ARTIGO

Sample size to estimate biomass and productivity of Canavalia ensiformis and Dolichos lablab

Tamanho da amostra para estimativa da biomassa e produtividade de Canavalia ensiformis e Dolichos lablab

Paulo E. Teodoro¹, Larissa P. Ribeiro¹, Flávia A. da Silva¹, Caio C. G. Corrêa¹, Roque A. A. da Luz Júnior¹ and Francisco E. Torres¹

 

¹ Departamento de Fitotecnia, Faculdade de Agronomia, Universidade Estadual de Mato Grosso do Sul, Unidade de Aquidauana. CEP: 79200-000, Aquidauana, Mato Grosso do Sul, Brazil. E-mail: eduteodoro@hotmail.com, author for correspondence

 

ABSTRACT

Green manure brings numerous benefits that promote essentially the maintenance and conservation of agro-systems, and its implementation is fundamental to Brazilian Cerrado region. In this scenario, the present research aimed to determine the sample size for the estimation of biomass and productivity expected values of Canavalia ensiformis and Dolichos lablab. The experiment was installed in the experimental area of the State University of Mato Grosso do Sul - Unit University Aquidauana (UEMS/UUA), located in the Brazilian Cerrado. Forty five plants were randomly selected in the experimental area of each crop to determine fresh mass, dry mass and yield. Measures of central tendency, variability, asymmetry and kurtosis were calculated and normality of the variables under study was checked by the Lilliefors test. In C. ensiformis and D. lablab, 259 and 362 plants, respectively, are sufficient for the estimation of evaluated variables, with confidence interval of 95% an estimation error equal to 5% of the observed average. C. ensiformis showed significant high mean values for the fresh mass production, dry mass and yield grain in comparison with the Dolichos lablab, being recommended for cultivation in Brazilian Cerrado.

Keywords: experimental design, green manure, number of plants.

 

RESUMO

A adubação verde traz inúmeros benefícios que promovem, essencialmente, a manutenção e a conservação de agrossistemas, sendo a sua implementação fundamental para a região do Cerrado brasileiro. Neste cenário, a presente pesquisa tem como objetivo determinar o tamanho da amostra para a estimativa da biomassa e da produtividade média de Canavalia ensiformis e Dolichos lablab. O ensaio foi conduzido na área experimental da Universidade Estadual de Mato Grosso do Sul - Unidade Universitária de Aquidauana (UEMS/UUA), localizada no bioma Cerrado. Foram selecionadas, aleatoriamente, 45 plantas na área experimental de cada cultura para determinação da massa verde, massa seca e produtividade, sendo posteriormente calculadas as medidas de tendência central, variabilidade, assimetria e curtose, e verificada a normalidade por meio do teste de Lilliefors. Em C. ensiformis e D. lablab, 259 e 362 plantas, respectivamente, são suficientes para a estimativa dos caracteres avaliados, com intervalo de confiança de 95% e um erro de estimativa de 5% da média observada. O C. ensiformis apresentou médias de produção de massa verde, massa seca e produtividade de grãos significativamente superiores ao D. lablab, sendo esta espécie recomendada para o cultivo no Cerrado brasileiro.

Palavras-chave: adubação verde, planejamento experimental, número de plantas.

 

Introduction

The Cerrado occupation has occurred quickly, based on intensive production systems, which have increased the processes of soil degradation (Silva et al., 1994). These processes result in impacts that often generate the inefficiency of agricultural systems, with frequent need for interventions through conservation practices to maintain the productive potential of the soil. In this context, green manure becomes fundamental to the Cerrado region, because it promotes protection, improvement and maintenance of soil quality, as well as substantial increases in soil organic matter and nutrients, benefiting agroecosystems (Carvalho et al., 1999; Leite et al., 2010).

Among the desirable characteristics for selection of species for green manure, stands out mainly dry mass production, because it is related to the capacity to increase nutrient by symbiosis with microorganisms, soil cover and recycling of nutrients (Chaves and Calegari, 2001). Among the various botanical families species that can be grown as green manures, stand out those of the Fabaceae family. The legumes, in addition to providing similar benefits to other species, have the ability to accumulate N through biological fixation (Silva et al., 2009). According to Teodoro et al. (2011), Canavalia ensiformis (L.) DC. and Dolichos lablab L. are, among the leguminous species, the most promising to green manure, owing to the high dry matter production, surpassing the productivity 6 to 12 t ha^-1 of dry mass recommended by Darolt (1998) and Alvarenga et al. (2001) for soil management programs in the Cerrado biome.

D. lablab, in Cerrado, adapts to cultivation during the rainy season, as it is sensitive to photoperiod and water deficit, with an approximate cycle of 156 days to flowering (Carvalho et al., 1999). C. ensiformis is well adapted to adverse environmental conditions, from the arid and semiarid regions to the rainforest climate. In the Cerrado of Central Brazil is sown until the end of the rainy season, because of drought resistance and little sensitivity to photoperiod (Calegari et al., 1993; Carvalho et al., 1999). In agricultural systems in areas of Cerrado are used intercropped with maize and perennial crops, because tolerates partial shade (Carvalho and Amabile, 2006).

Although the data can produce reliable results in a given study, it is important to determine the sample size in order to estimate the average of each variable, with high confidence and a given estimation error. The sample size is directly proportional to the variability of the variables under study and to the desired confidence degree in the estimate and inversely proportional to the estimation error allowed, initially established by the researcher (Bar-betta et al., 2004; Bussab and Morettin, 2004; Spiegel et al., 2004).

Thus, the objective of this research was to determine the sample size to estimate biomass and productivity of C. ensiformis and D. lablab average and identify the most recommended species to cultivation in the Brazilian Cerrado.

