Full Length Research Paper
ABSTRACT
Although sugarcane crop has moderate tolerance to water stress, it shows high response to irrigation. Thus, the improvement of irrigation techniques becomes necessary to achieve maximum water use efficiency. Several studies in Brazil and worldwide on different sugarcane varieties have shown the effect of irrigation on productivity. Moreover, nitrogen fertilization stands out as one of the cultural practices of higher research demand, since studies on nitrogen show very variable results and sometimes even contradictory. The aim of this study was to evaluate the biometric indices of sugarcane in different growing stages, stalk productivity, and water use efficiency at different levels of water replacement by subsurface drip system with and without nitrogen application during cultivation. The experiment was carried out in the experimental field of the Federal Institute of Goias - Campus Rio Verde, GO. The experimental design was randomized blocks, with 5 × 2 factorial design and four replications. Treatments consisted of five levels of water replacement (100, 75, 50, 25, and 0% soil moisture at field capacity) combined or not with nitrogen fertilization (0 and 100 kg urea ha-1). Plant height, stalk diameter, and leaf area of three plants were determined in the floor space of each plot in nine steps each month, from 90 days after planting (DAP), corresponding to the following periods: 90, 120, 150, 180, 210, 240, 270, 300, and 330 DAP. The results were subjected to analysis of variance by F test at 5% probability, and in cases of significance, linear and quadratic polynomial regression analysis was performed to the levels of water replacement. For nitrogen fertilization, means were compared by Tukey’s test at 5% probability. The subsurface drip irrigation provided suitable conditions for accelerating sugarcane growth. The levels of water replacement contributed linearly on the development parameters and stalk yield. The nitrogen fertilizer contributed to the development of biometric variables in the latter stages of crop development.
Key words: Subsurface drip irrigation, water deficit, growth, sugarcane.
INTRODUCTION
Currently, sugarcane crops have played an important role in economic development, once sugarcane is considered one of the major agricultural commodities in terms of productivity (Devos, 2010), cultivated in tropical or subtropical climate, mainly used to produce sugar and biofuel (Kajihara et al., 2012).
Although sugarcane has some tolerance to water stress, it also shows high response to irrigation (Singh et al., 2007). It is assumed that the irrigation increases sugarcane productivity (Wiedenfeld and Enciso, 2008; Gava et al., 2011); however, the improvement of management techniques becomes necessary to achieve maximum efficiency in use of water resources, focusing on maximum productivity with lower volumes of water. According to Gava et al. (2011), plant growth and development are affected either by a lack of or excessive water supply.
Although, the irrigated farming may improve the production environment (Carr and Knox, 2011), for water use efficiency by the sugarcane crop, it is essential to identify the water requirement, responsible for the maximum yields (Wiedenfeld and Enciso, 2008).
Several studies in Brazil and worldwide on different sugarcane varieties have shown the effect of irrigation on productivity. Moreover, nitrogen fertilization stands out as one of the cultural practices of higher research demand, since studies on N show very variable results and sometimes even contradictory (Korndörfer et al., 2002). However, there are many studies on the importance of N in sugarcane crops (Oliveira et al., 2013; Franco et al., 2011).
Drip irrigation stands out among the irrigation methods used to meet the water requirement for sugarcane crop. This method allows precise control of water supplied in small quantities at high frequency to the root zone, allowing the maintenance of favorable conditions to root proliferation in the soil partly moistened (Souza et al., 2009). In addition, subsurface drip irrigation allows nutrient application directly to the root zone, without damaging the crop, allowing topdressing applications rationally and parceled out, according to the crop needs at various plant development stages.
Studies on sugarcane development have been used as an important tool to assess the performance of varieties, once initial fast and uniform growth leads to a good stand of sugarcane crop (Silva et al., 2007). Thus, the aim of this study was to evaluate the biometric indices of sugarcane growth in various development stages, stalk productivity, and water use efficiency at different levels of water replacement by subsurface drip irrigation and without nitrogen fertilization during the cultivation.
MATERIALS AND METHODS
The experiment was carried in the experimental area of the Federal Institute of Goias - Campus Rio Verde, Goias, latitude 17° 48'28 "S and longitude 50° 53'57" O, with an average altitude of 720 m and slightly rolling slope (6%). The climate is classified as Köppen and Aw (tropical), with a rainfall season from October to May, and the dry months from June to September. The annual average temperature ranges from 20 to 35°C and precipitation range of 1500 to 1800 mm per year. The soil was classified as Red Latosol (LVdf) of medium texture. Table 1 shows the hydro-physical and chemical soil characteristics.
