Ripening Response of Sugarcane Varieties to Chemical Ripeners and Economic Benefits during the Early Period of Harvesting at Wonji-Shoa and Metahara Sugarcane Plantations, Central Rift Valley of Ethiopia

Ethiopian Sugar Corporation, Research Centre, Eastern Showa Zone P.O. Box 15, Wonji, Ethiopia Department of Crop Production, Faculty of Agriculture, University of Eswatini, P.O.Luyengo, M 205, Kwaluseni, Eswatini South African Sugarcane Research Institute, Mount Edgecombe, Blackburn 4300, South Africa Department of Plant and Soil Sciences, University of Pretoria, Pretoria 0028, South Africa School of Plant Sciences, Haramaya University, P.O. Box 138, Dire Dawa, Ethiopia


Introduction
Sugarcane (Saccharum spp., hybrid) is cultivated in Ethiopia at a commercial level [1], as well as by smallholder farmers [2]. It also has broad socioeconomic advantages [3]. However, the per-capita sugar consumption of Ethiopia is one of the lowest in the world with about 5 to 6 kg per annum, and future sugar consumption growth is expected to be around 3-4% annually, which was forecasted to grow by 0.4 million tons by 2030 [4]. e total annual sugar production in the country was reported to be 400,000 tons in 2018, which only covers 60% of the annual demand for domestic consumption [5]. e country is endowed with a favourable climate, enormous land, and water resources for large-scale irrigated development of sugarcane [1] with an average sugarcane yield potential of 162 tons per hectare [6]. Nonetheless, the value of sugarcane is determined by the amount of recoverable sucrose per weight of cane [7]. In Ethiopia, sucrose is the main product of sugarcane processing [8], and its quantity principally depends on the quality of sugarcane supplied [9]. is suggests the need for maintaining quality using appropriate ripening management as an indispensable means [10].
Although the dry matter yield potential of sugarcane in the country is high [6], the quantity partitioned to sucrose varies depending upon the variety, age [11], season [8], soil fertility, irrigation [12], weed, pest and disease control [13], and the length of crushing season [7]. Nevertheless, air temperature and soil moisture are the major factors affecting the partitioning of sucrose in sugarcane [14]. At a higher temperature, from the total carbon fixed and stored, sugarcane partitions less carbon to sucrose [15]. e availability of soil moisture also reduces the sucrose content of sugarcane during ripening due to the high growth sink demand [16]. In the ripening phase, there should be a synthesis and rapid accumulation of sucrose with a concomitant reduction of vegetative growth and a decline in the level of monosaccharides (fructose and glucose) in stalks [12].
Wonji-Shoa and Metahara sugarcane plantations have a total area of 10,342 and 10,235 ha, respectively. Conventionally, to maximise the sucrose content (%) of cane harvested from these plantation areas, harvesting of sugarcane is conducted after drying off the cane by withholding irrigation for a specified period ranging from 5 to 9 weeks before harvesting [17]. However, the sucrose content of cane in the beginning (September to November) period is lower due to the presence of residual soil moisture coupled with the high temperature in July and August, and this has been reported to be a persistent problem at both estates [8]. is indicates the inadequacy of natural ripening [18], which resulted in the need for accurate control over crop water supply [19] and the reduction in cane and sucrose yield from the extreme withholding of water during drier periods [20].
Chemical ripeners have become an important technology to tackle challenges related to low sucrose content in many sugar industries of the world [12]. Ripeners used in sugarcane are plant regulators whose action consists of modification of plant morphology and physiology, which can alter plant production quantitatively and qualitatively [21]. e use of ripeners can provide gains in sucrose quality above those achieved by natural ripening [12,19]. Ripeners can reduce plant height, increase sucrose content, advance plant maturation, and increase sucrose yield, and they can also present effects on enzymes that catalyse sucrose accumulation in the internodes [21]. e technology has potentially considerable impact in areas overwhelmed with poor natural ripening conditions [22]. e successful introduction of ripener technology could also facilitate the harvest of cane earlier in the season when it is relatively immature [23]. e chemical ripeners currently in use in many sugar industries include glyphosate (e.g., Roundup ™ ), 2-chloroethylphosphonic acid (e.g., Ethephon ™ ), fluazifopp-butyl (e.g., Fusilade Forte ™ ), and trinexapac-ethyl (Moddus ™ ) [12]. In terms of their modes of action, glyphosate and fluazifop-p-butyl are herbicidal and suppress new tissue formation at a sublethal dose [24], while 2-chloroethylphosphonic acid and trinexapac-ethyl have a hormonal mode of action [25,26]. e hormonal mechanism of improvement in sucrose content through 2-chloroethylphosphonic acid emanates from the active ingredient ethylene [25,26], which reduces the demand for sucrose for vegetative growth [24]; but the mechanism for trinexapacethyl is related to its ability of inhibition of elongation of internode, resulting from the reduction in the level of GA 1 [27]. Earlier works also confirmed the efficacy of sole application of glyphosate [24,28], 2-chloroethylphosphonic acid [29,30], fluazifop-p-butyl [30,31], trinexapac-ethyl [32,33], and combination of ripeners [23,29,30] in improving sucrose yield of sugarcane. However, to effectively use chemical ripeners at a commercial level, it is vital to generate information regarding the response of sugarcane varieties to these chemicals. In line with this study, reports from different sugar industries confirmed the need for evaluation of varieties for their response to chemical ripeners [23,28]. In this regard, a preliminary study conducted at Metahara sugarcane plantation in the 1980s using fluazifop-p-butyl showed that the sucrose content (%) of varieties B41-227 and NCo376 were improved by 0.47 and 0.57% units, respectively, when compared with untreated control plots [29].
Although these studies showed improvement in juice quality of some sugarcane varieties, the emergence of new chemical ripeners, combination treatments, and sugarcane varieties necessitated another study that can provide up-todate information to the estates. erefore, this study was conducted to evaluate the response of selected sugarcane varieties to chemical ripeners at Wonji-Shoa and Metahara sugarcane plantations.

