Single and split nitrogen dose in wheat yield indicators

Dose única e fracionada do nitrogênio nos indicadores de produtividade do trigo R E S U M O O fornecimento único ou fracionado do nitrogênio pode maximizar a expressão dos indicadores de produtividade do trigo. Objetivou-se no estudo avaliar a maior eficiência de uso do nitrogênio sobre os indicadores de produtividade do trigo pelo fornecimento único e fracionado do nutriente em condição de ano favorável e desfavorável ao cultivo, em sistema de sucessão de alta e reduzida liberação de N-residual. O estudo foi conduzido em 2014 e 2015, em delineamento experimental de blocos casualizados com quatro repetições em fatorial 4 x 3, para doses de N-fertilizante (0, 30, 60, 120 kg ha-1) e formas de fornecimento [dose única (100%) no estádio fenológico V3 (terceira folha expandida); fracionada (70 e 30%) no estádio fenológico V3/V6 (terceira e sexta folha expandida) e; fracionada (70 e 30%) no estádio fenológico V3/R1 (terceira folha expandida e início do enchimento de grãos)], respectivamente, no sistema de sucessão soja/trigo e milho/trigo. A maior eficiência do nitrogênio à produtividade do trigo é obtida no fornecimento em dose única em ano favorável de cultivo e com o fracionamento no estádio V3/V6 em ano desfavorável, independente do sistema de sucessão de alta e reduzida liberação de N-residual.


Introduction
Wheat is the main winter crop in Southern Brazil and the state of Rio Grande do Sul is responsible for 36% of the national production (CONAB, 2016).In favorable years for cultivation, under adequate management, profitability is easily obtained.On the other hand, restrictive climatic conditions compromise the development and limit the use efficiency of nitrogen (N), the main nutrient for the elaboration of the yield indicators (Ferrari et al., 2016;Silva et al., 2016).Besides increasing the costs, N losses through leaching or volatilization turn the nutrient into a pollutant, generating the need for technologies that improve N use efficiency by the plant, regardless of the cultivation condition (Benin et al., 2012;Mantai et al., 2016).
According to the recommendation of cultivation, N fertilization should be applied at sowing and as top-dressing (between the beginning of tillering and beginning of elongation), moments in which the wheat crop needs greater supply of the nutrient (Arenhardt et al., 2015;Cunha et al., 2016).However, greater efficiency with top-dressing N application is directly dependent on adequate soil moisture, a condition not always obtained at the moment of fertilization (Silva et al., 2016).Studies indicated that split N supply can reduce its losses by using partial doses in moments suitable for fertilization (Brezolin et al, 2016;Ferrari et al., 2016).Thus, the complex relationships between plant, climate and management must be considered in the definition of efficient managements for N supply in the elaboration of yield.
The objective of the study was to evaluate the greater N use efficiency on wheat yield indicators through the single and split application of the nutrient under favorable and unfavorable year conditions to the crop and succession system of high and reduced residual N release.

