Management of Soil Mulch in Weed Suppression and Sugarcane Productivity

The soil mulch is an agricultural practice that can benefit soil fertility and can be effective in suppressing weeds. The objective this research was to evaluate the mulching from legumes in weed control and sugarcane (first harvest/cut) productivity, comparing the results with the conventional application of herbicides. This research was carried out under field conditions. Five legumes were managed in two ways to form the soil cover: (1) mechanical topple, and (2) chemically desiccated. To compare the results, used treatments with herbicides applied in pre and pre + post emergence. The soil mulch from mechanical topple of Crotalaria spectabilis, C. juncea, C. ochroleuca, C. breviflora and Cajanus cajan presented lower efficiency in suppressing weeds than the treatment with herbicides applied in pre + post-emergence, however, were more efficient in controlling weeds in relation to the use of herbicides in pre-emergence, a fact observed at 60 days of sugarcane cultivation.


Introduction
In Brazil, the sugarcane as one of the crops which more agrochemicals are used, especially herbicide.Among the main damages caused by agrochemicals, we can mention destruction of soil microbiota balance (Mueller & Senseman, 2015), contamination of the water table, surface or underground; air contamination; death of insects and natural enemies and numerous diseases, including kidney diseases and cancers, are correlated with increased herbicide use in crops (Samsel & Seneff, 2013).
One alternative to control weed's in sugarcane that can reduce the use of herbicides is the formation of mulching over the soil.Plants of the Leguminosae family with high productivity of biomass can be cultivated and managed to form the cover.This practice interferes in infestation and establishment of weeds by physical, chemical and biological means (Teixeira et al., 2014).In the State of Alagoas, Northeastern Region of Brazil, the most used legumes in the renewal area of sugarcane, are of the genus Crotalaria and the species Cajanus cajan (popular name: bean-guandu-dwarf) (Silva et al., 2016).Among the benefits provided by this practice are increment in macro and micronutrients (Oliveira et al., 2017); increase in organic matter (Garcia-Franco et al., 2015); increase in nitrogen availability (Santos et al., 2017) and nutrient recycle (Sharifi et al., 2014).In addition to these benefits to soil fertility, the soil mulch from legumes can suppress weeds (Gomes et al., 2014;Mhlanga et al., 2015).
When legumes topple on the soil surface, without incorporation, they negatively interfere in the germination of positive photoblastic seeds, those that require light for germination (Yamauti et al., 2011).Tridax procumbens seeds, for example, are photoblastic positive; don't germinate without the presence of light and therefore the infestation of this weed is intensely reduced in the crops with soil cover (Guimarães et al., 2002).
Another effect of soil cover is the physical barrier that hinders the survival of seedlings of weeds with small quantities of reserves in the diaspores (seed dispersal structures).Often the reserves are not sufficient to ensure  The statistical design used was a randomized block design with four blocks and experimental plots of 6 m × 8 m.The collected data submitted to analysis of variance and the means tested by the Scott-Knott grouping test up to 5% probability.The calculations performed using Assistat version 7.7 beta (Silva and Azevedo, 2016).
Contrasts (C) also performed between some treatments, obtained by the t test in the Scheffé method: C1 (1 vs 13) corresponding to the contrast between C. spectabilis with mechanical topple and C. spectabilis desiccated with post-emergence herbicides; C2 (1 + 3 + 5 + 7 + 9 vs 11) corresponding to the contrast between the five legumes submitted to the mechanical topple and the treatment without cover and without herbicides; C3 (2 + 4 + 6 + 8 + 10 vs 12) corresponding to the contrast between chemically desiccated legumes and treatment with just herbicides and without soil cover; C4 (1 + 3 + 5 + 7 + 9 vs 14) corresponding to the contrast between the five legumes submitted to the mechanical topple and the additional treatment of pre-emergence herbicides and C5 (1 vs 14) corresponding to the contrast between C. spectabilis and the additional treatment of herbicide application in pre-emergence.

