Technical and agronomic efficiency of nitrogen use on the yield and quality of oat grains 1

: The efficiency of nitrogen use by oats in association with climatic conditions is fundamental to the development of more sustainable managements with yield and quality. The objectives of this study were to define the agronomic efficiency of nitrogen by the ratio of the dose provided and product obtained, estimate the maximum technical efficiency of the nutrient on grain yield; and for the optimum dose, simulate the expression of the straw and industry yields, protein and total fiber in different conditions of the agricultural year in a soybean/oat system. The study was conducted from 2011 to 2016, in Augusto Pestana, RS, Brazil, in a randomized block design with four repetitions in a 4 x 2 factorial referring to nitrogen doses (0, 30, 60 and 120 kg ha -1 ) and oat cultivars (Barbarasul and Brisasul) in a soybean/oat system. Nitrogen increased grain, straw, and industry yields and total grain protein, with agronomic efficiency of 7.8, 19.7 and 3.3 kg ha -1 and 0.10 g kg -1 , respectively, with reduction of the total fiber in 0.05 g kg -1 per kg of N supplied. The dose of maximum technical efficiency in the expression of grain yield is dependent on the weather conditions during cultivation. In general, the maximum efficiency of grain productivity was obtained with 86 kg ha -1 of N, with linear equations showing increased productivity of straw and industry yield, total protein, and reduction of the fiber content of oat grains by nitrogen use.


Introduction
Inadequate management of N is one of the factors mostly impacting grain yield and quality, thereby making the process unsustainable . For maximum expression of the yield and quality of oat grains, the adjustment of management technologies is recommended to improve the efficiency of nitrogen absorption and use .
Meteorological conditions directly influence nitrogen losses, either by nitrate leaching or ammonia volatilization, thereby affecting nitrogen absorption by roots and reducing nitrogen use efficiency (Mamann et al., 2020). In addition, when applied in small doses, it limits yield, but in high doses, although it maximizes yield, it also promotes the lodging of plants thereby making harvesting difficult. This is accompanied with losses in grain yield and quality, resulting in economic and environmental damage (Marolli et al., 2018).
Advances are required in the development of strategies that promote better use of nitrogen in oats, adding efficiency with less environmental impact . In this perspective, the agronomic efficiency of the input/product ratio obtained and the technical efficiency for estimating the optimal dose of the nutrient, can assist in decision making by promoting more sustainable processes of nitrogen management in oats.
The objectives of this study were to define the agronomic efficiency of nitrogen by the ratio of dose provided and product obtained, estimate the maximum technical efficiency of the nutrient on grain yield, and for the optimum dose, simulate the expression of the straw and industry yields, protein and total fiber in different conditions of the agricultural year in a soybean/oat system.

Material and Methods
The experiments were conducted in a field, from 2011 to 2016, in the county of Augusto Pestana, RS, Brazil, geographically located at 28° 26' 30'' S latitude and 54° 00' 58'' W longitude. Soil obtained from the experimental area was classified as Oxisol and the climate of the region, according to the Köppen classification, is of the Cfa type, with hot summer without dry season (Kuinchtner & Buriol, 2001;Santos et al., 2006). The area in which the experiment was installed had a consolidated no till farming system. During summer, the area was occupied by soybean, characterized as the most used crop precedent in Southern Brazil. Ten days before each sowing of oat, a soil analysis was carried out, identifying, on average, the following chemical characteristics of the site (Tedesco et al., 1995): pH = 6.3; P = 34.1 mg dm -3 ; K = 231 mg dm -3 ; OM = 3.2%; Al = 0 cmol c dm -3 ; Ca = 6.6 cmol c dm -3 and Mg = 2.9 cmol c dm -3 .
Sowing was carried out between the first and the second week of June with mechanized seeder-fertilizer. Each plot comprised five lines of 5 m in length and spacing between lines of 0.20 m, forming an experimental unit of 5 m 2 . The population density used was 400 viable seeds per square meter. The seeds of the selected genotypes were submitted to germination and vigor tests in the laboratory, in order to correct the density of plants to constitute the desired population. During sowing, 45 and 30 kg ha -1 of P 2 O 5 and K 2 O were applied, respectively, based on the levels of P and K in the soil with the expectation of grain yield of 3 t ha -1 , and with 10 kg ha -1 of N at sowing (except in the control treatment), with the remainder of each dose applied as urea (N = 45%) topdressing, to complete the proposed doses of N-fertilizer applied in the phenological stage of the expanded fourth leaf, with the source urea. During the study, tebuconazole fungicide was applied at a rate of 0.75 L ha -1 and weeds were controlled with metsulfuron-methyl herbicide at a dose of 2.4 g ha -1 and additional hoeing when necessary.