The experimental design was randomized blocks, 5 × 2 factorial design, with four replications. The treatments consisted of five levels of water replacement (100, 75, 50, 25, and 0% soil moisture at field capacity) combined with or without nitrogen fertilization (0 and 100 kg urea ha-1).
Sugarcane planting was performed in March 2012, using the variety RB 85-5453, as it has high sugar content and precocity as main characteristics. The experimental plots consisted of three double-furrow openings ("W" planting) with a spacing of 1.40 × 0.4 m between rows and 8 m in length, totaling 35.2 m2 of the total area per plot.
For treatments using water replacement, the subsurface drip irrigation method was used. The drip line was installed at 0.20 m above ground in the center of the double row and has the following characteristics: 16150 PC Dripnet model with thin wall, working pressure of 1 bar, nominal flow rate of 1.0 L h-1, and drip spacing of 0.50 m.
Irrigation was carried out based on digital tensiometer puncture with a sensitivity of 0.1 kPa, with probes placed at depths of 0.20, 0.40, 0.60 and 0.80 m and distances of 0.15, 0.30, 0.45, and 0.60 m from the drip line, with daily readings of the soil matrix potential (Ψm). A critical tension of 50 kPa was used to determine irrigation water needs. The physical and hydraulic characteristics of the soil were determined by the soil water retention curve (Oliveira et al., 2014).
From the daily results of soil moisture, the volume of water used for each replacement level was determined and the increase of soil moisture to field capacity was considered for the treatments with 100% water replacement. For the other treatments, water laminas were applied according to the percentage of the predicted water replacement. At the end of the experiment, the total volume of water applied for irrigation (VTA) was 0, 126, 252, 378, and 504 mm of water for 0, 25, 50, 75, and 100% water replacement, respectively.
According to the results of soil analysis, the experimental area was fertilized, with application of 30 kg N ha-1 (urea), 120 kg P2O5 ha-1 (superphosphate), and 80 kg K2O ha-1 (potassium chloride). In the plots where nitrogen application was predicted, it was fully applied through irrigation water (fertigation) parceled out in ten applications during the crop cycle. Potassium fertilization was partially performed to the furrow lines, representing 30% of total, and the remaining fertilizer was applied through irrigation water. For the treatment with 0% water replacement, the application of nutrients was performed by a fractioned way.
From the climatological data of the experimental period, the ten-day water balance was estimated for sugarcane grown under rain fed management, as described by Thornthwaite and Mather (1955), and the reference evapotranspiration (ET0) was calculated according to Allen et al. (1988) (Figure 1). The total rainfall during the crop cycle was 1479.6 mm and after discounting the volume of percolated water, an effective precipitation (PE) of 1019.2 mm was computed. The crop evapotranspiration (ETc) totaled 1817.5 mm.
The analysis of the biometric variables of sugarcane was performed once a month beginning at 90 days after planting (DAP), corresponding to 9 periods as follows: 90, 120, 150, 180, 210, 240, 270, 300, and 330 DAP. Plant height (AP, cm), stalk diameter (DC, mm), and leaf area (LA, m2) of three plants were determined in the floor space of each plot (linear meter in the center of the double row). The AP was measured with the aid of a measuring tape from the ground to the collar leaf +1 and the DC was measured using a digital caliper placed at the stalk base. The AF was calculated by measuring the leaf length and width +1, and counting the number of green leaves, using the following equation: AF = (C × L) × (N + 2) × 0.7, where C is the leaf length+1; L is the leaf width +1; N is the number of green leaves; and 0.7 is the correction factor.
At harvest, the stalk yield (PC, Mg ha-1), water use efficiency (EUA mm Mg-1 ha-1), and number of industrially tillers (NPI) were determined. The PC was determined by weighing the stalks within each plot using a digital hand scale, extrapolating to 1 ha. The EUA was calculated as the ratio between the total receiving water (calculated by the sum of PE and VTA) and the PC. The NPI was determined by counting plants in floor space.
