SUSTAINABLE MANAGEMENT FOR OAT GRAIN YIELD AND QUALITY BASED ON NITROGEN SUPPLY AND SEEDING DENSITY

Purpose: The objective of this study is to evaluate a sustainable management for oat crops with high nitrogen use efficiency combined with high seeding density to improve grain yield and quality while increasing oat competitive ability against ryegrass. Method/design/approach: The experiment was carried out in the years 2020, 2021 and 2022, with an experimental design of randomized blocks with four replications, in a 4 x 3 factorial arrangement, for sowing densities (100, 300, 600 and 900 viable seeds m -2 ) and form of nutrient supply: [single dose, (100%) at phenological stage V4 (fourth expanded leaf); fractional (70/30%) at the V4/V6 phenological stage (fourth and sixth expanded leaves) and; fractionated (70/30%) at phenological stage V4/R1 (beginning of anthesis)], respectively. Indicators of productivity and chemical quality of oat grains were analyzed, and lodging and ryegrass inflorescences exposed on oat plants were analyzed in the field. Results and conclusion: The application of a single dose of nitrogen at the V4 stage resulted in higher grain and oat straw yields, without changes in the crude protein and crude fiber content of the grains. The combination


INTRODUCTION
White oat has been an important alternative agricultural crop due to the importance and multiple uses of this species (Scremin et al., 2020;Kraisig et al., 2020).In Brazil, it is an excellent alternative for producing forage and grains during winter, as well as for soil cover in crop rotations under no-tillage systems, especially in the South region of the country (Dornelles et al., 2018;Silva et al., 2020).Oats has becoming important in human nutrition, as they can be used in several forms in the food industry, including flakes, flours, and biscuits (Arenhardt et al., 2017;Silveira et al., 2020).Oat is a highly nutritious cereal due to its presence of proteins, carbohydrates, lipids, and dietary fibers such as β-glucan.Fiber has an important function in reducing LDL cholesterol and, consequently, preventing diseases such as diabetes, obesity, and cancer (Malanchen et al., 2019;Bouchard et al., 2022).Therefore, oat consumption is connected to healthy and balanced diets (Scremin et al., 2017;Aparicio-García et al., 2021).
Grain yield and quality in oat crops is directly affected by genetic properties of the cultivars, weather conditions, and crop management practices (Kraisig et al., 2020;Pereira et al., 2023).Maintaining soil quality through application of nitrogen fertilizers and use of seeding density strategies are essential management technologies for crop development (Romitti et al., 2016;Mantai, et al., 2021).The maximum nitrogen absorption by plants is highly affected by weather conditions (Silva et al., 2016;Mantai et al., 2021).Nitrogen application on inappropriate times can decrease the nutrient use efficiency, and excessive rates can cause plant lodging, reducing product quality.In this context, nitrogen fertilizer losses due to excess or application under inadequate conditions result in environmental damage by air, soil, and water contamination, and increase in crop production costs (KraisiG et al., 2021;Trautmann, 2021).Challenges in managing nitrogen fertilizer applications have required a thorough evaluation to determine whether the highest nutrient use efficiency is achieved through single application or split rates during crop stages with high nutrient demand (Reginatto et al, 2021;Trautmann et al., 2022).
Another factor that affects nitrogen use efficiency and oat grain yield is competition with weeds, which can be attributed to improper seeding density managements (Lamego et al., 2013;Romiti et al., 2016).The Brazilian Oat Research Commission (Indicação Técnica Aveia, 2021) has recommended densities between 200 and 300 seeds m -2 since the 1990s.However, current oat cultivars have undergone significant changes in architecture, such as low plant size, and shorter growth cycles, requiring adjustments in planting density to improve crop yield (Silva et al., 2020;Bazzo et al., 2021).These factors also contribute to the emergence of weeds that hinder the agronomic performance of oat plants, including ryegrass, the most difficult weed to control in oat crops due to its potential to develop resistance to herbicides, which results in excessive use of pesticides and, consequently, significant environmental impacts and high production costs (Lamego et al., 2013;Pagnoncelli, Júnior et al., 2020).The search for more efficient agricultural systems has shown that increasing seeding density can promote rapid soil coverage, improve the use of light and nutrients by plants, and provide more effective control of weeds (Krüger et al., 2011;Rosa et al., 2023).