The experimental design was a randomized block with four replicates, following a 4 x 2 factorial scheme refering to doses of N-fertilizer (0, 30, 60 and 120 kg ha -1 ) and oat cultivars (Barbarasul and Brisasul), respectively. In each year of cultivation in the soybean/oat system, two experiments were conducted, one to quantify the total biomass yield (straw + grains) and the other to estimate grain yield. The biomass yield (BY, kg ha -1 ) was obtained by cutting the three central lines of each plot close to the soil at the stage of physiological maturity. The biomass samples were sent to a forced air oven at a temperature of 65 °C, until reaching constant weight and converted to kg ha -1 . Grain yield (GY, kg ha -1 ) was obtained by cutting the three central lines of each plot at the stage of harvest maturity with grain moisture around 22%. Thereafter, the plants were tracked with a stationary harvester and grains were sent to the laboratory to correct the moisture to 13%. Thus, the straw yield (SY, kg ha -1 ) was obtained by subtracting the grain yield from the biomass yield. The number of grains larger than 2 mm (NG > 2 mm, n) was obtained by counting 100 grains from the sample in each plot, which were placed in a 2 mm mesh sieve and those above this dimension were counted. The husking index (HI, g g -1 ) was determined by the ratio between the cariopsis mass of 50 grains larger than 2 mm and its grain weight. Industrial yield (INY, kg ha -1 ) was obtained by the product of grain yield with the number of grains larger than 2 mm and the husking index (INY = GY x NG > 2 mm x HI). The determination of total protein (TP, g kg -1 ) and total fiber (TF, g kg -1 ) was conducted using near infrared spectrophotometry (NIRs) on a sample of unhulled grains. The device used was of the Perten brand, model Diode Array DA7200. The air temperature (ºC) and rainfall (mm) information for analysis of the meteorological conditions of the agricultural years were obtained by the Automatic Station installed 500 m from the experiment. It is worth mentioning that meteorological conditions, together with grain yield, were used to classify the agricultural years as favorable, intermediate and unfavorable for the cultivation of oats. Data were subjected to analysis of variance to detect the main and interaction effects (not shown) and linear regression analysis, in the fitting of equations to estimate the agronomic efficiency of oats by the kilogram of nitrogen supplied per kilogram of product obtained. The grain yield data were also subjected to quadratic regression analysis, in the formulation of equations to estimate the maximum technical efficiency of nitrogen use by oats. Optimal nutrient doses were used to simulate straw and industry yields, protein and total fiber of the grains. Statistical analyses were performed with the aid of the GENES software.

Results and Discussion
Based on the temperature, rainfall and mean yield of oat grains presented in Table 1, the years 2011 and 2013 were favorable (FY) to oat cultivation. The year 2011 was marked by well-distributed rainfall during the growing cycle, with volumes similar to the historical average of the last 25 years . Rainfall precipitations were observed in the moments preceding the application of nitrogen, providing adequate soil moisture for urea solubilization, as shown in Figure 1A. The maximum, minimum and average temperatures were stable throughout the growing cycle. In 2013, rainfall distribution occurred regularly between the months of growing, with volume below the historical average. At the time of fertilization, the soil moisture from rainfall, in previous days, may have favored greater nutrient use by the plant. In addition, temperatures were milder, reducing possible nitrogen losses through volatilization ( Figure 1C).
The years 2012 and 2014 showed grain yield much lower than the desired expectation of 3 t ha -1 , justifying their classification as unfavorable agricultural years (UY) for oat growing. In 2012 ( Figure 1B), there was water restriction at the beginning of development, however rainfall increased some days prior to fertilization and temperatures reached close to zero degrees during nutrient management (Table 1). At the end of the growing season, rainfall was frequent with a high accumulated value, promoting days of lower radiation quality and delaying grain harvesting. In 2014 ( Figure 1D), the first days of the growing season were marked by above average rainfall and high air temperatures. These conditions affected the efficiency of photosynthesis and the consequent shoot formation and root growth.
