Sugarcane is one of the most important crops in Brazil due to its great socio-economic relevance in the production of several products, mainly biofuel used worldwide. Brazil is currently the biggest world producer of sugarcane with a total production of crushed sugarcane for the 2014-15 harvest estimated at 642.1 millions of tons, cultivated in approximately 9004.5 thousands of hectares in the producing states [1] .

According to Silva et al. [2] , for maximum yield, sugarcane needs adequate moisture, directly proportional to the transpired water, throughout its vegetal growth. Thus, irrigation becomes indispensable for greater productivity.

Useful information may be found in the literature on responses and benefits of nitrogen in sugarcane plants. Several authors report that responses to the nutrient are generally more frequent in sugarcane ratoon than in the sugar plant and that the crop absorbs between 50% and 60% of the N amount applied to the soil [3] -[5] . However, for higher productivity, the interaction of the plant’s genetic factors, climate, soil and management must be taken into account [6] .

The dry matter of the aerial segment is also relevant. In mechanized sugarcane harvest, part of the phytomass, mainly comprising leaves and pointers (leaves at the tip of the stem), is placed on the soil as a soil cover or chaff. Chaff is not only a protection but provides nutrients and improves the soil’s physical and chemical properties [7] [8] and, at the same time, may enhance the temporary immobilization of nitrogen [6] . The production of dry matter is one of the most studied variables in assays on agriculture. According to Faroni et al. [9] , sugarcane phytomass comprises all vegetal matter ranging from the aerial segment to the radicular system (rhizomes and roots).

In the medium and long terms, soil may accumulate organic C and N when sugarcane is managed without burning, even though, in the short term, the amount of residues with high C/N may demand more mineral N due to microbial immobilization. Several studies have shown that the best N dose should be 60 kg∙ha⁻¹, which is higher than that used in burnt sugarcane [10] . Current analysis may provide important information on nitrogen fertilization associated with irrigation and may contribute towards recommendations for nitrogen fertilizers and the rational use of water in sugarcane crops in the soil-climate conditions of the Brazilian savannah.

The method of harvesting sugarcane without the use of fire, mandatory by environmental legislation, implies in the management of nitrogen fertilization concerning doses, sources and methods of application. Non-burnt sugarcane produces residues of dry leaves and pointers on the soil which may vary between 10 and 20 t∙ha⁻¹ dry matter, a C/N ratio above 100 and N contents between 40 and 80 kg∙ha⁻¹ [11] [12] . Consequently, when straw decomposes, some N rates may be absorbed by the subsequent stubbles and others may supply the N stock in the soil [12] [13] .

Current analysis quantifies the dry matter of the aerial segment of the plant and evaluates the productivity of the sugarcane stem, pointers and straw in sugarcane ratoon at different levels of water replacement (WR) with and without nitrogen fertilization with surface drip irrigation system.

Table 2 shows that water replacement affected significantly all the variables under analysis, with the exception of PS. In N applications, there were significant differences for stem productivity (SP) and harvest index (HI), but no significant interaction between the factors studied for the assessed variables. There was an increase of 17.85 t∙ha⁻¹ for SP as a response for the application of N-fertilizer when compared to N-less treatments, where a 2.5% increase occurred for HI. As reported in current study and in several research works, sugarcane ratoon responds favorably to nitrogen application. Souza [26] , reports that treatment without nitrogen fertilization had an increase of 5.04 and 7.41 t∙ha⁻¹, when compared to 120 and 240 kg∙ha⁻¹ N doses, or rather, lower than the increase reported in current study. The result can be attributed to differences between cultivars, or even by soil type and/or nitrogen source applied.

Rípoli et al. [27] reported that sugarcane produces between 9% and 14% of straw (dry matter base), with approximately 10 t∙ha⁻¹. Although not affected by evaluated factors, straw production in current analysis ranged between 15.92 and 17.16 t∙ha⁻¹, higher than rates by the former authors.

