Structural characteristics, nutritional value of Tifton 85 grass under nitrogen doses at different ages of regrowth in the semiarid

Características estruturais, valor nutricional do capim-Tifton 85 sob doses de nitrogênio em diferentes idades de rebrota no semiárido

Weudes Rodrigues Andrade¹, Eleuza Clarete Junqueira de Sales¹, João Paulo Sampaio Rigueira¹, Virgílio Mesquita Gomes¹, Leidy Darmony Rufino², Daniel Ananias de Assis Pires¹, Marielly Maria de Almeida Moura¹, Gabriel Santos Souza David¹, Maria Catiane Araújo Silva¹, Mariana Oliveira de Jesus¹, Alisson Junior Moura Alves Barroso¹, Julieta Maria de Alencar Chamoné¹ and Flávio Pinto Monção^1,¥,*

¹Programa de Pós-graduação em Zootecnia, Universidade Estadual de Montes Claros, Unimontes, Av. Reinaldo Viana, 2630 – Bico da Pedra, 39440-000, Janaúba – MG, Brasil

²EPAMIG, Norte, Montes Claros, Minas Gerais, Brasil

(*E-mail: moncaomoncao@yahoo.com.br)

ABSTRACT

The objective of this study was to evaluate the structural, productive and nutritional value of Tifton 85 grass fertilized with two nitrogen doses at four regrowth ages. The experimental design was a randomized block in a factorial 4 x 2, with four regrowth ages (28, 35, 42 and 49 days) and two levels of nitrogen (100 and 300 kg ha^-1) in six replicates, totaling 48 experimental units, of 25 m² (5x5m) each. The blocking criterion was the variation in the experimental area. Dry matter production increased linearly, on average 56.4%, as the regrowth age increased from 28 to 49 days (P<0.01). Among the N rates, there was a 17.2% increase in dry matter yield when fertilized with 300 kg / ha of nitrogen. The height of the plants (P <0.01), as well as the number of live leaves (P <0.01), number of expanded leaves (P <0.01) and the final leaf length (P <0.01) increased linearly with the advancement of cut age. Fertilization of Tifton 85 grass with 100 kg ha^-1 of N, managed every 35 days improves the dry matter production, structural characteristics and nutritional value of forage for cutting and / or grazing.

RESUMO

Objetivou-se por meio deste estudo avaliar as características estruturais, produtivas e o valor nutritivo do capim-Tifton 85 adubado com duas doses de nitrogênio em quatro idades de rebrota. Foi utilizado delineamento em blocos casualizados seguindo esquema fatorial 4 x 2, com 4 idades de rebrota (28, 35, 42 e 49 dias) e duas doses de nitrogênio (100 e 300 kg ha^-1) com seis repetições, totalizando 48 unidades experimentais de 25 m² (5x5m) cada. O critério de blocagem foi à variação na área experimental. A produção de matéria seca incrementou linearmente, em média 56.4%, à medida que avançou a idade de rebrota de 28 para 49 dias (P<0,01). Entre as doses de N, houve aumento de 17.2% na produção de matéria seca quando adubado com 300 kg ha^-1 de nitrogênio. A altura das plantas (P<0,01), assim como o número de folhas vivas (P<0,01), número de folhas expandidas (P<0,01) e o comprimento final da folha (P<0,01) aumentaram linearmente com o avanço da idade corte. A adubação do capim Tifton 85 com 100 kg ha^-1 de N, manejado a cada 35 dias melhora a produção de matéria seca, características estruturais e valor nutricional da forragem para corte e/ou pastejo.

Palavras-chave: Adubação, manejo de corte, produção de massa, altura, valor nutricional

Introduction

Beef cattle production in Brazil is highly dependent on forage-based grazing systems. Therefore, strategically well-planned grazing systems are critical to optimize beef production (Barbero et al., 2015). Therefore, the search for forages with high production potential is of fundamental importance in the optimization of the production system. Grasses of the Cynodon genus have been noted for their high mass production, especially in tropical regions (Monção et al., 2016; Paris et al., 2016).