The results of both the biometric variables obtained at each development stage and productivity indices were submitted to analysis of variance by F test at 5% probability, and in cases of significance, linear and quadratic polynomial regression analysis was performed to the levels of water replacement. For nitrogen fertilization, the means were compared by Tukey test at 5% probability using the statistical program SISVAR® (Ferreira, 2011).
RESULTS
No significant interaction between the water replacement (RH) and nitrogen (N) was observed for the sources of variation evaluated (Table 2).
When analyzed separately, the response of the variable plant height (AP) for the factor RH was significant at 1% probability at all stages of crop development, as well as for the variable leaf area (LA), except at 90 DAP, where the significance level was 5% by the F test. For the stalk diameter (DC), no significant difference was observed at 90 DAP, since in general in the early stages of plant development, no differentiation is observed with respect to the RH levels. However, at 120 DAP, a significant difference was observed at 1% probability level for all development stages. The factor N caused no significant effect on the biometric variables at the beginning of the cycle. In contrast, a significant interference at 1% probability was observed only at 300 and 330 DAP for the variable AP as a function of N, according to the F test (Table 2). For the DC, a significant (5%) difference was observed in the development stages evaluated from 150 DAP. For the AF, 5% significance was observed only from 240 DAP, which exhibited 1% significance in the last assessment (330 DAP).
When the variable AP was analyzed facing the presence and absence of N in the development stages with significant differences by Tukey’ test, the sugarcane suffered an increase of 5.58 and 4.97 cm after N fertilization at 300 and 330 DAP, respectively, corresponding to an increase of 1.96 and 1.59% (Table 2).
The increase in DC was more pronounced with the N supply, since a significant difference was observed from the 150 DAP, with an increase of 4.38% (Table 2). In the subsequent development stages, the increase in DC of sugarcane grown with N was lower with values of 2.91, 2.66, 2.40, 2.54, 2.20, and 2.19% at 180, 210, 240, 270, 300, and 330 DAP, respectively. Thus, it is clear that the use of N by plant cane to provide an increase in the DC occurred in the early stages of maximal growth, and the growth rate was similar in the other stages, concluding that N was responsible for initiating the DC increment.
The N supply affected the AF development only from the 240 DAP, with an increase of 4.06 m2, which corresponded to an increment of 6.83%. At 270 and 300 DAP, the increase in AF provided by N was less expressive, 5.90 and 5.53%, respectively. However, the maximum AF development due to the N supply occurred at 330 DAP, with an increase of 6.76 m2, representing an increment of 8.51%, faced with the end of N application (Table 2).
The direct nitrogen application into the root zone may have contributed to the availability of this nutrient in the later development stages of sugarcane, by providing a direct contribution to the rapid N dispersion in the environment, allowing these plants to absorb a higher N level for a longer period of time.
The mean AP values as a function of RH behaved in linear models for all development stages (Figure 2). During the establishment and tillering, an increase of 1.60 cm at 90 DAP (Figure 2A) and 2.57 cm at 120 DAP (Figure 2B) was observed for each RH level, thus at 100% RH, yields 19.85 and 22.72% higher than the rain fed management (0% RH) were found at 90 and 120 DAP, respectively. From 150 DAP, a period that includes the beginning of the maximal plant growth, the AP development was influenced more significantly by RH. The AP in the treatment with 100% RH at 150 DAP was 33.77% higher when compared with the rain fed management (0% RH), with an increase of 5.13 cm for each RH level (Figure 2C).
At 180, 210 and 240 DAP, the AP development was similar within the RH levels, with differences of 21.89, 18.92, and 22.05% for the 100% RH when compared with the treatments with 0% RH and the AP growth rate remained uniformly among treatments.
In these stages, the increase in AP for each RH level was 4.83, 5.04, and 7.18 cm at 180, 210, and 240 DAP, respectively (Figure 2D, 2E, and 2F), which shows that when irrigated the sugarcane crop exhibits a growth rate much higher than those with water deficit.
In contrast, at the end of the maximal growth stage, the effect of RH on the AP was less expressive. The increments provided by the increased RH level were 6.19, 9.01, and 6.95 cm at 270, 300 and 330 DAP, respectively. The estimated AP at 100% RH was 13.46, 13.69, and 9.40% higher than the rain fed management (0% RH) (Figure 2G, 2H and 2I).