The variables stem productivity and pointer productivity had a linear behavior for WR (Figure 2). There was a 22.6 t∙ha⁻¹ increase in stem production for a 25% increase of WR, with the highest productivity at 233.4 t∙ha⁻¹ stems for 100% WR (Figure 2(a)). The above WR is equivalent to a water depth of 1475.33 mm (100% WR plus true rainfall). The above rates were higher than those by Haddad et al. [28] who studied stem productivity and sugar yields of sugarcane according to water depth. They also registered productivity rise in proportion to increase in water depth, with maximum rate at 125.9 t∙ha⁻¹ for the application of 1854.4 mm. Difference in productivity may have been caused by better water distribution throughout the sugarcane cycle coupled to different fertilizations, management, climate and varieties. However, results were similar to those given by Andrade Júnior et al. [29] and Barbosa et al. [30] with subsurface drip irrigation.

There was a 2.1 t∙ha⁻¹ increase of pointer productivity for every 25% WR increase, with highest productivity at 48.8 t∙ha⁻¹ at 100% water depth (Figure 2(b)). Increase in the productivity of pointers enhanced stem productivity, which may be due to the source-drain ratio, where the pointer is the energy source for the stems (drain), and to the unit increase (UI) of WR. In fact, UI was three times greater for stem productivity than UI for point productivity, and thus the greater development of the pointer caused a greater production of energy used by the sugarcane stems.

^**Significant at 0.01 probability by F-test; ^*Significant at 0.05 probability by F-test; ^ns not significant at 0.05 probability by F-test; Means followed by the same letter in the columns do not differ statistically at 0.05 probability by Tukey’s test; LSD (least significant difference); SE (standard errors); DF (Degrees of freedom); SP (stem productivity); PS (straw productivity); PP (productivity of pointer); HI (harvest index).

^** and ^* respectively significant at p < 0.01 and 0.05, according to F-test.

Table 3 shows that variables DMS, DMP and DMP/DMS were significantly affected by WR. Application of N caused a significant effect on variables DMS and DMP/DMS, with an increase of 8.16 t∙ha⁻¹ and 13.64% for the respective variables, when compared to non-application of N. Total dry matter (TDM) of the aerial segment was the only variable which significantly differed with regard to WR × N.

Cantarella [32] assessed sugarcane response to N application in a three-year period and verified that the production of the dry matter of the stem was significantly higher in urea-fertilized crops (100 kg∙ha⁻¹ N). Moreover, the residual effect of fertilization was also confirmed in the following year. Results demonstrate that the application of N in sugarcane crops increases the dry matter of the stem and final productivity.

Variables DMP and DMS exhibited a linear behavior for WR (Figure 4). The amount of dry matter accumulated in the pointer (DMP) due to water replacement varied between 10.96 and 14.77 t∙ha⁻¹. There was a 0.35% increase in DMP for every 1% increase in WP, or rather, a gain of 0.038 t∙ha⁻¹ (Figure 4(a)). Therefore, the accumulation of dry matter in the pointer increases in proportion to water increase. Oliveira et al. [33] reported rates between 7.5 and 12.0 t∙ha⁻¹ for DMP in irrigated sugarcane varieties, similar to that in current analysis.

^**Significant at 0.01 probability by F-test; ^*Significant at 0.05 probability by F-test; ^ns not significant at 0.05 probability by F-test; Means followed by the same letter in the columns do not differ statistically at 0.05 probability by Tukey’s test; LSD (least significant difference); SE (standard errors); DF (degrees of freedom); DMS (dry matter of stem); DMP (dry matter of pointer); TDM (total dry matter from the aerial segment).

Every 1% increase in WR, the increment for DMS is of 0.28 t∙ha⁻¹ (Figure 4(b)), corroborating results by Farias et al. [34] who reported that water supply through irrigation provided an increase in the accumulation rate of the sugarcane stem’s dry matter. Irrigated water supply for crops increases the production of dry matter of stem and pointer, underscoring that DMS production is 1.94 times greater than DMP production when there is a 1% WR increase by irrigation.

Therefore, irrigation increases the production of dry matter in the stem and, therefore, in sugarcane productivity. The above justifies the practice, especially when one observes in Figure 1 that true rainfall supplied the requirements of the crop (ETc) only during approximately 70 days within the entire cycle.

