Growth suppression of sandspur grass by cover crops 1

2010). Moreover, the presence of biomass on the soil surface has potential to assist in the integrated management of weeds (Silva et al. 2009, Moraes et al. 2011, Borges et al. 2014), due to the interception of solar radiation (Duarte et al. 2007) and release of allelopathic compounds, during the mineralization process of plant residues (Theisen et al. 2000). Weeds management in agricultural areas has been mainly sustained by chemical control. With Roundup Ready® technology, farmers intensified


INTRODUCTION
The search for cultivation techniques that offer higher yields and profitability, and that mitigate negative effects to the environment, are the main goals of agronomic research.
e- ISSN 1983-4063 -www.agro.ufg.br/pat -Pesq. Agropec. Trop., Goiânia, v. 45, n. 3, p. 319-325, Jul./Sep. 2015 the use of glyphosate, due to easy application, effective desiccation of weeds and post-emergence management of crops.However, the use of techniques that may improve the integrated weed management is essential to minimize the effects of selection pressure caused by the intensive use of the same active ingredient (Christoffoleti & López 2003).
Cenchrus echinatus L. is a Poaceae, popularly known as sandspur grass, that is widespread in almost all cropping regions of Brazil.This species may develop in different soil fertility conditions and maintains high growth rates, even under environmental stress conditions.It has been found infesting various commercial crop fields, such as peanut, citrus, bean, cassava, soybean, sugarcane, mint, onion and tomatoe, as well as in pastures (Kissmann & Groth 1999, Duarte et al. 2007).In crops such as cotton, it is particularly important, because C. echinatus seeds may fixate on the cotton fibers, causing significant reduction in quality and price (Lorenzi 2000).
The response to the use of cover crops on the germination of weed plants depends on the amount and distribution of the residue, as well as its allelopathic potential (Rice 1984, Chauhan et al. 2012).The allelopathic activity of mulch on weeds depends directly on the amount of biomass, soil type, microbial population, climatic conditions and composition of the weed community species (Monqueiro et al. 2009).
Sustainable practices of agricultural management are important to reduce the environmental costs of food production.Thus, this study aimed at evaluating the efficiency of biomass levels of different cover crops to control the sandspur grass (Cenchrus echinatus L.) weed.

MATERIAL AND METHODS
The experiment was conducted in a greenhouse at the Universidade Federal do Piauí (UFPI), in Bom Jesus, Piauí State, Brazil, from March to June 2013.
Each experimental unit consisted of pots with 8 dm 3 (volume) and 35 cm (diameter).Vessels were filled with substrate samples taken from the layer of 40-60 cm of a dystrophic Latosol.This depth was chosen in order to avoid the upper soil layers, which contain the largest amount of weed seeds.Dolomitic lime was added to the soil to reach the base saturation of 50 %, and NPK fertilization (10:20:20) was performed at a dose of 0.4 g dm -3 of soil, corresponding to 800 kg ha -1 .C. echinatus was randomly sown with 30 seeds per pot.Seeds were covered with a soil layer of approximately 1.0 cm deep.The fresh matter of cover crops was collected and fractionated when the experiment was installed, avoiding the possible loss of allelochemicals, and put over the soil surface at different amounts of straw corresponding to 0 t ha -1 , 2 t ha -1 , 4 t ha -1 , 8 t ha -1 , 12 t ha -1 and 16 t ha -1 of dry biomass.
To obtain the cover crops biomass, the seeds were sown manually in a sandbox with 5 m², where plants developed, and their shoots were collected at the full flowering stage.Plant residues were grouped into sections of 2-3 cm, weighed and fixed by dry basis reference (dried in oven at 140 ºF, for 72 hours and/or until constant weight).Fresh biomasses were adjusted on the basis of the desired dry biomass per hectare, being subsequently homogenized and kept on the soil surface according to the treatments.Irrigation was performed on a daily basis.
The variables evaluated for the C. echinatus were: total number of emerged plants, germination speed index, leaf area, shoot dry biomass, root volume and root dry biomass.The germination speed index was calculated using the equation described by Maguire (1962): where N 1 , N 2 , N 3 ... N n correspond to the number of seedlings identified in the first, second, third, ... and N th days after sowing (DAS).
At 52 DAS, when the majority of the sandspur grass reached the pre-flowering stage, leaves were separated from shoots for measuring the leaf area, Growth suppression of sandspur grass by cover crops e-ISSN 1983-4063 -www.agro.ufg.br/pat-Pesq.Agropec.Trop., Goiânia, v. 45, n. 3, p. 319-325, Jul./Sep.2015 which was determined with a LI-3100 (LI-COR, Inc.Lincoln, NE, USA) equipment and expressed in cm 2 plot -1 .Roots were washed and separated from shoots (Basso 1999), in order to measure the root volume, expressed in cm 3 plot -1 .Both shoots and roots were dried at 140 ºF, until constant weight, to obtain their dry biomasses.
Statistical analyses were performed with Anova (p ≤ 0.01) and the comparison of means for the qualitative factor (cover crops) was conducted using the Tukey test (p ≤ 0.01) and Sisvar software, version 4.2 (Ferreira 2011).A regression analysis, using the Sigma Plot version 10.1 software, was performed for the quantitative factor (equivalent amount of straw).