The growing conditions showed the grain yield obtained in the years 2015 and 2016, as intermediates (IY) for oat cultivation. In 2015 (Table 1), the accumulated rainfall was close to the historical average. The rains before fertilization guaranteed soil moisture for nitrogen management, however, a long period of water restriction after fertilization, possibly affected the efficiency of nutrient use in yield components preparation. High temperatures during anthesis may impair the development of the reproductive system ( Figure 1E). In the year 2016, milder temperatures and stability were recorded throughout the growing season, but with reduced rainfall in the grain filling period, and significant rainfall in the final phase of the season, when the grain yield was defined. This occurred with the possibility of losses on grain quality ( Figure 1F). White oats are a highly adaptable species; however, the occurrence of leaf diseases and meteorological restrictions are limiting the expression of the maximum crop yield . Among the meteorological factors, temperature and rainfall maximized the yield and quality of oat grains the most, as the environmental conditions improved (Klink et al., 2014;Trautmann et al., 2020). In oats, a favorable environment is characterized by rainfall with small volumes Rev. Bras. Eng. Agríc. Ambiental, v.25, n.8, p.529-537, 2021. adequately distributed during the growing season and with mild temperatures from the vegetative phase until grain filling (Souza et al., 2013;.

Min
The analysis of variance results showed significant effects in the variables analyzed both for the main effects years and nitrogen doses and for the interaction, configuring the need to analyze the efficiency of nitrogen use in each year of cultivation (data not shown). Table 2 presents analysis of the agronomic efficiency of the kilogram ratio of nitrogen supplied per kilogram of product obtained. In these conditions, grain yield showed efficiency range between 7.0 and 9.8 kg ha -1 of grains per kg of N among the agricultural year conditions, with average trend of 7.8 kg ha -1 . In general, grain yield showed a reduced variation in efficiency due to nitrogen use; however the linear coefficient was significant in indicating the starting point for nitrogen use. This resulted in the classification of 2011 and 2013 as favorable years for oat production.
In the expression of straw yield (Table 2), agronomic efficiency showed greater amplitude, ranging from 8.6 to 28.8 kg ha -1 . The most significant values of this efficiency for nitrogen to straw yield were recorded in the intermediate years. Regardless of the agricultural condition, each kilogram of nitrogen supplied produced a return of 19.7 kg ha -1 of straw yield. In general, in the expression of straw yield, the most expressive intercepts were observed in the intermediate and favorable years to oat cultivation.
In the analysis of industry yield (Table 2), the agronomic efficiency of nitrogen use did not show any relationship with the agricultural year, showing for example, that the favorable year of 2013 and the unfavorable year of 2012, showed similar agronomic efficiency of 5 kg ha -1 of industrial yield per kg of N supplied. Although the year 2016 recorded high agronomic efficiency on grain and straw yield, it showed the lowest efficiency on industry yield. A fact that highlights the importance of the individualized analysis of the variables that make up the estimate of industrial grain yield, such as the husking index and the number of grains greater than 2 mm.
In the expression of total protein (Table 2), the most expressive values of agronomic efficiency were obtained in favorable (2011), unfavorable (2012) and intermediate (2016) years of growing, indicating no relationship with the year of cultivation. In the general equation, regardless of the condition of the agricultural year, the observed agronomic efficiency was 0.10 g of protein per kilogram of grain for each kilogram of nitrogen supplied per hectare, with an initial concentration of FY -Favorable year; UY -Unfavorable year; IY -Intermediate year; R² -Coefficient of determination; P (bix) -Probability of the slope parameter of the line; * -Significant at p ≤ 0.05, by t test; ns -Not significant at p ≤ 0.05, by t test; Averages followed by the same lowercase letters in the column and uppercase letters in the row, constitute a statistically homogeneous group using the Skott-Knott model at p ≤ 0.05 Table 2. Equations of agronomic efficiency and average values of yield and industrial and nutritional quality of oat grains in different years of cultivation 100.9 g kg -1 . However, the increase in nitrogen dose resulted in a decrease in total fiber, indicating an average reduction of around 0.05 g of fiber per 100 kg of grains for each kilogram of the nutrient added per hectare in the oat crop, regardless of the growing year condition. Thus, higher fiber contents were found under more restrictive nitrogen use, especially in years unfavorable to cereal cultivation.
Evaluating the agronomic efficiency of urea,  observed the existence of genetic differences in the expression of grain yield, where the greatest efficiency was obtained with the cultivar URS Taura with 4.68 kg of grains produced per kg of N supplied. Although more efficient, it produced the lowest grain yield, starting from a lower linear coefficient when compared to cultivars with lower angular coefficients. Agronomic efficiency is obtained by the slope of the linear equation, indicating the relationship with which the variable of interest answers per unit of nitrogen (Moll et al., 1982). However, the linear coefficient of the equation must also be considered, as it determines the nutrient performance starting point .