Description of the Study Areas.
e present investigation was carried out at Wonji-Shoa and Metahara sugarcane plantations from December 2017 to October 2018. Wonji-Shoa Sugarcane plantation is located in the Rift Valley of Ethiopia (8°31′N and 39°12′E), at an average elevation of 1550 masl. e plantation has a mean maximum and minimum air temperature of 26.9 and 15.3°C, respectively. Similarly, Metahara sugarcane plantation is located in the Rift Valley of Ethiopia (8°51′N and 39°52′E), at an average elevation of 950 masl. e plantation has a mean maximum and minimum air temperature of 32.6 and 17.5°C, respectively. e soils of the experimental fields of Wonji-Shoa and Metahara were clay (>54%) in texture.
During the study period, Wonji-Shoa and Metahara sugarcane plantations obtained a total rainfall of 859 and 317 mm, respectively. As compared to the long-term annual average, both plantations experienced lower rainfall. Moreover, the distribution was not even at both locations ( Figure 1). Maximum rainfall of 267 and 120 mm were recorded in April and August at Wonji-Shoa and Metahara, respectively. e mean annual minimum temperature recorded at Wonji-Shoa was 13.9°C and Metahara was 17.0°C. e mean annual maximum temperature recorded at Wonji-Shoa was 29.0°C and at Metahara was 33.2°C.

Experimental Procedure.
e experiment was planted using three budded sets originated from properly managed seed source fields. roughout the growing period, furrow irrigation was provided until two weeks before harvesting. Urea (46% N) was applied once at 200 kg ha − 1 manually after a mechanical earthing-up operation was conducted at two and a half months of cane age. Weeding was conducted manually as required. Irrigation and weeding were conducted similar to commercial practice on the estates. Field inspections were conducted every 15 days, and there were no diseases and insect pests encountered throughout the growing period.
Each plot had 4 cane rows of 6 meters long and 1.45 m row spacing with a total plot size of 34.8 m 2 . Samples were collected from the centre 2 rows. Applications of the ripeners were conducted using a high-clearance boom frame using a motorised power sprayer operating at 100 kPa pressure at an average height of 50 cm above the canopy with two flood-jet nozzles, used to reduce chemical drift effects, spaced at 50 cm apart. e spray mixtures were delivered in water volumes of 431 L ha − 1 . Spraying was conducted early in the morning when the wind was calm. e age of harvesting was 10 months at both locations.

