining the information of weeds present in the late cycle, with those present at soybean pre-sowing, it is possible to outline the best drying system that precede the first growing season, as well as understand the dynamics of weed populations.
In the drying period, it is essential to select the best active ingredient to control weeds.Among the most widely used active ingredients, glyphosate stands out, which is a post-emergence herbicide, which belongs to the chemical group of substituted glycines, classified as non-selective and of systemic action, with a broad spectrum of action, which enables excellent control of annual and perennial weeds, both narrow and broad-leaved species (Galli and Montesuma, 2005).However, the weed floristic monitoring allows understanding the selection pressure process of herbicides, such as glyphosate, thus serving as indicators to prevent the increase of species resistant to certain molecules.
The high IVI of Parietaria spp.and low IVI of other species can be assigned to interspecific competition with Parietaria spp. in this period (possibly due to better environmental conditions for their development), which can be dominant over other species, thereby inhibiting their development (Jakelaitis et al., 2003).
In areas planted with second crop corn, 5 m (Figure 3A) inside the crop again, Parietaria spp.showed the highest IVI (26.83), in that range of 5 m, the total percentage of plants was 18.89% of the total plants of this species found in the areas.Another species with high IVI was Conyza spp.(16.28) and plants concentrated in this first analysis range with 66.88% of plants found in the areas.Stachys arvensis L. showed IVI of 14.11 and about 91.22% plants were found in the first five meters into the crop, with little representation inside the crop.
Conyza spp.and Stachys arvensis L. were found in the first 5 m (edges) of the winter corn crop, which can be justified by higher traffic of vehicles beside the crop, which can cause compaction, preventing seed germination or resulting in smaller plants as it involves a higher incidence  of light.In addition, the service vehicles of the farmer or even vehicles that travel on the side of the farm, can bring seeds from infected areas, thus favoring their spread in these five meters.In the case of Conyza spp., the lightness can result in higher germination of seeds, which can reach 78% germination at 20°C (Vidal et al., 2007).In corn crops, these species can cause economic damage because they compete with the crop for nutrients and light, and these plants can be a source of proliferation and dispersal of seeds throughout the area, so, the proper management of weeds in the crop edges is necessary.Thus, monitoring of different distances from the edge allows understanding the process of dispersal and colonization of species within the agro-ecosystem.At 25 m (Figure 3B), Parietaria spp.showed the highest IVI (91.83), representing, in this range, 61.58% of total plants.Field observations indicate that this species prefers areas with greater moisture content and preferably shade.Conyza spp.had the second highest IVI in this sampling range (6.44), with 25.96% of plants found in the areas.At 50 m (Figure 3C), some species showed large differences, Parietaria spp.presented 61.58% in the 25 m of sampling, demonstrating that this species may prefer areas closer to the edges of the crops; also, for the same species, at 5 and 50 m, percentage of individuals is 18.89 and 9.52%, respectively.Of the total of 10 main species sampled at the end of the second crop corn cycle, 5174, 14596 and 8395 were in the range of 5, 25 and 50 meters with a total percentage of 18.4; 51.8 and 29.8, respectively.
In areas that are in the off-season period, at the distance of 5 m inside the crop (Figure 4A), again the species Parietaria spp.had the highest IVI (46.71); in this range 21.73% of all plants of this species were found.Gnaphalium spicatum Lam. had the second IVI (10.30) and the total in this range was 66.07%.At 25 m (Figure 4B), Parietaria spp.Exhibited the highest IVI (54.49),

Figure 2 .
Figure 2. Importance value index of the major weed species in six areas in the off-season period in four municipalities of Western Paraná State.

Figure 3 .
Figure 3. Importance value index of the major weed species located at 5 m (A), 25 m (B) and 50 m (C) inside the crop in nine areas planted with second crop corn in four municipalities of Western Paraná State.

Figure 4 .
Figure 4. Importance value index of the major weed species located at 5 (A), 25 (B) and 50 m (C) inside the crop in six areas in the off-season period in four municipalities of Western Paraná State.

Table 1 .
Floristic composition of weeds in nine areas at the end of the second crop corn cycle in four municipalities in Western Paraná State.Importance value index of the major weed species in nine areas planted with winter corn in four municipalities of Western Paraná State.