Screening of Fifteen Sugarcane Genotypes under Irrigation based on Genetic Variations and Heritability for Agro-morphological Traits at Early Selection Stage in Ferké, Ivory Coast

The study aimed to determine the best performing early-season sugarcane genotypes tested at the first selection stage under sprinkler irrigation, in comparison with a check variety (R579). The experimental design used was a randomized complete block (RCB) with 17 cane genotypes in three replications. Every plot consisted of five dual rows of ten meters with 0.5 and 1.90 m of inter-row spacing, i.e. 95 m2 per plot and about 5000 m2 for the whole experiment. It was carried out on a commercial sugarcane plantation of Ferké 2 located in the northern part of the country, over two seasons (plant cane and first ratoon) as an early-season crop from November 18, 2015 to December 05, 2017. Over each micro-plot, data based on different agro-morphological traits were collected at harvest from three central dual rows. The study showed that based on sugar yields, six genotypes higher or equivalent to check varieties used, namely N47, R95-2202, ECU01, R97-0391, N49 and CP98-1029, were found promising for the next and final advanced selection stage, with respectively 20.3, 16.5, 15.2 , 14.2, 13.3 and 13.3 t sugar/ha. Their cane yield performances ranged from 115.8 to 160.8 t/ha compared to 160.4 and 134.0 for check varieties R579 and SP70-1006, respectively. Not only sugar yield, but also, recoverable sucrose, cane internode infestation rate and number of tillers per hectare were found as the most relevant agro-morphological traits in genetic variation of sugarcane genotypes tested in line of their higher broad sense heritability values.


Introduction
In addition to be a source of sucrose, sugarcane is an important bioenergy crop, with an energy ratio of ethanol production five times higher than that of maize [1][2][3]. It used to be considered as a feedstock for production of advanced biofuel due to its superior contribution to reduce the life cycle greenhouse gas emission in the fight against global warming and climate change [4]. As a result of its advantages as cash crop like very high biomass production and well-established technologies regarding farming, harvesting as well as processing, sugarcane is a leading candidate for bioenergy production and a feedstock for bio-refineries. However, productivity improvements in sugarcane have been negligible in the past three decades, and production statistics are reflecting decreased yields globally [5]. In all cases, increased sugarcane production is linked to expansion of land surface rather than increase in yield [6][7][8].
In all sugarcane breeding programs worldwide, the key targeted traits are resistance to important local diseases and pests, commercially extractable sucrose content, cane yield, acceptable fiber content and ratooning performance. In some programs, other traits affecting costs of harvesting or crop management are of importance. Sugarcane varieties tend to run out or decline after some years of cultivation in a specific area [9]. To obtain high yields on a sustainable basis, it has been essential to substitute varieties regularly grown with new clones. Sugarcane varieties are clonally propagated and therefore are not expected to undergo genetic changes as it may occur in a seed propagated crop except for the variety decline over several ratoons due to disease incidence and other environmental constraints with therefore a need for replacement [10].
Breeding high performing commercial cultivars is crucial for maintaining sugarcane production, which will benefit from research in sugarcane genome sequencing and genetic mapping. These research areas focused on understanding sugarcane's genome structure, organization and inheritance patterns. They also help in understanding genetic variations within sugarcane populations or germplasms that control important agronomic traits [7]. Often, the ultimate objective of sugarcane breeding programs is to release varieties which improve the profitability of the sugar industry being targeted. That is why breeders need to determine the optimal weightings that should be applied to each agro-morphological to be investigated. A first step towards this involves identifying all traits influencing industry stakeholders and determining the relative economic value of variation in each trait, preferably in quantitative terms [8]. As industries change, the economic value of traits may change. In recent decades, weightings of some traits have changed in response to developments such as the introduction of mechanized harvesting, increased use of sugarcane for energy production and change in agronomic practices.
The objective of study was to determine the best performing early-season sugarcane genotypes under irrigation at the first selection stage for different agro-morphological traits based on their genetic variations and heritability.

Site Characteristics
The study was carried out on Ferké 2 sugarcane field N1-005, sprinkler irrigated with center pivot, in northern Ivory Coast (9°20'-9°60' N, 5°22'-5°40' O, 325 m a.s.l.). The prevailing climate is tropical dry with two seasons: one, starting from November to April, is dry and the other, from May to October, is wet. The dry season is marked by the boreal trade wind which blows over mid-November to late January. The rainfall pattern is unimodal and centered on August and September which total amount of rainfall reaches almost half of the average annual rainfall (1200 mm) with an average daily temperature of 27 °C. Average maximum and minimum daily air temperatures are 32.5 and 21 °C, respectively.
To meet sugarcane crop water requirements, the total amount of irrigation water required reaches 700 mm/year [11][12][13] as well as genetic advance are very essential to improve any trait of sugarcane because this would help in knowing whether the desired objective can be achieved from the material to be investigated [14].