However, the use of high seeding densities for benefiting from years with favorable rainfall conditions for the crop, combined with application of soil nitrogen fertilizers, may cause plant lodging (Silva et al., 2012;Hawerroth et al., 2015).Lodging is a complex phenomenon that causes the plant to change its vertical position by leaning and falling to the ground, affecting grain yield and quality, and making harvesting difficult (Berry, et al., 2003;Krysczun et al., 2017).
Studies focused on increasing oat seeding density combined with nitrogen supply methods are essential for developing more efficient and sustainable crop managements.Therefore, the objective of this study is to evaluate a sustainable management for oat crops with high nitrogen use efficiency combined with high seeding density to improve grain yield and quality while increasing oat competitive ability against ryegrass.
The experiment was conducted in a soybean-oat system, using a randomized block design with four replications, in a 4 x 3 factorial arrangement.The factors consisted of four seeding densities (100, 300, 600, and 900 viable seeds m -2 ) and three methods of nutrient supply: single rate, with 100% at the V4 phenological stage (four expanded leaves); split application, with 70% at the V4 and 30% at the V6 (six expanded leaves) phenological stages; and split application, with 70% at the V4 and 30% at the R1 (beginning of anthesis) phenological stages.The total nitrogen rate was 70 kg ha -1 , for an expected grain yield of 4,000 kg ha -1 .
The seeds were sown in the first half of June, using a seed-fertilizer drill, in plots consisting of five 5-meter rows spaced 0.20 m apart, forming an experimental unit of 5 m².The oat cultivar used in the experiment was URS Guará, which belongs to the Federal University of Rio Grande do Sul (UFRGS); it is characterized by early cycle and high yield potential, grain quality, and yield stability.Fertilizer application at planting consisted of 45 and 30 kg ha -1 of P2O5 and K2O, respectively, based on soil P and K contents and the expected grain yield of 4,000 kg ha -1 .The fungicide tebuconazole were applied throughout the experimental period at a rate of 0.75 L ha -1 .
Grain yield (GY) was estimated by mechanically harvesting the three central rows of each plot when the grains reached a moisture content of approximately 15%.The grains were taken to the laboratory for moisture correction to 13%, cleaned to remove impurities, and weighed; the results were converted into kg ha-1.Crude protein (g kg -1 ) and crude fiber (g kg - 1 ) contents were determined in husked-grain samples using a near-infrared reflectance spectrometer (NIRS) (Perten Diode Array DA7200).Straw yield was obtained by cutting the plant material close to the ground in one linear meter of three rows of each plot, at the crop physiological maturity (approximately 120 days after emergence).The samples were dried in a forced air-circulation oven at 65 °C until constant weight for estimating the dry biomass yield (BY; kg ha -1 ).Straw yield (SY; kg ha -1 ) was calculated by using the equation SY = BY-GY.Lodging and exposed ryegrass inflorescences in oat plants were also analyzed in the field.Lodging (%) was visually estimated by considering the angle formed in the plant stem's vertical position relative to the ground and the area of lodged plants, according to the methodology suggested by Moes & Stobbe (1991).The number of ryegrass inflorescences was obtained by visual counting of exposed ryegrass inflorescences in the experimental unit.
Meteorological data, including air temperature and rainfall, were collected by an automatic weather station of the Regional Institute for Rural Development (IRDeR) of the Regional University of the Northwest of the State of Rio Grande do Sul (UNIJUI), installed approximately 200 meters from the experimental area.
Data that met the assumptions of homogeneity and normality by Bartlet's tests were subjected to analysis of variance to detect interaction between the studied factors.The means were compared by the Scott & Knott test and polynomial regression test at 5% probability level of error.The statistical analyses were carried out using the software GENES (Cruz, 2013).