The differences between results of agronomic efficiency have been attributed to the predecessor crop, cultivar used and meteorological conditions (Prando et al., 2013). Some authors (Martinez et al., 2010;Hawerroth et al., 2013) have reported that the protein and fiber content of oat grains are also influenced by nitrogen availability and cultivation conditions with high temperature and reduced air humidity during maturation of grains (Monteiro, 2009). Contradictory results of this research were obtained by Zakirullah et al. (2017), indicating gradual increments in the percentage of crude fiber in oats with an increase in the nitrogen level. Figure 2 shows the estimates of the maximum technical efficiency of nitrogen use for grain yield. In this perspective, the favorable year (2011) indicated maximum technical efficiency similar to the unfavorable year (2012), with 82 and 86 kg ha -1 of N, respectively. However, the 2011 simulation shows grain yield of 4200 kg ha -1 , compared to 2012 with simulation for 2841 kg ha -1 . Although the nitrogen dose is similar, the product efficiency was very expressive, indicating the importance of environmental relationships and the greater In addition to the need for greater nitrogen use in 2014 for maximum yield, the result obtained was much lower compared to 2013, with the lowest dose of the nutrient (Figure 2). It is noteworthy that the intermediate years of 2015 and 2016 showed similarity of the optimal dose of nitrogen use with similar values of maximum yield. The results presented suggest that the use of optimum doses for the expression of yield, takes into account the environmental conditions at the time of application of the nutrient and is based on meteorological forecasts during the cultivation cycle, in search of greater economic return and reduction of environmental impacts due to the ease of losses by volatilization or leaching under restrictive growing conditions.
The maximum technical efficiency of nitrogen use is given by the response of higher yields with less supply of the input . Using 75 kg ha -1 of N, Kolchinski & Schuch (2003) determined the maximum technical efficiency of nitrogen use for the yield of oat grains. Silva et al. (2016) observed that the maximum technical efficiency of nitrogen use is strongly dependent on environmental conditions. These authors reported a technical efficiency of nitrogen use with 86 kg ha -1 , generating an expected grain yield of 4181 kg ha -1 in a favorable year. However, under restrictive conditions, they found maximum technical efficiency in the use of nitrogen with 119 kg ha -1 , with yield of 2930 kg ha -1 of grains. The results are similar to those obtained, corroborating that the wide range of grain yield is associated with the high variability of growing conditions, with the year factor being the component of greatest influence on the variations in expectation (Storck et al., 2014).
In Table 3, the nitrogen doses indicated by the maximum technical efficiency for the expression of grain yield by agricultural year condition were used to estimate the expression of the straw and industry yields, protein and total fiber, from the equations that established the behavioral trend. Therefore, the biological interpretation of nitrogen use in these variables is sought considering the optimum dose of grain yield regardless of agricultural year. In this perspective, with 86 kg ha -1 of N there is an expectation of 7539 kg ha -1 of straw, 1412 kg ha -1 of industry yield and 109.5 and 124.9 g kg -1 of protein and total fiber, respectively.
Studies have reported positive results from the use of nitrogen for straw yield; however, high doses of fertilization result in the lodging of plants, causing significant yield losses (Zakirullah et al., 2017;Marolli et al., 2018). Hawerroth et al. (2013), Sunilkumar & Tareke (2016) and Lima et al. (2017) confirmed that the application of nitrogen as topdressing increases the protein concentration in oat grains. However, the nutrient absorption capacity varies between cultivars, soil fertility and meteorological and environmental factors such as humidity, temperature, photoperiod and radiation. Moreira et al. (2001) highlighted that the fiber content of oats does not change with increased nitrogen supply. Silveira et al. (2016) stated that the fiber concentration of oats is predominantly dependent on the cultivar and on the weather conditions during cultivation.

Conclusions
1. Nitrogen increases grain, straw, and industry yields and total grain protein, with agronomic efficiency of 7.8, 19.7 and 3.3 kg ha -1 and 0.10 g kg -1 , respectively, with reduction of the total fiber by 0.05 g kg -1 per kg of the nutrient supplied.
2. The dose of maximum technical efficiency in expressing grain yield is dependent on the weather conditions during the growing season. In general, the maximum efficiency of grain productivity is obtained with 86 kg ha -1 of N, with linear equations that show increase in the productivity of straw and industry yield and the total protein, and reduction of the fiber content of oat grains by use of nitrogen.