However, in regions with semi-arid climate, studies with Cynodon are still incipient, especially when it comes to cutting management. In the semi-arid region of northern Minas Gerais, Brazil, forage production has fluctuated considerably in the last five years due to low rainfall and long dry periods. Consequently, the productive potential of forages such as Cynodon has not been reached, which has altered the pasture stocking rate. In this sense, the use of forage adduction strategies in these regions has been an alternative to maximize forage production (Sales et al., 2014; Machado et al., 2017). However, gaps still exist regarding the association of nitrogen fertilization (N) and cutting management on the structural characteristics, forage production and nutritional value.

According with Machado et al.\ (2017), among these factors, soil fertility can be manipulated through pasture fertilization. N fertilization is the most striking in terms of gains in forage production (Sales et al., 2014; Coblentz et al., 2017). Its application is of fundamental importance for the maintenance of productivity and sustainability of the pasture, being its deficiency considered an important factor to trigger the process of degradation (Sales et al., 2014; Machado et al., 2017). The main function of this nutrient is to be the constituent of proteins, besides interfering directly in the photosynthetic process (Gastal and Lemaire, 2002). Furthermore, the soil must be corrected for pH and supplied with phosphorus (P), potassium and other macro and micronutrients for the N to maximize forage production in intensive production systems (Hajighasem et al., 2016).

Based on the above, the objective of this study was to evaluate the structural, productive and nutritional value of Tifton 85 grass fertilized with two nitrogen doses at four regrowth ages.

MATERIAL AND METHODS

The municipality of Janaúba is located in the northern region of Minas Gerais, at 15º 47 'south latitude, 43º 18' west longitude and 516 m altitude. The climate of the region, according to the classification of Köppen (1948), is Aw type with summer rains and dry periods well defined in winter (Antunes, 1994). The average annual rainfall is 876 mm, with an annual average temperature of 24°C. The climate is tropical mesothermal, almost megathermic, due to altitude, subhumid and semi-arid, with irregular rains, causing long periods of drought (Figure 1).

The experiment was carried out in a flat area with Tifton 85 grass (Cynodon dactylon cv. Tifton 85), already established since 2007, on dystrophic red-yellow soil with clay texture with the following chemical characteristics: pH in water, 6.0, P (Mehlich): 6.0 mg.dm^-3; K (Mehlich): 68 mg.dm^-3; Ca²⁺ (KCl 1 mol L ^-1): 3.7 cmolc.dm^-3; Mg ²⁺ (KCl 1 mol L ^-1): 1.0 cmolc.dm^-3; Al3 + (KCl 1 mol / L) 0.1 cmolc.dm^-3 H + Al (calcium acetate 0.5 mol / L^-1): 2.6 cmolc.dm^-3 base sum: 4.9 cmolc.dm^{- 3}; cation exchange capacity: 7.4 cmolc.dm^-3 V: 65%.

The experimental design was a randomized block in a factorial 4 x 2, with four regrowth ages (28, 35, 42 and 49 days) and two levels of nitrogen (100 to 300 kg ha^-1) in six replicates, totaling 48 experimental units, of 25 m² (5 x 5m) each. The blocking criterion was the variation in the experimental area. At regrowth ages (28, 35, 42 and 49 days), the mean heights were 21, 22, 28 and 30 cm, respectively. The regrowth ages were chosen because of the high growth of grass Cynodon, which reach physiological maturity early, as shown in previous researches of Monção et al. (2014) and Oliveira et al. (2016).

On October 29, 2015, a standardization cut was carried out on Tifton 85 grass close to the soil, using costal brush cutters. Then, the plots were fertilized with 100 kg of P₂O₅ and 100 kg of K₂O in the single superphosphate and potassium chloride forms, respectively. Nitrogen fertilization was done in split-plot three times after the cut of uniformity and in the two subsequent cuts. Urea (45% N) was used as the source of N and the doses tested were calculated equivalent to the area of each plot. During the experimental period the Tifton 85 grass was irrigated once a week as recommended by Mota et al. (2010). The irrigation remained during the whole experimental period, being activated in the absence of rain.

The mean height of the tifton 85 grass was obtained by measuring the base of the plant up to the curvature of the last newly expanded leaf, measuring 10 points per plot before each cut.