RESULTS AND DISCUSSIONS
Rainfall was well-distributed throughout the oat crop cycle in 2020, similar to the 25year historical average for the region, with higher rainfall depths during the vegetative cycle, up to 56 days after emergence (Table 1; Figure 1).Nitrogen applications at the V4 (four expanded leaves) and V6 (six expanded leaves) phenological stages were carried under adequate soil moisture conditions due to prior rainfall.However, during the split nitrogen application at the beginning of grain filling stage (R1), conditions were restrictive for application due to a prolonged period with insufficient rainfall, although air temperature conditions were adequate for the crop, with small temperature variations during the cycle.The mean grain yield obtained was 3,083 kg ha -1 (Table 1), classifying 2020 as an intermediate year (IY) for oat crops, considering the expected grain yield of 4,000 kg ha -1 .
The rainfall depths in 2021 were below the historical average, but with adequate distribution throughout the crop cycle (Table 1; Figure 1).Nitrogen applications at different phenological stages (V4, V6, and R1) were carried out under suitable soil moisture conditions and milder temperatures, which are basic requirements for efficient nutrient absorption by the plant and the photosynthesis process.The mean grain yield was approximately 4,000 kg ha -1 , corresponding to the expected yield, classifying 2021 as a favorable year (FY) for oat crops.In 2022, satisfactory rainfall did not occur from the beginning of crop development to 50 days after emergence (Table 1; Figure 1), resulting in significant water restrictions for vegetative development, hindering crop emergence and production of tillers, which are structures directly linked to grain yield.Therefore, nitrogen application at the V4 phenological stage was conducted under insufficient soil moisture, reducing nutrient absorption efficiency.The other nitrogen managements, consisted of split applications at the V6 and R1 (early grain filling stage) phenological stages were conducted under more favorable soil moisture conditions; however, significant rainfall occurred after the applications, contributing to nutrient leaching.The total rainfall depth during the 2022 crop cycle was 1,173 mm (Table 1), which was higher than the 25-year average.Significant rainfall depths coincided with the grain filling stage, which may have hindered the allocation of photoassimilates to grains.The low grain yield (2,291 kg ha -1 ) found compared to previous years and the expected yield (4,000 kg ha -1 ) reinforces the classification of 2022 as an unfavorable year (UY) for oat crops.
The efficiency of applying urea, the most used nitrogen source, depends on favorable weather conditions, mainly adequate rainfall distribution and milder temperatures (Arenhardt et al., 2015;Costa et al., 2017).Milder air temperatures and adequate solar radiation favor tillering and grain filling.However, the occurrence of temperatures below 2 or 3 °C near the flowering stage can cause leaf and stem damage and flower sterility (Marolli et al., 2017;Mantai et al., 2021).Furthermore, low temperatures combined with frost during grain formation can stop the grain growth, resulting in grains with low industrial quality (Macedo- Cruz et al., 2011;Mantai et al., 2016).The occurrence of mild temperatures during the vegetative stage is a decisive factor for increasing the number of grains per panicle (Castro, Costa, Ferrari Neto, 2012;Scremin et al., 2020).Thus, a favorable climate for growing oats should have milder temperatures and high solar radiation, without high intensity and volumes of rainfall, which should be well-distributed to maintain an adequate soil moisture (Silva et al., 2015;Trautmann et al., 2021).
The analysis of variance showed significant effects of the main sources of variation (nitrogen application and seeding density) on grain yield, straw yield, lodging, crude protein and crude fiber contents, and ryegrass inflorescences (Table 2).Significant effects of the interaction between the factors on these variables was also found in all evaluated agricultural years.The mean comparison test for 2020 showed that the application of a single nitrogen rate at the V4 phenological stage resulted in higher mean grain and straw yields and lower mean lodging of oat plants and ryegrass inflorescences (Table 3). of a single nitrogen rate (100%) at the V4 (four expanded leaves) phenological stage; V4/V6 = split nitrogen application (70%-30%) at the V4 and V6 (six expanded leaves) phenological stages; V4/R1 = split nitrogen application (70%-30%) at V4 and R1 (beginning of grain filling) phenological stages; GY = grain yield; SY = straw yield; LD = lodging; CP = crude protein content; CF= crude fiber content; RI = ryegrass inflorescence; IY = intermediate year, FY = favorable year, and UY = unfavorable year for oat crops.Source: Prepared by the authors (2023).
The nitrogen management with application of a single rate at V4 and split application at V4 and V6 phenological stages in 2021 had similar effects on oat grain and straw yields, which had the highest means (Table 3).The lowest mean lodging and number of exposed ryegrass inflorescences found in 2021 support the management of supplying nitrogen at a single rate at the V4 stage.Grain and straw yields in 2022 presented similar results, with higher means for application of a single nitrogen rate at V4 and split nitrogen application at the V4 and V6 phenological stages (Table 3).These nitrogen supply methods had low contributions to lodging; however, the exposure of ryegrass inflorescences under split nitrogen application resulted in a lower competitive ability of oat plants.Crude protein and crude fiber contents were not affected by nitrogen applications, regardless of seeding density and agricultural year.