Data Collection.
At harvest, millable stalk height and stalk weight were calculated from 20 stalks, while 10 stalks were used for juice quality viz. Brix (%) and Pol (%) measurements. e stalks were randomly selected from the net plot. Millable stalk height was determined by measuring the height of stalks from the ground to the top visible dewlap leaf. Stalk weight was determined using a weighing balance. e number of millable stalks was counted before sampling and harvesting. Cane yield was determined from the net plot area by weighing all stalks using a weighing balance and then converted to tons per hectare.
Brix (%) was determined by crushing the stalks using a crushing mill, and the juice was analysed in the laboratory using a bench refractometer (Rudolph Research, model J157). Similarly, pol (%) was determined from the same juice using a saccharimeter (Analytical Autopol 880; Rudolph  Research). Purity (%) was calculated as the ratio of pol%/brix % and multiplied by 100. Finally, the sucrose content (%) was calculated according to the equation described by Berg [30]: where 0.61 was the nonsucrose factor and 0.75 was the crop factor. en, sucrose yield (t ha − 1 ) was determined by multiplying the cane yield (t ha − 1 ) obtained by the sucrose content (%) of cane.

Data Analysis.
After verifying the homogeneity of error variances, a combined analysis of variance was done using PROC GLM procedure in SAS, version 9.2 [31]. Comparisons of the treatment means with significant differences for the measured parameters were done using Tukey's studentised range (HSD) test at 5% level of significance. Normality of the data was assessed using the Kolmogorov-Smirnov test.
e economic feasibility of the ripener treatments was assessed using partial budget analysis following the procedures of CIMMYT [32]. e average experimental sucrose yield results were adjusted downwards by 10% to reflect the difference between the experimental plot yield and the yield that the sugarcane plantations would expect from the same treatment under their own management [32].
Sales revenue was determined by multiplying the adjusted sucrose yield by the selling price of sucrose in USD (Unite States Dollar). en, the gross field benefit for each treatment was determined by adding the saving from harvest and transport and sales revenue. Saving from harvest and transport refers to the cost of harvesting and transport saved due to reduction in cane yield by some of the ripener treatments. us, savings were determined by multiplying the amount of yield reduced in each ripener treatment and the cost of harvesting and transport. e cost of harvest and transport was fixed at USD 4.5. e variable cost (cost of ripening) includes chemical cost and spraying cost. Cost of chemical ripeners per hectare for Ethephon ™ , Fusilade Forte ™ , and Moddus ™ were USD 30.00, 23.00, and 33.33, respectively. e cost of spraying per hectare (including the labour cost), which was assumed to be applied using a drone, was estimated to be USD 5.625. Selling price of sugar was taken to be 0.62 USD kg − 1 .
Net benefit (NB) was calculated by subtracting the total costs that vary (total cost of ripening) from the gross field benefits for each treatment.
e marginal rate of return (MRR) was calculated by dividing the difference between the net benefit of the treatment and the control (unsprayed) to variable cost (cost of ripening). For a ripener treatment to be regarded as a worthwhile option to the sugarcane plantations, the minimum marginal rate of return (MRR) was assumed to be 100% [32]. To check the marginal analysis results, residuals (returns on investment) were calculated for each treatment. e residuals calculated were used to indicate the difference between the net benefits and the cost of the investment.

