Productive potential of superior genotypes of Paspalum notatum Flügge in response to nitrogen fertilization

The Paspalum genus forms the natural pastures of the tropical and subtropical regions of America and includes several species of recognized forage potential. The aim of this study was to evaluate the influence of different levels of nitrogen fertilization on the agronomic traits and nitrogen use efficiency in superior genotypes of Paspalum notatum Flügge. Four nitrogen fertilization levels (0, 60, 180 and 360 kg N ha -1 ) and nine genotypes were evaluated, six from the United States Department of Agriculture (USDA) (30N, 36N, 48N, 70N, 83N and 95N), collected in South America, two native genotypes of Rio Grande do Brazil Rocha and Bagual) and cv. Pensacola. The experimental design was a randomized complete block in a 4 x 9 factorial, with four field replications. From the variability of the responses found for the agronomic traits, the genotypes 48N, André da Rocha and Bagual are indicated to continue in the breeding program because they reach the maximum productive efficiencies according to the doses of N tested. The genotype 36N obtained the highest efficiency of nitrogen utilization, being biologically viable with the lowest N dose, equivalent of 60 kg of N ha -1 .


INTRODUCTION
Species of the genus Paspalum are the main constituents of natural pastures in the tropical and subtropical regions of South America (Sartor et al., 2011). In southern Brazil, these species predominate in the physiognomy and are critical for livestock production in the natural fields of the Pampa Biome. Studies have demonstrated the forage potential of different Paspalum species (Pereira et al., 2012;Motta et al., 2017;Steiner et al., 2017). Among Paspalum species, P. notatum Flüggeis salient, with native genotypes of recognized forage potential, which surpasses that of the commercial cultivar Pensacola predominantly used in the southern region of the country (Fachinetto et al., 2012, Weiler et al., 2018.Knowledge of the productive potential of promising genotypes released as new cultivars coupled with the appropriate use of input technologies can be used to intensify livestock production systems. In this sense, the use of nitrogen (N) is one of the main factors responsible for increased biomass production in natural ecosystems and is an important agronomic tool to improve the performance of warm season grasses. In natural pasture areas of the Pampa Biome, where P. notatum is one of the predominant species, a positive biological and economic response was observed with application of up to 200 kg N ha -1 (Santos et al., 2008). On the other hand, the high cost of N fertilization has hindered efforts to increase production on small farms, especially in marginal cultivation regions (Do Vale et al., 2012). In developed countries, where larger amounts of N are applied to soils, environmental problems occur owing to its high pollutant potential (Ahlgren et al., 2008). Among the future challenges of ruminant production is reduction in the use of non-renewable resources through the development of more productive and sustainable livestock. In order to achieve this goal, it is essential to develop forage cultivars adapted to various soil and climate conditions, with higher biomass production and efficient use of fertilizers. Therefore, the evaluation of different P. notatum Flügge genotypes in response to different N fertilization levels should be performed within a forage breeding program. Obtaining more productive genotypes with more efficient Nuse will enable more profitable and sustainable livestock production. The aim of this study was to evaluate the influence of different levels of N fertilization on agronomic traits and N use efficiency of superior P. notatum Flügge genotypes.

MATERIAL AND METHODS
The experiment was conducted at the Agronomic Experimental Station belonging to the Federal University of Rio Grande do Sul. The climate is humid subtropical (Cfa), according to the Köppen classification (Moreno, 1961).During the experimental period, precipitation was 649.8 mm (accumulated in 5 months) in the first productive cycle and 1429.5 mm (accumulated in 11 months) in the second cycle, and the average temperature was 19.4 °C and 22.4 °C, respectively. The soil is classified as Typical Dystrophic Red Argisol (Embrapa, 2013).According to the Soil Chemistry and Fertility Commission (CQFS) -RS / SC (2004), soil samples for forage cultivation were collected at 0-10 cm depth and had the following chemical: characteristics: clay = 15%; pH (H 2 O) = 5.4; SMP index = 6.3; P (mg dm -3 ) = 15.6; K (mg dm -3 ) = 151.4; M.O. = 2.7%.The area was fertilized at the time of seeding with phosphorus and potassium, according to the technical indications for warm season perennial grasses, following the recommendations of CQFS (2004). In the fall of 2011, clones of the genotypes were generated and kept in 1L pots with commercial substrate in a greenhouse. Subsequently, the seedlings were transplanted to the field on 10/12/2011 as plots formed by five rows of 1.5 m in length, spaced 0.15 m apart, totaling 50 plants per plot. The experimental design was a randomized block design in a 4 × 9 factorial scheme, with four field replications. The N fertilization treatments included four levels equivalent to 0, 60, 180 and 360 kg N ha -1 , and nine P. notatum genotypes (30N, 36N, 48N Viamão/RS -Brazil Diploid N was applied as urea and fractionated in three applications per production cycle. In the first production cycle, the applications were performed on 02/24, 03/16, and 04/26/2012.In the second cycle, applications occurred on 10/24, 11/12, and 12/11/2012.All genotypes were selected in the preliminary evaluation of forage production (Fachinetto et al., 2012). The evaluation was performed by cuts, using two squares of0.25 m 2 per plot.