Oat plant lodging in 2020 was lower at the lowest seeding density (100 viable seeds m - 2 ), regardless of the nitrogen application method.The lowest number of exposed ryegrass inflorescences was found for the highest seeding densities, mainly 600 and 900 viable seeds m - 2 .
The highest oat grain yields in 2021 were found at densities of 300, 600, and 900 viable seeds m -2 and applications of a single N rate at V4 and split N application at V4 and V6 stages, which did not differ from each other (Table 4).Lodging was also lower at the lower seeding density (100 viable seeds m -2 ), regardless of the nitrogen application method.In 2022, the highest mean grain yields were obtained at densities of 600 and 900 viable seeds m -2 , regardless of the nitrogen application method.Lodging reduced when compared to the previous agricultural year (Table 4).Romitti et al. (2016) evaluated oat cultivars and emphasized the need for further investigations for appropriate seeding density recommendations, considering the evolution of cultivars towards earlier cycles and shorter plant heights.They found an ideal seeding density of approximately 500 seeds m -2 , regardless of the year and cultivar evaluated, with improved biomass production and grain yield; these results are similar to those of the present study.They also reported that the use of this ideal seeding density in a soybean-oat rotation system may have favored plant lodging, whereas it was efficient in improving oat grain yield with a significant reduction in lodging when used in a corn-oat system, despite a lower residual N release, even when using higher seeding densities.Abreu et al. (2005) evaluated late-cycle oat genotypes and found a linear increase in biomass and grain production as the plant population was increased from 100 to 400 plants m -2 .According to Silva et al. (2015), the use of a seeding density higher than the technical recommendations for oat crops can increase grain yield, if it does not cause lodging.Additionally, they highlighted its benefits for oat crop management due to greater vegetation cover for weed control and maintenance of soil moisture.
The regression analysis showed quadratic functions when estimating the seeding density for maximum grain yield, regardless of the nitrogen application method (Table 5).Overall, the adjusted seeding density was approximately 490 viable seeds m -2 for an optimal nitrogen supply condition (application of a single rate at V4 stage), regardless of the agricultural year evaluated.Considering the split nitrogen applications at V4 and V6 and at V4 and R1, the adjusted seeding density increased to 518 and 545 viable seeds m -2 , respectively, denoting the need for more seeds to compensate for the reduced tiller production due to the lower fertilizer supply at the V4 stage by split application.Silva et al. (2012) evaluated two oat cultivars (Taura and Brisasul) in two rotation systems (soybean-oat and corn-oat) and found an adjusted density of approximately 550 viable seeds m -2 for a maximum grain yield of 3,353.5 kg ha -1 .Silveira et al., (2010) obtained a significant increase in grain yield when using 500 to 600 viable seeds m - 2 for wheat crops.These seeding densities for these species are well above those currently recommended.
The regression functions for straw yield, lodging, and ryegrass inflorescences were linear (Table 6).The simulation of these variables was based on seeding densities adjusted for maximum grain yield.In the split nitrogen application at the V4 and R1 stages, increases in seed density per square meter was necessary, reaching a density of 545 viable seeds m -2 for a straw yield of 6,411 kg ha -1 .The highest estimated straw yield (7,343 kg ha -1 ) was found for a density of 490 viable seeds m -2 , when applying a single nitrogen rate at the V4 phenological stage.Increasing seeding density with split nitrogen applications resulted in higher lodging of oat plants.Lodging should be minimized in the field as it makes harvesting difficult and reduces the quality of oat grains (Table 6).The method of applying nitrogen at a single rate at the V4 stage resulted in a lower number of ryegrass inflorescences compared to the other nitrogen application methods.Controlling weeds in the crop field is essential, as they compete with the crop for water, nutrients, and solar radiation and increase impurities at harvesting.
Plant population is associated with the plant potential to produce fertile tillers, affecting the number of panicles produced per area (Castro et al., 2012;Silva et al., 2012).Additionally, rapid soil coverage by the canopy can favor better light and nutrient utilization, providing more effective control of weed species (Fleck et al., 2009;Lamego et al., 2013).Plant population is an important factor for promoting biomass production and grain yield (Ceccon, Grassi Filho, Bicudo, 2004;Dornelles et al., 2018).
Several cultures studies have been incorporated or defined ideal seeding densities for different crops based on changes involving genetic and environmental factors.Moreover, increasing the spacing between rows or between plants in the row is an alternative to increase plant uniformity and grain yield (Momoh & Zhou, 2001;Romiti et al, 2016).In early-cycle cultivars, for example, large plant populations have favored rapid soil coverage, reducing weed infestation, as highlighted in studies conducted with wheat species in a wide range of environments.Increasing seeding density from 100 to 200 plants m-2 can reduced weed dry matter by half and wheat grain yield loss from 23% to 17%, improving crop uniformity at the harvest (Lemerle et al., 2004).Valério et al. (2008) found that wheat genotypes with low tillering were more dependent on increasing seeding density, and those with high tillering grown under high seeding densities underwent higher competition for water, light, and nutrients, reducing grain yield and favoring lodging (Ozturk et al., 2006).