It is noteworthy that some recovery in the AP development was observed in the treatments with water restriction, probably due to the soil water availability provided by rainfall, which began at the end of September.
The mean DC for the different RH levels showed a linear tendency in all development stages with significant differences by the F test (Figure 3). The difference in the DC development as a function of different RH levels was similar in the different periods. At 120 DAP, the DC suffered an increase of 3.92% for each RH level, with an estimated average of 27.28 mm with full irrigation (100% RH) (Figure 3A).
In the early stage of maximal sugarcane growth, the increases in DC with increasing the RH levels were 3.62, 3.07, and 3.39% at 150, 180 and 210 DAP, thus achieving an increase of 3.77, 3.49, and 4.1 mm at 100% RH as compared to the rain fed management (0% RH) (Figure 3B, 3C and 3D).
At 240 DAP, certain recovery in the DC development was observed in the treatments with water restriction, with differences of only 2.66% for each RH level studied (Figure 3D). In addition, at 270 and 300 DAP, differences of 2.56 and 2.94% were observed for the RH levels. However, it is noteworthy that it was a high rainfall period (Figure 1), which generally favors the maintenance of soil moisture (Figure 3F and 3G).
However, soon after, a gap in the mean DC is observed in analogy of the different RH levels, showing that, at 330 DAP, an increase of 4.15 mm was achieved at 100% RH when compared with rain fed management (0% RH), which represents an increase of 3.13% for each level RH, obtaining a maximum value of 37.26 mm (Figure 3H).
The AF responses of sugarcane subjected to different RH levels showed linear behavior at all development stages (Figure 4). The different AF values found for the RH levels were higher according to the crop development stages. At 90 DAP, the AF suffered an increase of 1.85 m2 for each RH level, with a maximum response of 22.69 m2 at 100% RH (Figure 4A).
From the early stage of maximal growth, the AF development of the plants subjected to conditions of higher soil moisture was more significant, which was 58.20% higher at 100% RH when compared with the fed rain management (0% RH) (Figure 4B). At 150, 180, and 210 DAP, the AF development was quite similar, with increases of 11.22, 10.72, and 10.57% for each RH level.
Throughout the sugarcane development stages, it is observed that the difference in AF as a function of different RH levels gradually increased. At 240, 270, 300, and 330 DAP, increments of 11.37, 15.36, 17.77, and 19.79 m2 were observed for each RH level, respectively. The full irrigation (100% RH) provided increments of 45.59, 61.46, 71.08, and 79.16% as compared to rain fed management (0% RH).
The significant difference between the RH levels is mainly related to the low development of AF in the treatments with water restriction, evidenced by the growth rate at 0% RH of only 4.81 and 5.70 m2 at 240 to 270 and 270 to 300 DAP, respectively, and a reduction of 2.84 m2 at 300 to 330 DAP. It is noteworthy that, for all RH levels, the maximum AF values were found at 300 DAP, reaching maximal response of 98.05 m2 at 100% RH.
Table 3 shows the summary of the analysis of variance for the variables: stalk yield (PC), water use efficiency (EUA), and number of industrially tillers (NPI). Both the factor N and the interaction RH × N caused no significance in the variables analyzed. The variable PC showed significance at 1% probability for the factor RH. The factor RH caused a 5% significant effect on NPI.
The factors evaluated did not significantly affect the variable EUA, probably due to the high rainfall throughout the experimental period (PE = 1019.2 mm). However, it is noteworthy that the rainfall in the region is concentrated in a period of the year, resulting in sharp decline in the PC yield in the rain fed management (0% RH) (Figure 5). The average PC of sugarcane followed a linear model(R2 = 0.7471) as a function of the RH levels (Figure 5). The RH allowed an increase of 17.74 Mg ha-1 for each level evaluated, corresponding to an increase of 9.96%. With respect to the rain fed management (0% RH), the PC was 28.5% lower than that observed at 100% RH, with an estimate yield of 249.02 Mg ha-1.
The NPI of sugarcane showed a linear growth as a function of RH (R2 = 0.9282) (Figure 6). The maximum NPI response at 100% RH was 24 tillers m-2. The full irrigation system (100% RH) allowed increments of 5.22, 10.44, 15.66, and 20.88% at 0, 25, 50 and 75% RH levels, respectively. The plant height as a function of water replacement was much more expressive than the stalk diameter. In respect to leaf area, although higher sensitivity to the different volumes of irrigation have been observed, the water replacement had less effect on the number of tillers.