Material and Methods
The experiment was carried out at the field, in the 2014 and 2015 agricultural years in the municipality of Augusto Pestana, RS,Brazil (28° 26' 30'' S;54° 00' 58'' W).The soil of the experimental area is classified as typic dystroferric Red Latosol and the climate of the region, according to Köppen's classification, is Cfa, with hot summer without dry season.In the study, soil analysis was made ten days before sowing and the following chemical characteristics were identified in the average of the years (Tedesco et al., 1995): i) maize/wheat system -pH = 6.5, P = 34.4mg dm -3 , K = 262 mg dm -3 , OM = 3.5%, Al = 0.0 cmol c dm -3 , Ca = 6.6 cmol c dm -3 and Mg = 3.4 cmol c dm -3 ; and; ii) soybean/wheat system -pH = 6.2, P = 33.9mg dm -3 , K = 200 mg dm -3 , OM = 3.4%, Al = 0.0 cmol c dm -3 , Ca = 6.5 cmol c dm -3 and Mg = 2.5 cmol c dm -3 .Sowing was performed using a seeder-fertilizer machine, composing plots with five 5-m-long rows spaced by 0.20 m, to form the experimental unit of 5 m 2 .At sowing, fertilizations with 30 and 15 kg ha -1 of P 2 O 5 and K 2 O, respectively, were applied based on the contents of P and K in the soil for an expected grain yield 3 t ha -1 , and N fertilization was applied at planting as 10 kg ha -1 in the form of urea (except in the standard experimental unit) and the rest was applied to contemplate the doses proposed in the study.The seeds were subjected to tests of germination and vigor at the laboratory, to correct plant density to the desired value of 400 viable seeds m -2 .The fungicide Tebuconazole was applied at the dose of 0.75 L ha -1 and weeds were controlled with the herbicide metsulfuron-methyl at dose of 4 g ha -1 .The experiment used the wheat cultivar 'BRS Guamirim' , with small size, early cycle, resistance to lodging, commercially classified as bread wheat and of high productive potential, representing the standard biotype desired by the wheat producers in Southern Brazil.
In each cultivation condition of high and reduced residual N release (soybean/wheat and maize/wheat systems), two experiments were conducted, one to quantify biomass yield and the other to estimate grain yield.Therefore, in the four experiments, the design was randomized complete blocks with four replicates in 4 x 3 factorial scheme, for N-fertilizer doses (0, 30, 60 and 120 kg ha -1 ) and forms of N supply [single application (100%) in the phenological stage V 3 (third expanded leaf); split application (70 and 30%) in the phenological stages V 3 /V 6 (third and sixth expanded leaves) and split application (70 and 30%) in the phenological stages V 3 /R 1 (third expanded leaf and early grain filling)], totaling 192 experimental units.The experiments to estimate biomass and grain yields were manually harvested by cutting the three central rows of each plot, in the stage close to the harvest point (125 days), with grain moisture of 15%.Plots intended for grain harvest were threshed with stationary threshing machine, taken to the laboratory to correct grain moisture to 13% and weighed to estimate grain yield (GY, kg ha -1 ).Plots intended for biomass analysis were dried in a forced-air oven at temperature of 65 ºC until constant weight and weighed to estimate biomass yield (BY, kg ha -1 ).The obtained values of total biomass yield and grain yield were used to estimate straw yield (SY, kg ha -1 ) through the difference BY-GY and harvest index (HI, kg kg -1 ) through the relationship GY/BY.
After meeting the assumptions of homogeneity and normality via Bartlett's test, analysis of variance was made to detect main and interaction effects.Based on these data, the linear equation (Y = b 0 ± b 1 x) and quadratic equation (Y = b 0 ± b 1 x ± b 2 x²) were fitted as a function of N doses in each supply condition (single and split dose).Biomass, grain, straw yields and harvest index were estimated based on the N dose indicated for expected grain yield of 3 t ha -1 , considering the organic matter content in the soil and the crop in succession, according to the recommendation of cultivation.Therefore, the soybean/ wheat and maize/wheat systems considered the total N doses of 60 and 90 kg ha -1 added in the crop cycle, respectively.In addition, Scott-Knott tests were applied to compare the means relative to the different N supply conditions, regardless of the total N dose applied as top-dressing.The program Genes was used for these analyses.