FINAL CONSIDERATIONS
The application of a single nitrogen rate at the V4 (four expanded leaves) phenological stage resulted in higher oat grain and straw yields, with no changes in the crude protein and crude fiber contents in the grains.The combination of this nitrogen application method with the seeding density of 490 seeds m -2 increased the competitive ability of oat plants, reducing the number of ryegrass inflorescences.
at 5% probability level by the F test; DF: degrees of freedom; GY: grain yield; SY: straw yield; LD: lodging; CP: crude protein content; CF: crude fiber content; RI: ryegrass inflorescence; IY: intermediate year, FY: favorable year, and UY: unfavorable year for oat crops; CV: coefficient of variation.Source: Prepared by the authors (2023).
determination; YE = estimated value of technical efficiency; IY = intermediate year, FY = favorable year, and UY = unfavorable year for oat crops.Source: Prepared by the authors (2023).

Table 1 .
Air temperature and rainfall depths over the months and oat growing years, mean grain yields, and classification of the agricultural year.rainfall depth from May to October of the last 25 years; GY = grain yield; Means followed by the same letter in the column are not significantly different from each other at 5% probability level by the Scott & Knott test; IY = intermediate year, FY = favorable year, and UY = unfavorable year for oat crops.Source: Prepared by the authors (2023).

Table 2 .
Analysis of variance for nitrogen (N) application methods and seeding densities on grain yield, grain quality, and ryegrass inflorescences.

Table 3 .
Means of oat agronomic and quality indicators and ryegrass inflorescence as a function of nitrogen application methods and seeding densities (viable seeds m -2 ).

Table 4 .
Means of oat agronomic and quality indicators and ryegrass inflorescences as a function of seeding densities (viable seeds m -2 ) and nitrogen application methods.

Table 5 .
Regression functions for estimating seeding density in each year and nitrogen application method, based on maximum grain yield (GY).

Table 6 .
Regression functions for simulating straw yield, lodging, and ryegrass inflorescence for each year and nitrogen application method, based on seeding densities adjusted for maximum grain yield.