Agronomic performance of sweet cassava cultivars

Brazilian sweet cassava consumers are becoming more and more demanding in relation to sanitary conditions, organoleptic and nutritional characteristics of this vegetable products and their derivatives. Thus, more nutritious, early, productive, adapted to mechanized planting and good culinary quality sweet cassava cultivars were developed. This study aimed, therefore, to evaluate the performance of four sweet cassava cultivars in Uberaba, in Triângulo Mineiro region. The experiments were carried out in the experimental field of Faculdades Associadas de Uberaba (FAZU) during two harvest seasons. We adopted a randomized block design with three replicates, each plot consisting of four lines with ten plants. Trait averages were grouped by the Scott & Knott test. The results showed that cultivars BRS 399, BRS 397 and IAC 576-70 have potential to be grown in the region, since they showed root yield higher than 32 t ha-1 and up to 30-minute cooking time. BRS 399 stood out for its performance, showing root yield of 37 t ha-1 and 48 t ha-1 in 2015/2016 and 2016/2017 harvest seasons, respectively, with short-time cooking and presenting shoot productivities which allow its use even as animal feed.

Palavras-chave: Manihot esculenta, aipim, macaxeira, produtividade de raízes, qualidades culinárias. application, selection and prepare of planting material, planting systems, planting times, spacing and planting density, intercropping, crop rotation, weed control, shoot pruning, harvest, pest control, disease control, among others . The research also developed, for the Brazilian Savannah biome, an irrigation management which takes into account the daily soil water balance, to determine the ideal moment for irrigation and the amount of water applied in cassava crop (Antonini et al., 2017).
R e g a r d i n g p o s t -h a r v e s t i n g , strategies to maximize useful life of the roots submitted to minimum processing and frozen (Rinaldi et al., 2015a(Rinaldi et al., , b, 2017 and different packing systems  were developed. In the last years, more nutritive, early, productive, adapted to mechanized planting and with good cooking quality cassava cultivars were developed for the Brazilian Savannah conditions, in special for Distrito Federal Region (Vieira et al., 2011(Vieira et al., , 2018.
However, despite the edaphoclimatic conditions and market potential of Triângulo Mineiro region for cassava cultivation , systematic studies on agronomic cassava cultivars in the region are still scarce in literature, and there is no cultivar recommended for this environment.
Thus, this study aimed to evaluate the agronomic performance of four cassava cultivars in the municipality of Uberaba-MG, in Triângulo Mineiro region.

MATERIAL AND METHODS
The experiments were carried out during two harvest seasons in the experimental field of the school-farm at Faculdades Associadas de Uberaba (FAZU), in Uberaba-MG (19º44'45"S, 47º57'415"W, 771 m altitude), from August, 2015, to May, 2016, and from August, 2016, to May, 2017. The soil was classified as Red-Yellow Latosol, medium texture (Embrapa, 2018) and, according to Köppen-Geiger, the local climate is Aw (tropical with dry season). During the experiment, 2015/2016 and 2016/2017 harvests, the weather variables were measured and soil physicochemical composition was determined (Table 1).
Four sweet cassava cultivars were selected, to be grown in Distrito Federal, BRS 399, BRS 397, BRS Moura and IAC 575-70 (Fialho et al., 2009;Vieira et al., 2015Vieira et al., , 2018, with still no reported information on agronomic performance in Uberaba-MG in Triângulo Mineiro region, though. The experimental design was randomized blocks, with three replicates, being each plot composed of four lines with 10 plants. The spacing used was 0.80 m between plants and 1.00 m between rows, each plot consisted of 16 central plants. The selection of propagating material and fertilization followed the recommendations for cassava cultivation in the Brazilian Savannah region . The experiments were conducted under conventional sprinkler irrigation up to the beginning of rainy season, based on the daily soil water balance at the effective depth of the cassava root system (0.40 m), according to the management proposed by Antonini et al. (2017).
At harvest, five agronomic traits were evaluated: i) plant height in meters (PH); ii) shoot weight without original stem cutting in kg ha -1 (ShW); and iii) root yield in kg ha -1 (RY); iv) percentage of starch content in roots using the hydrostatic balance method (RSC), described by Grosmann & Freitas (1950); and v) cooking time in minutes (CT), according to the method described by Borges et al. (2002).
The obtained data were submitted to variance analysis according to randomized block design, following the model: Yijk = m + Gi + Bk + Aj + GAij + Eijk, in which: Yijk = observed value of genotype i in block k from environment j; m = general average; Gi = effect of genotype i; Bk = effect of block k; Aj = effect of the environment; GAij = effect of the interaction of genotype i with environment j; Eijk = experimental error. In order to analyze hypothesis of data normality, Shapiro-Wilk's test at 5% probability was used (Shapiro & Wilk, 1965). Trait averages were grouped using Scott & Knott's agglomerative test at 5% probability of error (Scott & Knott, 1974). Statistical analyses were performed using Softwares R (R Core Team, 2019) and Genes (Cruz, 2016).