DMP/DMS ratio (Figure 5) had a quadratic behavior: all rates decreased till 66.6%; from this point, rates increased due to WR. Relationship between pointer and stem revealed that the decrease of this index caused a proportional increase in the production of stems and a decrease in the production of pointers. Machado et al. [35] reported that after 100 days the dry matter of leaves and of pointer provided 70% of the plant’s biomass and that, after this period, there was a progressive decrease in the accumulation of dry matter, or rather, 9% of dry matter accumulated by the plant at the end of the cycle.

Figure 6 shows total dry matter from the aerial segment behavior in sugarcane ratoon: when nitrogen was applied, productivities were estimated at 53.13; 62.88; 72.63; 82.38; 92.13 t∙ha⁻¹ respectively for water replacement 0%, 25%, 50%, 75% and 100%; without the application of nitrogen, productivities were 51.91; 58.11; 64.31; 70.51; 76.71 t∙ha⁻¹, respectively for the same WR percentages. All rates were higher in treatments with nitrogen, or rather, increase in water irrigation depth provided a proportionate increase in the difference between rates. In other words, the amount of available water for the plant affected the absorption of nitrogen.

Figure 6(a) demonstrates that the evolution of the WR × N interaction increased linearly the productivity of total dry matter of the aerial segment. With regard to the evolution of WR for each N level, a 0.39 and 0.25 t∙ha⁻¹ increase occurred in the production of TDM in treatments with and without N for every 1% increase in WR. Consequently, irrigated sugarcane crops were more efficient in increasing TDM production when N-ferti- lizer was applied.

Above results disagree with those given by Arantes [36] who evaluated the production capacity of sugarcane cultivars with and without irrigation and reported but the cause of different results can be well attributed the cultivars used in the study of Arantes, since none were the same used in the current study. That there occurred no significant effect on the accumulation of total dry matter in each evaluation period.

There was only a significant difference for TDM in WR 100% with a 20.1% increase, according to the supply of the nutrient (Figure 6(b)). As a rule, there was a 12.93% gain in the application of N at WR levels (Table 3).

Otto et al. [37] assessed the phytomass of roots and the aerial segment of sugarcane related to N fertilization and reported that splits which did not receive N fertilization showed great energy expenditure in the growth of the roots. This fact established a competition for photoassimilates in the growth of the aerial segment and also explained the low TDM rates in treatments without N-fertilizers in current assay.

Increase in water replacements significantly raised the productivity of dry matter in stems and pointers by the sugarcane.

There was an increase in harvest index up to water replacement 70.22%; for the DMP/DMS ratio, harvest index decreased up to water replacement 66.6%.

There was an increase of 90.61 t∙ha⁻¹ in the productivity of stems and 8.40 t∙ha⁻¹ in the productivity of pointers between treatments water replacement 0% and water replacement 100%.

Nitrogen fertilization significantly increased the productivity of stems, harvest index, dry matter of stem and DMS/DMP ratio.

Nitrogen fertilization increased productivity by 8.16 t∙ha⁻¹ for dry matter and 20.85 t∙ha⁻¹ for stems; in the case of DMP/DMS, increase reached 13.64% and 2.5% for harvest index.

Total dry matter of the aerial segment was affected by RH × N interaction: highest rates occurred in water replacement 100% with the application of nitrogen. Straw productivity was not affected by any of the factors evaluated.

The authors would like to thank the Brazilian National Council for Scientific and Technological development (CNPq) for funding current assay. Thanks are also due to the Brazilian Coordination for the Updating of Higher Education Personnel (CAPES), to the Foundation for Research in the State of Goiás (FAPEG), to Financing Agent for Studies and Projects (FINEP) and to the Federal Institute of Goiás, Campus Rio Verde GO Brazil.

Alefe VianaSouza Bastos,Renato Camposde Oliveira,Nelmício Furtadoda Silva,Marconi BatistaTeixeira,Frederico AntonioLoureiro Soares,Edson Cabralda Silva, (2015) Productivity and Dry Matter Accumulation of Sugarcane Crop under Irrigation and Nitrogen Application at Rio Verde GO, Brazil. American Journal of Plant Sciences,06,2374-2384. doi: 10.4236/ajps.2015.614240