The varietal characteristics of the variety RB 85-5453, when defined under irrigated management increased with the increasing volume of water supplied, affecting the photosynthetic efficiency and consequently the stalk yield.
DISCUSSION
Dantas Neto et al. (2006) found similar results for the variables of sugarcane growth, with no interactions between fertilization levels and irrigation regimes despite the top-dressing, which was significant. Opposite results were reported by Silva et al. (2008), who found that the stalk diameter may not reflect the differences between sugarcane grown on different water regimes. These results are in agreement with Franco et al. (2011), who reported that studies on nitrogen fertilization have very variable results regarding the effect of N application, mainly on stalk yield, with heterogeneous responses for the plant cane and relatively homogeneous responses for the ratoon. Nevertheless, it is known that among the main factors that limit the productivity of Brazilian sugarcane include water and nutrients availability, mainly nitrogen (Wiedenfeld and Enciso, 2008; Oliveira et al., 2013).
According to Roberts (2008), the use of fertigation generally improves nutrient use efficiency, since they are applied in a fractioned way, according to the nutrient absorption of the crop. Dias et al. (2012) reported that the shoot height and biomass production were sensitive to water restriction, reaching the highest values under the full irrigation regime. Freitas et al. (2012) reported that the significant difference between sugar cane height as a function of irrigation occurred at 116 DAP, evidencing that, from then on, the water availability has become a limiting factor to the crop vegetative development. These results corroborate the findings of Oliveira et al. (2010), who demonstrated that RB varieties grown under full irrigation reached plant height values higher than 300 cm.
This response is similar to results obtained by Dantas et al. (2006) in irrigated sugarcane crop in the Brazilian Northeast, once these authors found an increase of 30.0% with a total water of 1343 mm in relation to the rain fed management, reaching an average of 24.67 mm in stalk diameter. Farias et al. (2007) observed an increase of approximately 46.0% in leaf area index of sugarcane subjected to full irrigation.
The crop canopy is an important factor in crop yield, since it intercepts the solar radiation which promotes photosynthesis and evaporation, and causes shading on weeds (Smit and Singels, 2006). Studies have shown that the photosynthetic capacity of sugarcane decreases drastically due to the reduced leaf area (Inman-Bamber et al., 2009).
Oliveira et al. (2011) found a mean increase of 145.0% in sugarcane yield varieties subjected to full irrigation in the Pernambuco State, reaching yields of up to 255.6 Mg ha-1. Gava et al. (2011) observed a mean increase in sugarcane yield of 24% using the irrigation system in São Paulo state. The sugarcane production can be affected significantly by the reduced tiller emission and survival, especially when water deficit occurs during crop establishment, the final number of stalks is significantly affected (Mauri, 2012).
According to Inman-Bamber and Smith (2005) and Ghannoum (2009), the morphological and physiological characteristics modified by water stress are of great importance to achieve high plant productivity. However, the productivity of the irrigated sugarcane depends on the ratio between the amount of water applied and the amount of available fertilizer, in addition to the variety, cutting cycle, soil, and climate (Gava et al., 2011).
When subjected to water stress conditions, plants exhibit several morphological and physiological changes, such as leaf rolling, changing in leaf angle, and reduction in leaf area (Chaves et al., 2008). Farias et al. (2007) observed an increase of approximately 46.0% in leaf area index of sugarcane subjected to full irrigation.
CONCLUSIONS
The subsurface drip irrigation provided suitable conditions for the establishment and faster growth of sugarcane. The water replacement levels contributed linearly to both the growing parameters at most stages of crop development and stalk yield.
Despite the nitrogen fertilization by fractioned way through the irrigation system has contributed significantly to the development of biometric variables in the latter stages of crop development, no significant changes were observed in stalk yield.
CONFLICTS OF INTERESTS
The authors have not declared any conflict of interests.
ACKNOWLEDGEMENTS
The authors would like to thank the Ministry of Science and Technology (MCT), the Foundation for Research Support of the State of Goiás (FAPEG), the Coordination for Upgrading Higher Institution Personnel (CAPES), the Brazilian Council for Scientific and Technological Development (CNPq) and FINEP for funding the current scientific project.
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