The forage plant was collected in two points per plot with the aid of a metal frame (0.25 m²), and cut with a knife, at a height of 5 cm from the soil surface. The collected material was packed in a plastic bag, identified and sent to the laboratory, where it was weighed to determine the green mass per plot, after which a sample of approximately 400 grams was taken, then the material was placed in a forced air ventilation oven, with temperature of 55º for 72 hours to determine the partial dry matter. After drying the samples were milled in Willey type mills in 1 and 2 mm sieve, and stored in pots with lid properly identified.

The dry mass production per hectare (MDP) was determined by weighing all material harvested in a metal frame of (0.25m²) and corrected for the dry matter content obtained after the samples were processed.

The total number of tillers was obtained by counting all the tills present in the metal frame of (0.25 m²). From these tillers 10 units were separated per plot to determine the structural characteristics. To determine the final length of the blade, the completely expanded leaves were measured, from their insertion in the ligule to the leaf apex. The number of live leaves was determined as the fraction of total leaves that had no senescence signal, and the final stem length calculated based on the soil level up to the last expanded leaflet of each tiller.

Part of the collected tillers were separated on a blade, stem + hem and whole plant, later packed in a paper bag, weighed and fed to the forced circulation oven at 55ºC for 72 hours to evaluate the dry mass content for determination of the blade: stem ratio.

The obtained data were submitted to the analysis of variance using PROC MIXED of SAS (SAS Institute Inc., Cary, NC). When the "F" test was significant for the treatments, the regrowth ages were analyzed by means of orthogonal contrasts. As ages are not equidistant, the SAS PROC IML was used to estimate the Linear, Quadratic and Cubic order matrix. Nitrogen doses were analyzed by the T test. For all the tests the probability of 5% was used.

RESULTS AND DISCUSSION

There was interaction between doses of N x age of regrowth only for the final length of the stem (P = 0.02), being the highest average verified at age 49 days and at the dose of 300 kg ha^-1 of N. The dry matter production (DMP) linearly increased, on average 56.4%, as the regrowth age increased from 28 to 49 days (P <0.01). Among N rates, there was a 17.2% increase in DMP of Tifton 85 grass when fertilized with 300 kg / ha of N in relation to the dose of 100 kg ha^-1 (average of 3.835 kg dry matter [DM] ha^-1; Table 1).

The height of the plants (P <0.01), as well as the number of live leaves (P <0.01), number of expanded leaves (P <0.01) and the final leaf length (P <0.01) increased linearly with the advancement of cut age. The increase of 36.3% in the height of the plant is justified mainly by the limitation of the photosynthetic rate, in smaller height, resulting from the competition between the plants. In this way, the plant lengthens the stem, as verified in this study, as a strategy to expose the leaf blade to sunlight to capture solar energy. The growth speed of the stem and, consequently, of the height of the plant was increased by 23.5% with the presence of N in the dose of 300 kg ha^-1 in relation to the lower dose. With this, the number of live leaves was increased by 34.3% and expanded by 20.4% with the advancement of plant age. As the plant grows and reaches physiological maturity, changes in the cellular content and cell wall ratio occur as a strategy for plant survival and propagation (Monção et al., 2016). The increase in height is a reflection of the increase of cell wall components in detriment of the dilution of the constituents of the cellular content. In addition, the speed with which these changes occur is dependent on several factors, being this, mainly, and the N content in the soil. This is because the N stimulates the cell division causing the plant to reach the growth potential, consequently, increases in dry mass production as verified in this research.

Contrary behavior was verified on the number of tillers (P = 0.03) and leaf: stem ratio (P <0.01) that reduced linearly with the advancement of regrowth age. With the expansion of tiller growth, shading occurs in the basal area of the plant, which implies a reduction in the appearance of new tillers. Therefore, there was a decrease of 11.5% in the number of tillers. The variations in N rates did not change the number of tillers (P = 0.16), with a mean of 723 tillers / m². With the increase in plant height due to the age of regrowth, the proportion of stalk increases in relation to leaf blade, which, consequently, reduces the blade: stem ratio. In this study, this reduction was 36.5% when the regrowth age increased from 28 days to 49 days. Some studies (Monção et al., 2016; Oliveira et al., 2016) reported that the management of grasses of the genus Cynodon present a reasonable nutritional value when management blade: stem ratio of 1. This value in this research was verified when the plants were managed at 35 days of regrowth or 23.7 cm of average height. The application of N did not modify the blade: stem ratio of Tifton 85 grass (P = 0.41). Nutritionally, the management of Tifton 85 grass at ages greater than 42 days of regrowth is not interesting, because the stem ratio is higher than leaf blade as observed in the blade: stem ratio and for ruminants, the highest amount of nutrients comes from leaves in relation to the stem.