Results and Discussion
There was a significant effect of the interaction between soil mulch and management, for dry mass production and weed density at 30 and 60 DAP (Tables 2 and 3).At 30 DAP the suppression efficiency of C. spectabilis mulch, has passed from 27% with topple to 33% with post + pre herbicides, but there was no difference.Contrast C1 indicates that there was also no difference in relation to the additional treatment 1, only with post emergence herbicide.Observed from the above exposed that the common residual effect in pre-emergent herbicides may not be observed at 30 days in weed suppression in sugarcane (Table 2).High infestation of one or some weed species is an undesirable condition in which losses occur only for the interest crop.However, the correct management doesn't search the total eradication of weeds, only search to diminish the denser populations (Varella & Rocha, 1999).In the additional treatment 2, pre-emergence herbicides, the weed density was significantly higher (146 plants m -2 ) in relation to the treatment without mulch and without herbicides at 30 DAP.However, the opposite occurred with the dry mass of weeds (Table 2).This shows that plant density shouldn't be analyzed apart from the dry mass of weeds; this could lead to wrong conclusions about the suppressive effect of soil mulch (Oliveira & Freitas, 2008).3).The main effects of soil mulch on weed suppression are cited in Oliveira Neto et al. ( 2011): the physical barrier causes mechanical resistance to the emergence of weeds, prevents the light that interferes in the germination of positive photoblasts weed seeds, and this management of mulch requires a minimum soil cultivation that contributes to the maintenance the weed seeds dormancy on the soil.
The main weeds occurring in the area belonged to the family Poaceae: Eragrostis ciliares, Eleusine indica and Dactyloctenium aegyptium.However, other species also stood out: Emilia coccinea (Asteraceae) and Mollugo verticillata (Molluginaceae).
The weed density at 60 DAP was significantly higher in treatments that received herbicides compared to soil mulch with mechanical topple (Table 3).Although herbicides provide intense control of weeds, other species of the soil seed bank may be establishing in the crop area (Timossi et al., 2011).There was an interaction effect of the factors studied for culms production per hectare (TCH) and industrial yield (TPH) of sugarcane (Table 4).Note.Average followed by equal letters, lowercase in the column and uppercase in the line, don't differ by Scott-Knott test up to 5% probability.** Significant up to 1% probability; * significant up to 5% probability; ns not significant up to 5% probability; (C) contrasts obtained by the t-test.
The agricultural productivity (TCH) was significantly higher for treatments with soil mulch from C. spectabilis, C. ochroleuca and Cajanus cajan with mechanical topple, in relation to the uncovered area, in the option of mechanical management.This productivity increase isn't only related to the efficiency of suppressing weeds of the soil mulch, since the coverage of C. juncea with topple had the same suppression efficiency at 60 DAP as the mulch's cited (Table 3).However, it hasn't now presented the same agricultural productivity.Another research verified that the root system of the legumes: C. spectabilis, C. ochroleuca and Cajanus cajan, are very extensive and deep, in the formation of the soil mulch the decomposed roots of the legumes serve as biopores in the soil, that allow greater infiltration and availability of water for sugarcane cultivated in the same area, which would justify the increase of agricultural productivity (Cerqueira, 2011) (Table 4).
The use of herbicides in the formation of mulch seemed to contribute to a faster mineralization in the stage of decomposition of mulch in Crotalaria spectabilis, for example, this would justify the increase of productivity with the use of herbicides (Tables 3 and 4), because the suppression of weeds presented the same efficiency for the two treatments.Boer et al. (2008) also observed that when the soil cover was managed with chemical desiccation, an increase in the decomposition velocity of the vegetal remains occurred, causing a greater availability of nutrients to the soil, which would favor the production of the sugarcane under the herbicide management.
The absence of weed control until the 60 days of sugarcane cultivation, in the treatment without soil mulch and without herbicides, reduced the sugar production per hectare by 35%, in relation to the control with herbicides (pre and post), also reduced by 45% compared to treatment of soil mulch from C. spectabilis with herbicides.The treatments without soil mulch and without herbicides presented agricultural losses and losses of industrial yield around 35% (Table 4).

Conclusion
The soil mulch from C. spectabilis, C. juncea, C. ochroleuca, C. breviflora and Cajanus cajan was less efficient in suppressing weeds than the treatment with herbicides applied in pre-emergence + post-emergence.
On the other hand, they were more efficient in the control of weeds in relation to the use of herbicides in pre-emergence, fact observed at 60 days of sugarcane cultivation.
Soil mulch from C. spectabilis showed the same weed control efficiency and the same effect on sugarcane (first harvest/cut) productivity, both with mechanical management and with additional management with post-emergence herbicides.
Treatments (Nº)-Soil Mulch (SM) and Management (M) Description Dry mass (t ha -1 ) Herbicide (dose) (g ha -1 ) 1 = C. spectabilis + post-emergence (13) 6.5 Paraquat (200) 2 = Without Soil Mulch + pre-emergence (14) -Metribuzin (960) + Diurom (1066) + Hexazinone (134)During two seasons: at 30 and 60 days after planting (DAP), evaluated the effects of mulching on weed suppression between the lines of sugarcane plantation.Evaluated also two variables in the weeds: phytomass and plant density, using a methodology adapted from the square inventory(Braun-Blanquet, 1950), which based on the use of a square of 1.0 × 1.0 m, randomly placed inside the crops, however in this experiment a rectangle 0.5 × 1.0 m used, randomly placed in each experimental plot.At 12 months of cultivation, the sugarcane harvested and the data of agricultural productivity (TCH) and industrial yield (TPH) obtained.

Note.
Average followed by equal letters, lowercase in the column and uppercase in the line, don't differ by Scott-Knott test up to 5% probability.** Significant up to 1% probability; * significant up to 5% probability; ns not significant up to 5% probability; (C) contrasts obtained by the t test, [ ] the brackets bring the percentage efficiency of weed control in relation to the treatment without cover and with mechanical management.

Note.
Average followed by equal letters, lowercase in the column and uppercase in the line, don't differ by Scott-Knott test up to 5% probability.** Significant up to 1% probability; * significant up to 5% probability; ns not significant up to 5% probability; (C) contrasts obtained by the t test, [ ] the brackets bring the percentage efficiency of weed control in relation to the treatment without cover and with mechanical management.At 60 DAP of sugarcane cultivation, the soil mulch from C. juncea with mechanical topple overcame by almost 15 times the capacity to control weeds in relation to herbicides in pre-emergence (Table

Table 1 .
List of treatments and respective descriptions mulch and the m ilis, C. juncea r hand, the tw 11 and cultivat The other plots ways: (1) topp he plants, or (2 anagements: m t).

Table 2 .
Influence of management soil mulch on the production of dry mass and weed density in the area, 30 days after planting sugarcane, Sugarcane Industry Sinimbu, AL

Table 3 .
Influence of management soil mulch on the production of dry mass and weed density in the area, 60 days after planting sugarcane, Sugarcane Industry Sinimbu, AL

Table 4 .
Influence of soil mulch on Agricultural Productivity [tons of culm per hectare (TCH)] and Industrial Yield [tons of sugar per hectare (TPH)] of sugarcane, at 12 months of cultivation, Sugarcane Industry Sinimbu, AL