Experimental design
The experiment was carried out over two consecutive years, as early-season trial in plant and first ratoon crops, from November 18,

Agronomic Traits investigated
Data were collected at harvest from three central dual rows for millable stalk number/ha, cane yield, agro-morphological traits (stalk weight, stalk diameter, stalk height, number of internodes), juice quality traits (sucrose, purity, and recoverable sucrose), fiber content, and damaged internodes by stem borer Eldana saccharina.
At harvest, burned cane fresh production of the three dual rows of each plot was weighed separately to determine crop yield.
Moreover, 50 millable stalks were randomly chosen within every plot and split longitudinally with a machete in order to determine the percentage of bored or attacked internode and cane (%BIN, %BC) by stem borer.
Thirty millable cane stalks were sampled per plot for sucrose analyses in the laboratory. Prior to sample grinding operations in the laboratory for sucrose analyses, every stalk was cut into 3 pieces of almost equal length while separating them in basal, median and top parts. This allowed to randomly reconstitute 3 batches of 10 stalks for a better homogenization of the initial field sample by permutation of the pieces so that each reconstituted stalk was composed of parts coming from 3 different cane stalks.
Eventually, only one batch of 10 reconstituted stalks over 30 (1/3 of initial sample) were ground for a series of sucrose analyses digital polari-meter SH-M100. Methods used in the determination of required technological parameters were reported by Hoarau [15].

Phenotypic and Genotypic Coefficients of Variation, Heritability and Genetic advance
The phenotypic and genotypic variances for each trait were estimated from the RCBD analysis of variance ( Table 1). The where Covp and Covg are phenotypic and genotypic covariances, respectively.
Phenotypic correlations between traits were determined following the Pearson correlation coefficient calculated from means of observed traits for each cane genotype [22]. Genetic improvement in cane and sugar yields may be achieved by targeting traits closely associated to them. Several attributes have been proposed as indirect selection criteria for genetic improvement of yields in plant breeding programs [23][24]. Heritability represents the relative importance of genetic and environment factors in the expression of phenotypic and genotypic differences among genotypes within a population [25][26][27]. Consequently, the knowledge of heritability related to important traits and the correlations among them are key issues to determine the best selection strategy [28][29]. Genotypic coefficient of variation (GCV) is another measure of relative genetic variation of a trait within a population [30]. Chaudhary [31] reported high GCV for single stalk weight and millable cane number per unit area. Genotype x environment interactions (GxE) are a serious concern in breeding programs as they affect selection decisions. When a rank of a genotype changes across environments, it requires evaluation of genotypes across environments to determine their real value [32]. Studies in various sugarcane breeding programs have reported significant GxE interactions for cane and sugar yields [33][34].

Statistical analyses
The quantitative data recorded in this study were subjected to the analysis of variance using statistical procedures described by Gomez and Gomez [35] and reported by Shitahum et al. [36] with the assistance of R software package version 3.5.1 (Table 1).

Climatic conditions over plant and first ratoon crops
Total amount of rainfall and reference evapotranspiration recorded in first ratoon decreased by 5 and 2.4%, respectively, compared to that of plant cane ( Figure 1). In absolute values, they varied from 1274 to 1216 mm and from 1468 to 1433 mm,
In first ratoon crop, except for stem borer infestation rates in terms of cane and internode bored, and number of internodes with no significant differences within genotypes (P>0.5), highly significant differences (P<0.01) were observed for all agro-   Table 5: Phenotypic and genotypic correlation matrix of agro-morphological traits investigated in first ratoon crop (respectively below and above diagonal). Over both crop cycles (Tables 4&5), all juice quality traits   (pol juice, purity, sucrose content, and recoverable sucrose)

Genotypic correlations between pairs of agromorphological traits
Over both crop cycles (Tables 4&5),
In first ratoon crop, all PCV and GCV values determined which ranged respectively from 2.8 to 33.6%, and from 2.2 to 20.5%, varied from low to high. Particularly, stalk internode infestation which exhibited a relatively high GCV estimate (20.5%), might respond favorably to selection in line of findings reported by Ebid et al [27].
Regardless the trait considered in both crop cycles, the phenotypic coefficient of variation was higher than the genotypic one, suggesting that apparent variations were not only due to genetics but also to environmental influences. However, differences between PCV and GCV for most traits were small in line of observations made by different investigators [22,46,47], indicating high prospects for genetic progress through selection under conditions of this study.

Phenotypic, genotypic and environmental variance
In both crop cycles, phenotypic variance data obtained were higher than the genotypic ones regardless the trait considered. This

Heritability and genetic advance
Estimates of broad-sense heritability (h²) are categorized according to Robinson [48] cited by different authors [14,39,41] as low (<30%), moderate (30≤-<60%) or high (≥60%   As indicated by Vidya et al. [51], knowledge of variability and heritability of characters is essential for identifying those relevant to genetic improvement through selection. Moreover, the effectiveness of selection will depend not only on heritability but also on genetic advance [52,53]. Higher levels of mean genetic advance observed for sugar yields, millable stalk number, recoverable sucrose content and internode infestation rate were the result of moderate or high broad sense heritability combined with high GCV for these traits in line of findings reported by Bakshi [54]. According to this author, heritability estimates together with expected genetic gain were more useful than heritability values alone in predicting the effects of selecting best genotypes. Chaudhary [31] also reported high heritability and genetic gain for single cane weight followed by number of millable cane in a study of 36 clones indicating substantial scope for cane yield improvement. On the other hand, sucrose content recorded low heritability and genetic gain suggesting little scope for improvement in this character [55]. Patel et al [56] also reported high heritability estimates for single cane weight, number of internodes, tiller number, hand refractometer brix, cane diameter and millable cane length, which were associated with moderate to high genetic advance (23-190%). Findings indicated that these characters might be improved through selection. From the literature, findings on heritability, genetic gain, PCV and GCV for the same traits look sometimes controversial depending on locations, crop cycle (plant cane or ratoon), soil types, water regime (rainfed or irrigated), etc. [19,37,38,57,58]. But still, this is all about the scope of experimentation in agronomy, findings being mostly sitespecific.

Conclusions
The study showed that based on sugar yields, six genotypes