On the nutritional parameters, Table 2, with the exception of CP content (P <0.01), neutral detergent insoluble nitrogen (P = 0.01), there was no interaction between ages x N regrowth doses (P = 0.81). The DM content, neutral detergent fiber corrected for ash and protein, acid detergent fiber (ADF), hemicellulose, cellulose and lignin did not differ (P> 0.05) due to the regrowth age and not a function of N levels, with an average of 33.2, 75.3, 35.2, 42.3, 29.8 and 5.4%, respectively.

For the ash content (P = 0.19) and organic matter (P = 0.16) there was no effect of the N doses, with the averages of 8.6% and 91.4%, respectively. However, the means of these two variables adjusted to the quadratic regression model as a function of the increase of the regrowth ages. Several studies (Bassegio et al., 2013, Monção et al., 2016; Benjamin and Bradford, 2017) with forage plants have verified effects of cutting age on structural and nutritional characteristics. These changes reported by the same authors have been justified by the change in the proportion of cellular components that occur as a function of the physiological maturity of the plant. In this study, the interval between cuts associated to the soil and climatic characteristics of the northern semiarid region may have influenced the non-alteration of most components related to nutritional value. Monção et al. (2016) verified the effect of regrowth age on grasses of the genus Cynodon at ages up to 79 days. Wilson (1993, 1997) also reported that thickening of the secondary cell wall with maturation of plant tissues (increased cut age) increases the concentration of neutral detergent fiber, ADF and lignin at the expense of cellular content, a fact not verified in this search.

The N is well known for accelerating the growth of the forage so that it reaches in less time the maturity, which consequently changes the proportion of the cellular constituents (Velásquez et al., 2010; Coblentz et al., 2017). However, in this study, the shortest interval was not enough to allow modification in some nutritional parameters as a function of nitrogen fertilization. However, the content of CP 13.1% when handled at 28 days and fertilized with 300 kg ha^-1  of N in relation to the dose of 100 kg ha^-1 (P <0.01). At the ages of 35 and 42 days there was no difference between the N doses on the CP content, the average being 15.2%. At age 49 days, CP content was 24.7% higher when fertilized with 100 kg ha^-1 of N than the 300 kg ha^-1 dose (9.8%). This difference is a strong indicative of losses of N applied, mainly having urea as source. The highest concentration of nitrogen neutral detergent insoluble nitrogen was verified at a dose of 300 kg ha^-1, when the grass was managed at the age of 35 and 42 days. This means that 6.5% CP is associated with the cell wall of the plant with slow ruminal degradation. Velasquez et al., (2010) observed in several tropical forages such as Tifton 85 grass that the fraction A of CP, variable not quantified in this study, reduces with the increase of regrowth age, which means lower isoacid supply required by rumen microorganisms (Russell et al., 1992).

However, with nitrogen fertilization, the fraction A content increases as verified in the study of Neumann et al., (2017) with tropical forages. In ruminant nutrition, Detmann et al. (2014) verified that the minimum CP of the diet should be 8% of the dry matter, implying that Tifton 85 grass fertilized with N, independent of dose 100 or 300 kg ha^-1, and handled up to 49 days meets those requirements. However, it is interesting to adjust the energy levels to maximize the ruminal synthesis of microbial protein.

CONCLUSIONS

Fertilization of Tifton 85 grass with 100 kg / ha of N, managed every 35 days improves the dry matter production, structural characteristics and nutritional value of forage for cutting and / or grazing.

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Acknowledgements

Received/recebido: 2017.11.13

Accepted/aceite: 2018.06.16

 ^¥Bolsista de Pós-Doutorado da CAPES e professor colaborador do programa de pós-graduação em Zootecnia da Unimontes