RESULTS AND DISCUSSION
The total number of emerged plants, germination speed index, leaf area, root dry biomass, shoot dry biomass and root volume showed significant interactions (p > 0.01) between cover crops and straw levels (Table 1).
All cover crops showed potential to reduce the total number of emerged plants (Table 2 and Figure 1).Similar results were observed by Pacheco et al. (2013) with a significant control of Bidens pilosa, using 4.0 t ha -1 of biomass from U. ruziziensis and Fagopyrum tataricum.These results suggest that the physical effects of straw reduce the solar incidence on the soil surface, modifying the quality and reducing the amount of light, as well as inhibiting seeds germination.Monqueiro et al. (2009) also reported that the physical effect of straw interferes on the germination and seedling survival rate of some weed species.
U. ruzizienis was more efficient to control C. echinatus, in relation to other cover crops, and promoted significant reduction of C. echinatus emergence, starting at 4 t ha -1 of straw (Table 2 and Figure 1).Gimenes et al. (2011) analyzed the effect of U. decumbens on weed control and found a decrease from 30 to 2 C. echinatus plants m -2 , when compared to the control treatment.The Urochloa species has high amounts of phenol group substances and flavonoids (Waborne & Williams 2000, Lisboa 2009), which can act as suppressors of C. echinatus germination.
Germination of C. echinatus may be influenced by the release of allelochemical substances during biomass decomposition (Alvarenga et al. 2001, Salton 2004, Vidal & Trezzi 2004).Additionally, the biomass on soil surface provides great physical control of weeds and reduces the light necessary to stimulate the C. echinatus germination (Theisen et al. 2000, Severino & Christofoletti 2001, Sodré Filho et al. 2008).
Leaf area and shoot dry biomass of C. echinatus L. were reduced in 72 % and 90 %, respectively, when the seedlings were below 8 t ha -1 of U. ruziziensis biomass (Table 3 and Figure 2).2011) also demonstrated that 10 t ha -1 of U. decumbens biomass at 60 days after germination were sufficient to reduce more than 80 % of the leaf area of Digitaria horizontalis and C. echinatus.The symptoms of allelopathic effects from cover crops are yellowing or chlorosis of leaves, which causes their fall, and a decrease in shoot dry biomass (Almeida 1985).
U. ruziziensis also promoted the greatest reduction in root growth of C. echinatus (Table 4 and Figure 3).The reduction of the C. echinatus root system should result in a reduction of the competitive ability of the weed to uptake water and nutrients, especially under water stress conditions.
C. ochroleuca exponentially reduced the root volume of 60 % of weeds with up to 2 t ha -1 (Table 4 and Figure 3).This effect may be due to the different allelopathic responses that might be attributed to phenols and flavonoids, such as inhibition and activation of enzymes (Simões et al. 2004), and the Table 4. Root volume and root dry weight of Cenchrus echinatus L., at 50 days after sowing, as a function of cover crops and amount of straw on the soil surface (Bom Jesus, Piauí State, Brazil, 2013).
1 Means followed by the same letter in the column do not differ significantly by the Tukey test at 1 %.attraction or repulsion of microorganisms (Andrade et al. 2007).Furthermore, it is known in the literature that growth inhibition may result in reduction of root elongation (Baziramakenga et al. 1994, Ferrarese et al. 2000).The amount of 2 t ha -1 of biomass residues of C. ochroleuca significantly reduced the weed root volume.This result may be explained by the oxidizing of substances released during the mineralization process of plant residues (Cunha & Roque 2005).The concentration of phenols and flavonoids in the soil were reduced over 80 % in only 14 days, because of their fast oxidation (Lisboa 2009).
Our results indicate that the cover crops U. ruziziensis and C. ochroleuca are effective in controlling the development of C. echinatus.Thus, these species can be used as tools in the integrated management of this weed, reducing the use of herbicides and the production cost.CONCLUSIONS 1.The increase in the amount of cover straws on the soil surface improves the control of C. echinatus.2. All cover crops tested reduce the emergence and development of Cenchrus echinatus, when a minimum of 4 t ha -1 of biomass is on the soil surface.

Table 2 .
Total number of emerged plants and germination speed index as a function of cover crops and straw amounts on the soil surface(Bom Jesus, Piauí State, Brazil, 2013).
1 Means followed by the same letter in the column do not differ significantly by the Tukey test at 1 %.

Table 3 .
Shoot dry biomass and leaf area of Cenchrus echinatus L. plants as a function of cover crop and amount of straw on the soil surface(Bom Jesus, Piauí State, Brazil, 2013).
1 Means followed by the same letter in the column do not differ significantly by the Tukey test at 1 %.