 

Material and Methods

The experiment was installed in the experimental area of the State University of Mato Grosso do Sul -Unit University Aquidauana (UEMS/UUA), in the municipality of Aquidauana (MS), located in the Brazilian Cerrado (or Savanna), comprising the coordinates 20º27’ S and 55º40’ W, with an average elevation of 170 m.

The soil was classified as Ultisol sandy loam (Embrapa, 2006), with the following chemical characteristics in the layer 0 – 0.20 m: pH (H₂O) = 6.2; Al exchangeable (cmol_c dm^-3) = 0.0; Ca+Mg (cmol_c dm^-3) = 4.31; P (mg dm^-3) = 41.3; K (cmol_c dm^-3) = 0.2; Organic matter (g dm^-3) = 19.7; V (%) = 45.0; m (%) = 0.0; Sum of bases (cmol_c dm^-3) = 2.3; cation exchange capacity (or CEC) (cmol_c dm^-3) = 5.1. The climate of the region according to the classification described by Köppen-Geiger is Aw (Savanna Tropical) with average annual rainfall of 1200 mm and maximum and minimum temperatures of 33 ºC and 19 ºC, respectively (Schiavo et al., 2010).

C. ensiformis and D. lablab crops were sown manually on April 2013 in an experimental area with 50 m². The spacing used was 0.45 m between rows at a density of 15 plants per meter. No base fertilization and coverage for any culture were used.

At maturation, 45 plants were randomly selected in the experimental area of each crop to determine fresh mass, dry mass and productivity. Data were extrapolated to t ha^-1, and measures of central tendency, variability, asymmetry and kurtosis were calculated and normality of the variables under study was checked by Lilliefors test (Campos, 1983). To each variable, in each crop, the sample size was calculated (h), considering the 45 plants for a semi-amplitude of the confidence interval (estimation error) equal to 1%, 2%, ..., and 30% of the estimated average (a), with a confidence degree (1-a) of 0.95, by the expression (Fonseca and Martins, 1995; Barbetta et al., 2004; Bussab and Morettin, 2004; Spiegel et al., 2004), where t_a/2 is the critical value of Student t distribution, with (n-1) degree of freedom, and defined as P(t>t_a/2) = a/2 and variance estimate (s²).

The statistical analysis was performed with assistance of the Genes computational program (Cruz, 2006) and Office Excel® software.

 

Results and Discussion

The calculation of central tendency, variability, asymmetry, kurtosis and Lilliefors test relatively to morphological variables fresh mass, dry mass and C. ensiformis and D. lablab yield, from 45 plants evaluated, data deviation was identified to normal distribution (Table 1).

However, according to the central limit theorem, even if the basic population is not normal, the distribution of the sample mean will be approximately normal for samples greater than 30 observations (Fonseca and Martins, 1995; Bussab and Morettin, 2004). Given these considerations in relation to normality, it can be inferred that the data from these variables provide credibility to the scaling of the sample size study.

Analyzing the variation coefficient of both green manures verifies a superior magnitude of fresh and dry masses and productivity, suggesting the need of higher sample sizes for the estimation of the fresh and dry mass relative to average productivity.

Moreover, the statistical values revealed existence of genetic variability among C. ensiformis and D. lab-lab plants. Variables evaluated in C. ensiformis plants were statistically higher than those observed in the D. lablab being within the limit recommended by Darolt (1998) and Alvarenga et al. (2001) for cultivation in the Brazilian Cerrado region. These results showed lower magnitude than those observed by Melo (2012) that in evaluating the fresh and dry mass production in different green manures did not identify statistical difference in the C. ensiformis and D. lablab, more productive crops.

It is important to mention that given the observed results, the jack bean provides greater dry mass production and consequently greater soil cover in relation to D. lablab, which entails numerous benefits such as increased weed suppression, greater soil humidity, and smaller temperature oscillation, among others. Moreover, this is a crop that can be considered for family farms because of its high seed production and high prices in the market.

Sample size to estimate the average of each character in C. ensiformis, with semi-amplitude of the confidence interval equal to 1% of average estimate and a confidence of 95% degree, ranged between 292 and 6473 plants (Table 2), whereas for the D. lablab this variation was 334 to 9039 plants. These results confirm the variability among the variables and among green manure species, corroborating the results obtained by Cargnelutti Filho et al. (2010), that identified sample size variability of C. ensiformis and gray mucuna (Stizolobium cinereum Piper & Tracy).

From a practical standpoint, it can be inferred that it is difficult to obtain average estimates of the variables evaluated in C. ensiformis and D. lablab, with estimation error of 1% of the sample mean, due to the large plants number to be measured. It does not fit in this study the judgment of maximum estimation error acceptable, getting the use this information to the researcher for his experimental design. If the option is to obtain estimated averages with 5% estimation error of the sample mean, taking into account the density of 15 plants m^-1, 0.45 m spacing and the evaluation these variables in three rows useful length of 5 m, plots with rows of 5 and 7 m length would be suitable for experiments with C. ensiformis, according to Figure 1. Considering these same parameters for the D. lablab, plots of 7 rows with 7 m long would be suitable. However, other factors must be analyzed for planning experiments with these green manures, such as seed germination, plant density, spacing to be used and type of harvesting and weeding (manual or mechanical).

 

Conclusions

Canavalia ensiformi showed significant higher values for the mean fresh mass production, dry mass and productivity in relation to Dolichos lablab, being recommended for cultivation in Brazilian Cerrado. In Canavalia ensiformi and Dolichos lablab, 259 and 362 plants, respectively, are sufficient for the estimation of evaluated characters, with confidence interval of 95% and an estimation error equal to 5% of the sample mean.

 

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Recebido/Received: 2014.01.23

Aceitação/Accepted: 2014.06.21