RESULTS AND DISCUSSION
Shapiro-Wilk's test showed that the residues for the traits evaluated in the experiment presented normal distribution at 5% probability of error and they can be evaluated using parametric tests ( Table 2). The coefficients of variation of the variance analyses ranged from 7.02% for starch content in roots (RSP) to 11.18% for root yield (RY), showing good experimental accuracy (Ferreira, 1991).
The variance analysis showed significant differences between the average harvests, 2015/2016 (S1) and 2016/2017 (S2), for shoot weight without original stem cutting (ShW), root yield (RY) and cooking time (CT) (Table2). This result indicates the influence of the year in phenotypic manifestation of these traits and it is in accordance with what was widely reported in literature (Vieira et al., 2009(Vieira et al., , 2015Silva et al., 2014). The influence of the year can be explained by weather variations, such as, rainfall, maximum and minimum temperatures, wind speed, insolation, solar radiation, among others (Silva et al., 2017).
Significant differences among averages of cultivars for all evaluated traits were verified (Table 2). This variation can be explained by the genetic variability verified in the group of the evaluated cultivars (Vieira et al., 2011(Vieira et al., , 2018Fuhrmann et al., 2019), which probably shows that the sweet cassava cultivars chosen to be tested in this study were suitable.
The presence of significant interaction between crop and cultivar factors for PH, ShW, RY and CT (Tables  2 and 3) (Table 3). Higher averages for PH are important for recommending cultivars for cultivation due to: i) it facilitates cultural practices (weeding, field inspections and pesticide applications); ii) for greater availability of stem cuttings and iii) to facilitate mechanized planting. The superiority of cultivar BRS 399 for PH had already been reported by Vieira et al. (2018) in a study carried out during three harvests in 18 locations in Federal District and surroundings and by Fuhrmann et al. (2019) in a study carried out during two harvests in Planaltina-DF.
For shoot weight (ShW) in 2015/2016 harvest, cultivar BRS 397 was the one which showed superior average when compared with the others, 26.83 t ha -1 , whereas cultivar BRS 399 showed superior average in 2016/2017 harvest, 26.46 t ha -1 (Table 3). This trait is important when selecting a cultivar, since it is related to the possibility of using cassava shoot as protein source for animal feed (Fernandes et al., 2016) and, greater soil cover efficiency (erosion control, soil humidity maintenance and weed control). We highlight that in the study carried out by Fuhrmann et al. (2019) both cultivars showed average ShW of 20 t ha -1 , whereas in the experiments conducted by Vieira et al. (2018) these cultivars showed ShW higher than 30 t ha -1 .
All cultivars showed averages similar for percentage of starch content in roots (RSC) in 2015/2016 harvest, whereas, in 2016/2017 harvest, the cultivars BRS Moura, IAC 756-70 and BRS 397 showed similar averages of root starch content (RSC) among each other and superior averages in relation to cultivar BRS 399 (Table 3). Although being important for the use of roots in the production of flour and starch, this trait presents a secondary importance considering sweet cassava breeding program. RSC shows importance only when the goal is the use of roots for dual-purpose (in natura and industry), when, for example: the goal is to produce yellow cassava flour. Lower percentages of starch in roots of cultivar BRS 399 comparing with cultivars BRS 397 and IAC 576-70 had already been reported by Fuhrmann et al. (2019) in a study carried out during two harvests in Planaltina-DF, in which the cultivar showed RSC of 22.96 and 24.83% in harvests at 11 months after planting, rainfed conditions. No significant differences were 72 65 Organic matter in soil (g kg -1 ) 28 30 Clay in soil (g kg -1 ) 300 210 Sand in soil (g kg -1 ) 550 600 Silt in soil (g kg -1 ) 150 190    (Fialho et al., 2009), 53 t ha -1 (Vieira et al., 2009), 24 t ha -1 (Vieira et al., 2011) 26 t ha -1 (Silva et al., 2014), 44 t ha -1 (Vieira et al., 2015), 41 t ha -1 (Vieira et al., 2018) and 35 t ha -1 (Fuhrmann et al., 2019), showed the high potential of this sweet cassava cultivar in Uberaba-MG.
In the group of the evaluated cultivars, all of them showed up to 30-minute CT (Table 3), which is an indispensable factor for the commercialization of cassava roots for culinary use . Both in 2015/2016 and in 2016/2017 harvests, cultivar BRS 397 showed CT lower than cultivar BRS 399, which indicates that all the evaluated cultivars showed high culinary quality, being excellent to be commercialized.
The results obtained in this study showed that all evaluated cultivars, except BRS Moura, showed potential for cultivation in Uberaba-MG, since: i) showed averages of RY higher than 32 t ha -1 , which is an optimal RY for Brazilian Savannah conditions in Central Brazil (Vieira et al., 2018); ii) presented averages of CT of up to 30 minutes, within the acceptable limit for the commercialization of sweet cassava ; iii) showed average PH of 2.73 m, which is considered a good height for cassava plants (Vieira et al., 2018); and iv) presented ShW higher than 19 t ha -1 , which is considered a good forage yield, as cassava shoot area showed an average of 13% of crude protein (Fernandes et al., 2016).
The fact that more than one cultivar stood out is important because it contributes to the maintenance of genetic variability under cultivation. However, we highlight that cultivar BR 399 stood out comparing with the others, in relation to agronomic performance, RY of 37.62 t ha -1 and 48.53 t ha -1 in 2015/2016 and 2016/2017 harvests, respectively, with CT within acceptable limits for the commercialization and shoot yield which allow the producer to use the surplus production as animal feed.
However, before recommending any of the evaluated cultivars for commercial planting in Triângulo Mineiro region, it would be interesting to validate their performance in a greater number of locations, using the methodology of participatory selection of cassava cultivars (Fialho & Vieira, 2011), in which, in addition to agronomic data, information related to producers' preferences would be studied. Table 3. Comparison of averages of plant height in meters (PH), shoot weight without original stem cutting in kg ha -1 (ShW), root yeld in kg ha -1 (RY), starch content in roots in percentage (RSC) and cooking time in minutes (CT), evaluated in four sweet cassava cultivars, in 2015/2016 (S1) and 2016/2017 (S2) harvests in Uberaba (MG). Uberaba, FAZU/Embrapa, 2019.