Results and Discussion
Data of rainfall and maximum temperature along the wheat crop cycle are presented in Figure 1.In 2014 (Figure 1A), high temperatures occurred in the early development, without rainfalls in previous days and after fertilization.This R. Bras.Eng.Agríc.Ambiental, v. 22, n.1, p.16-21, 2018.fact leads to greater N loss through volatilization, resulting in lower development of tillers and ears per area.
N-fertilizer application in the phenological stage V 6 was followed by high rainfall volume, which favored N loss through leaching.These factors along with the mean yields (Tables 1  and 2) qualify 2014 as unfavorable year (UY) for cultivation.In 2015 (Figure 1B), the occurrence of rainfalls in days preceding N-fertilizer application led to soil moisture favorable for N management.In addition, lower temperatures in the vegetative cycle favored tiller production and distribution of photoassimilates for yield.This fact, combined with the mean yields in the year, characterize 2015 as favorable year (FY) for wheat cultivation.Arenhardt et al. (2015) point out that rainfall volume and distribution combined with air temperature are potential variables in the classification of favorable and unfavorable years for wheat cultivation.Espindula et al. (2010) comment that climatic variation influences N use by the plant.Favorable climate for wheat is determined by mild temperatures without the occurrence of rainfalls in large amounts and intensity (Valério et al., 2009).The excess of rainfalls after fertilization causes N loss through ammonia leaching (Ercoli et al., 2013).High temperatures promote reductions in yield and biomass accumulation due to the increase in respiration rate and N loss through volatilization, contributing to the events related to global warming (Mandal et al., 2016).
In the soybean/wheat system in the unfavorable year ( 2014), regardless of N fertilization dose, the highest mean grain yield was obtained in the split application in V 3 /V 6 (Table 1).
Although the mean values of straw yield and harvest index are similar between the N supply forms, the joint expression of straw and grain through the biological yield was also favored by the split application in V 3 /V 6 , a contribution from the higher grain production under such condition.The equations obtained by the N doses in each supply condition, either linear or quadratic, indicated significant slope parameter (b i x), qualifying the effectiveness of these equations on the inferences to be formulated.Therefore, the simulation with the N dose of 60 kg ha -1 in soybean/wheat system for an expected grain yield of 3 t ha -1 led to value closer to this expected yield in the V 3 /V 6 condition.The indicated dose also promoted greater contribution to biological yield and harvest index.
In the favorable year ( 2015), the mean values of grain yield, regardless of N doses, were more expressive in the application of single N dose in the phenological stage V 3 (Table 1).It should be highlighted that the mean values of the other yield indicators qualify the N supply in single dose and split dose in V 3 /V 6 , except for straw yield, which did not show alteration.In the simulation with the N dose of 60 kg ha -1 for expected yield of 3 t ha -1 , the condition of favorable agricultural year promoted values higher than this expected yield.However, the most expressive contribution of grain yield was obtained with the N supply in single dose, which also occurred for the expression of the biological yield.
The presented results indicate that the expected grain yield based on N dose is dependent on the weather conditions of the V 3 -Full nitrogen dose (100%) at stage of third expanded leaf; V 3 /V 6 -Split nitrogen dose (70 and 30%) at stages of third and sixth expanded leaves; V 3 /R 1 -Split nitrogen dose (70 and 30%) at stages of third expanded leaf and early grain filling Source: Weather Station of the Rural Development Regional Institute A.
B. cultivation year.Additionally, there is different contribution to the yield indicators for the N supply in single and split doses by the condition of the cultivation year.In a system with reduced C/N ratio, such as the soybean/wheat succession, N supply in split dose at V 3 /V 6 in the unfavorable year, and in single dose at V 3 in the favorable year are recommended under this cultivation condition.
Nitrogen is one of the nutrients most required by the wheat crop and is mostly not supplied in ideal quantity and period to guarantee high yield and good quality of the final product (Todeschini et al., 2016).The definition of the N dose in wheat depends on organic matter content in the soil, previous crop and expected yield (Siqueira Neto et al., 2010).However, N management to obtain the desired expected yield depends on the weather conditions of the cultivation year (Arenhardt et al., 2015).Studies have demonstrated that the split N application under unfavorable condition for cultivation results in higher N use by the plant, reducing its losses to the environment with increase of yield (Brezolin et al., 2016;Ferrari et al., 2016).The use of leguminous species as source of residual N in cereals such as wheat and oat has also favored crop yield with reduction in the use of N-fertilizer (Pinnow et al., 2013;Mantai et al., 2016).Although the greater absorption of N-fertilizer in the vegetative period promotes significant increments in biomass production, the use efficiency is defined by the capacity of transformation into biomass and grains, conditioning the expression of the harvest index (Viana & Kiehl, 2010;Todeschini et al., 2016).