The cuts were performed whenever the majority of genotypes reached 20 cm of canopy height, such that the post-cut residue height was 5 cm. Seven cuts were made during the evaluation period (03/16, 04/26, 11/12, and 12/11/2012; 01/22,02/20, and 03/18/2013).After the cuts, the samples were transported to the laboratory for morphological separation of leaf blades, stems, and stems and inflorescences. Subsequently, the samples were placed in a forced air oven at 65 °C until a constant mass was achieved. The measured variables included accumulated total dry mass production (TDMP, kg ha -1 ), accumulated leaf blade dry mass production (LBDMP, kg ha -1 ), and accumulated stem dry mass production (SDMP, kg ha -1 ). The leaf: stem ratio (LSR) was calculated by the ratio between LBDMP and SDMP. Nitrogen use efficiency (NUE, kg of MS kg -1 of N applied) was calculated according to the following equation: NUE = (TDMP in N fertilized plot -TDMP in unfertilized plot) / (applied N level) (Marriott;Haystead, 1993;Silveira et al., 2013). Data were subjected to analysis of variance and the F test at 5% probability. When differences between treatments were detected, comparison of means was performed by the Scott-Knott test at 5% probability. In addition, the variables were subjected to Pearson correlation analysis. For N fertilization levels, regression analysis was performed. Data were evaluated using the GENES statistical package (Cruz, 2007).

RESULTS AND DISCUSSION
The Paspalum genotypes and N levels were correlated (p <0.05) for the TDMP, LBDMP, LSR, and NUE variables. The TDMP and LBDMP parameters fit linear and quadratic regression models (Figure 1). For TDMP, genotypes 30N, 36N, 70N, 83N, and 95N fit the linear regression model. The results indicated that these P. notatum genotypes responded positively up to the maximum N level tested (360 Kg N ha -1 ) and demonstrated that the soil N supply did not meet the needs of the genotypes. For each kg of N applied, there was a conversion of 11.6, 7.8, 23.4, 12.0, and 16.2 kg TDMP for genotypes 30N, 36N, 70N, 83N, and 95N, respectively. Although the maximum TDMP was not reached, the 70N genotype exhibited the best conversion to TDMP with the tested N levels. Total accumulated dry mass production and leaf blades of P. notatum genotypes treated with different levels of nitrogen fertilization.
Genotypes 48N, André da Rocha, Bagual, and cultivar Pensacola fit the TDMP quadratic regression model (Figure 1), reaching the maxima with the N levels tested. The highest and lowest TDMP corresponded to the Bagual (20.349 kg DM ha -1 ) and 48N (19.347 kg DM ha -1 ) genotypes when 320 and 350 kg N ha -1 were applied, respectively.