In the maize/wheat system in the unfavorable year (2014), regardless of the N fertilization dose, the highest mean value of grain yield was obtained in the split application at V 3 / V 6 (Table 2).The mean values of biological and straw yields were similar between the different forms of N supply, without alteration of the harvest index in the conditions V 3 and V 3 /V 6 .The equations obtained for the maize/wheat system (Table 2) through the N doses in each supply condition, either linear or quadratic, also indicated a significant slope parameter (b i x), qualifying these equations on the obtained inferences.Thus, in the simulation with the N dose of 90 kg ha -1 in maize/wheat system for expected grain yield of 3 t ha -1 , the value closest to this expected yield was obtained in the condition V 3 /V 6 , a result similar to that for the soybean/wheat system.It worth highlighting that the simulation of straw yield was not altered by the forms of N supply, a condition that favored greater expression of the harvest index in the use of split dose at V 3 / V 6 due to the higher grain production, although the biological yield is similar in the supply at V 3 and V 3 /V 6 .
In the maize/wheat system in the favorable year ( 2015), the mean values of grain yield, regardless of N doses, were more expressive with the N supply in single dose in the phenological stage V 3 (Table 2).Although there is similarity between the mean values of biological yield in V 3 and V 3 /V 6 , and without alteration of straw yield due to the forms of N supply, the greater expression of the harvest index in V 3 brings the contribution from grain yield.In the simulation with the N dose of 90 kg ha -1 for expected yield of 3 t ha -1 , the condition of favorable agricultural year also led to values higher than this expected yield, a fact also evidenced in the soybean/wheat system.The most expressive contribution of grain yield was obtained with the N supply in single dose, leading to greater expression of the harvest index in this condition, because straw Table 1.Mean and regression with the estimation of biological, grain and straw yields and harvest index according to the form of nitrogen supply in the soybean/wheat system in 2014 unfavorable year and in 2015 favorable year V 3 -Collar formed on the 3 rd leaf of the main stem; V 3 /V 6 -Collar formed on 6 th leaf of the main stem and V 3 /R 1 -Ear differentiation; BY -Biological yield; GY -Grain yield; SY -Straw yield; HI -Harvest index; UY -Unfavorable year; FY -Favorable year; P(b i x) -Parameter that measures the significance of the line; R² -Coefficient of determination; Y E -Estimated value for nitrogen fertilization dose of 60 kg ha -1 yield was not altered by the form of N supply and the biological yield was similar in the conditions V 3 and V 3 /V 6 .
The results in the maize/wheat system were similar to those of the soybean/wheat system, indicating that higher N use efficiency was obtained with the split application in the stages V 3 /V 6 in unfavorable year and single application under favorable condition for cultivation.In the different conditions analyzed, the use of split dose in the stages V 3 /R 1 did not promote benefits in N use for the elaboration of the yield indicators, a condition that, despite not leading to increase of yield, can contribute to the technological and chemical quality of the grains through the N supply in the grain filling stage.
Split N application has been recommended for promoting higher efficiency in N assimilation, especially when the weather conditions are not adequate for application (Sangoi et al., 2007;Mantai et al., 2016).For Bredemeier et al. (2013), the adequate moment for N-fertilizer application as top-dressing depends on plant phenology, determined by the period of highest demand for the nutrient, which reflects the period between the production of third and sixth leaves (Arenhardt et al., 2015).Brezolin et al. (2016), evaluating N doses and supply to the wheat crop in maize/wheat system of succession, observed effects more favorable to grain yield with the split N application in the stages V 3 /V 6 .In contrast, Sangoi et al. (2008) observed that early N application stimulates higher grain yield.Teixeira Filho et al. (2010) reported that N doses of up to 120 kg ha -1 increase biomass and grain yields in wheat, regardless of the application period and source of N.However, high doses do not guarantee yield, due to losses of the nutrient caused by unfavorable weather conditions.

Conclusion
The highest efficiency of nitrogen for wheat yield is obtained with the supply in single dose in favorable cultivation year and with the split application in the stage V 3 /V 6 in unfavorable year, regardless of the system of succession of high and reduced residual N release.

Figure 1 .
Figure 1.Rainfall and maximum temperature along the wheat crop cycle and the moments of nitrogen application in year 2014 (A) and 2015 (B)

V 3 -
Collar formed on the 3 rd leaf of the main stem; V 3 /V 6 -Collar formed on 6 th leaf of the main stem and V 3 /R 1 -Ear differentiation; BY -Biological yield; GY -Grain yield; SY -Straw yield; HI -Harvest index; UY -Unfavorable year; FY -Favorable year; P(b i x) -Parameter that measures the significance of the line; R² -Coefficient of determination; Y E -Estimated value for nitrogen fertilization dose of 60 kg ha -1

Table 2 .
Mean and regression with the estimation of biological, grain and straw yields and harvest index according to the form of nitrogen supply in the maize/wheat system in 2014 unfavorable year and in 2015 favorable year