In this context, the André da Rocha genotype, which had a TDMP of 19.829 kg DM ha -1 using 250 kg N ha -1 , was remarkable. On the other hand, Pensacola only reached the maximum TDMP (16.163 kg DM ha -1 ) when 330 kg N ha -1 was applied, and this TDMP was lower than the maximum achieved by genotypes 48N, André da Rocha, and Bagual (Table 2). Pensacola, respectively. The highest nutritional quality of forage plants is in the leaf blades, which is the structure preferred by grazing animals (Bratti et al., 2009).Thus, the highest proportion of leaf blades is one of the main criterion for genotype selection in forage breeding programs. There was a positive correlation between TDMP and LBDMP (r = 0.99; p <0.0001), confirming the positive increments obtained between the variables. The high correlation between the variables described above was also observed in Brachiaria ruziziensis S. genotypes (Borges et al., 2011) and P. notatum genotypes and hybrids (Weiler et al., 2018).This result is particularly important, because selecting genotypes for high TDMP will result in selection of genotypes with high LBDMP. Pereira et al. (2012), who evaluated genetic variability in the genus Paspalum, emphasized that TDMP and LBDMP were the most effective traits for identifying genotypes with higher forage traits. Thus, the selection of genotypes with higher TDMP remains, in general, the main focus in forage breeding programs. Moreover, the high correlation between TDMP and LBDMP suggested that the laborious task of separating morphological components, at least in this species, could be avoided, saving time and resources. The applied N levels had the lowest influence on the LSR variable. Most genotypes did not show increased LSR with increasing N fertilization levels, except for genotypes 83N and André da Rocha, which presented responses to the level equivalent to 60 kg N ha -1 (Table  3).Selection for reduced stem production and increased leaf production has been strongly advocated for forage breeding (Pereira et al., 2011). In the present study, notably, the LSR of cultivar Pensacola remained at 0.9 without N fertilization. A high LSR is desired in forage plants as it confers better grazing adaptation or tolerance to mowing because it presents a phenological moment when apical meristems are closer to the ground and therefore less vulnerable to grazing elimination (Silva et al., 2013). Table 3 Leaf: stem ratio Paspalum notatum genotypes in response to different of nitrogen fertilization For the NUE variable, the highest value observed at the level equivalent to 60 kg of N ha-1, decreasing with increasing N levels applied (Table 4). The reduction in efficiency can be explained by the reduced capacity of the plant to absorb and use the nutrient for production, as well as possible soil leaching. N is one of the most difficult nutrients to manage effectively. In many agroecological systems, a substantial portion of applied N is lost from soil to groundwater, rivers, and oceans (Glass, 2003), because plants convert only 30% to 40% of the applied N into useful products. Thus, the variability between the studied genotypes allows the selection of plants with higher genetic potential for the NUE trait. According to Seepaul et al. (2016), several factors contribute to the amount of N removed by plant biomass, including the genotype and amount of N applied.NUE has been used to describe a plant's ability to acquire and use N to produce biomass, and is expressed as the yield of biomass produced per unit of N applied (Seepaul et al., 2016).In this sense, including this information as a tool for selecting genotypes of this species in forage breeding programs is important. Indirect calculation of NUE has been widely used owing to its practicality and low cost (Silveira et al., 2013;Obour et al., 2017). The most commonly used source of N in Brazil is urea; however, research data indicate volatilization losses of up to 30% of N. Measures that increase the NUE should be implemented in order to promote the management of an economically sustainable, forage quality production system with minimal negative environmental impact. The results of this study showed that N fertilization influences the agronomic traits of different genotypes, increasing biomass production. The use of NUE as a selection criterion may help the breeder obtain plants with higher forage yields with lower N levels. This may favor more profitable and sustainable livestock farming owing to reduced spending on N fertilizer and reduced impacts on the environment, such as the use of non-renewable fossil fuel reserves, effects on global warming, and water contamination.
The results of this work indicated that the evaluation and selection of native genotypes may produce new cultivars that are more productive and use N more efficiently than currently available genotypes on the market, such as P. notatum 'Pensacola'. Thus, based on the variability observed in the evaluated agronomic traits, the 48N, André da Rocha, and Bagual genotypes were indicated to be promising for continued use in the breeding program because they reached the maximum productivity efficiencies according to the N rates tested. The 70N and 83N genotypes are also indicated for new evaluations, because they showed ability to express forage yields with higher N rates than those tested in the present study, which may indicate their applicability in intensive livestock systems. The 36N genotype had the highest N use efficiency, proving to be biologically viable with the equivalent of 60 kg N ha -1 . These genotypes could be introduced in rural areas to recover degraded natural pastures or directly as cultivated pastures, with higher forage production due to positive responses to N application.
In addition, this introduction could reduce the use of exotic species and preserve the natural ecosystem, as P. notatumis native to this environment and better studied; thus, more information on P. notatum management practices is available.