Stalk Height.
e combined analysis over the two locations showed a significant (p � 0.025) interaction effect of ripener with variety on stalk height. However, none of the remaining interactions with ripeners were significant (Table 1) respectively. In contrast, the longest stalk height (1.64 m) was recorded in the control (unsprayed) treatment of variety NCo334 (Figure 2). e difference in response among the varieties to chemical ripeners might have been due to their innate genetic differences. In line with this, reports from different sugar industries confirmed the difference in response to chemical ripeners [27,33]. Physiologically, 2-chloroethylphosphonic acid reduces the growth sink demand for sucrose due to the reduction in the lamina size and mass as a result of the release of ethylene [24]. e current research results agree well with previous studies wherein 2-chloroethyl-phosphonic acid did not cause significant stalk height reduction in sugarcane [19,27]. According to Rostron [34], a reduction in stalk growth by shortening of one or two internodes may occur, but it was confirmed to be transitory. Contrarily, Abo El-Hamd et al. [35] showed a significant reduction in stalk height due to 2chloroethylphosphonic acid treatment. e reduction in stalk height due to Fusilade Forte ™ treatment was caused by the translocation of the active compound fluazifop-p-butyl to the stalk apical meristem where it terminates stalk growth [24,34] and restricts leaf growth [34]. Abo El-Hamd et al. [35] also reported that fluazifop-p-butyl treatments reduced stalk height independent of the rates considered.
In the same way, the reduction in stalk height due to the treatment with Moddus ™ was caused by the reduction of internode elongation, which resulted from the blockage of gibberellic acid GA 20 to GA 1 conversion process within the sugarcane stalk [19,27]. Similarly, van Heerden et al. [19] revealed that trinexapac-ethyl applied at the rate of 200, 250, and 500 g ai ha − 1 caused a rapid, and near-complete, inhibition of stalk growth up to 56 days after spraying. However, subsequent resumption of growth depended on the application rate. e stalk height reduction due to treatment with the Ethephon ™ + Fusilade Forte ™ combination treatment (Figure 2) predominately originated from fluazifop-p-butyl contained in Fusilade Forte ™ [24,34]. On the other hand, the reduction in stalk height due to treatment with the Moddus ™ + Fusilade Forte ™ combination ( Figure 2) could be due to the reduction in stalk elongation from the synergistic effect of both the active ingredients fluazifop-p-butyl and trinexapac-ethyl [19,24].

Stalk Weight.
e ripener main effect on stalk weight (  (Figure 3), although actual 4 International Journal of Agronomy reduction in stalk weight, relative to the control treatment, was relatively small. However, Ethephon ™ was statistically at par with the control (unsprayed) treatment ( Figure 3). e largest reduction in stalk weight was recorded in the Moddus ™ + Fusilade Forte ™ combination treatment (8.36%), while the lowest reduction in stalk weight was obtained from the Moddus ™ treatment (6.31%).
e reason for the relatively small reduction in stalk weight might have been due to varietal differences because not all varieties showed significant reductions in stalk height in response to ripener treatments ( Figure 2). Additionally, the effective ripening (increase in sucrose mass per stalk), as indicated by the increase in sucrose concentration by the various ripener treatments (Figure 4), could also have partially counteracted the effects on stalk height (Figure 2).

Number of Millable Stalks.
e number of millable stalks (stalk population) was not significantly affected by the ripener main effect as well as by its interactions (Tables 1 and Figure 3). Similar to the current result, Lu et al. [36] also demonstrated the negligible effect of 2-chloroethylophsphonic acid on stalk population. In contrast, Abo El-Hamd et al. [35] reported a significant reduction in stalk population due to 2-chloroethylphosphonic acid applied at a higher rate (1143 g ai ha − 1 ). Likewise, Abo El-Hamd et al. [35] also demonstrated that fluazifop-p-butyl at a higher rate (88.3 g ai ha − 1 ) had resulted in a significant reduction in stalk population. e combination of 2-chloroethylphosphonic acid (1143 g ai ha − 1 ) + fluazifop-p-butyl (88 g ai ha − 1 ) also reduced stalk population [35].

Cane Yield.
e ripener treatments did not result in a significant reduction in cane yield (Table 2 and Figure 5). erefore, cane yield did not reflect the observations of ripener treatment effects on stalk height and weight (Figures 2 and 3). e absence of significant effects on cane yield among the ripener treatments might have been due to effective ripening (increase in sucrose mass per stalk), as   indicated by the increase in sucrose content induced by the various ripener treatments (Figure 4), as well as the relatively shorter spray-to-harvest intervals [37] and lower rates used in this study [35]. Similar to the current results, different authors also reported the lack of significant cane yield reduction due to treatment with fluazifop-p-butyl [37] and trinexapac-ethyl [27,28]. Contrarily, Abo El-Hamd et al. [35] reported a significant cane yield reduction due to treatment with fluazifop-p-butyl. Similarly, Orgeron [38] reported a significant reduction of cane yield due to treatment with trinexapac-ethyl applied at 300 and 350 g ai ha − 1 .

Sucrose Content.
e sucrose content (%) was highly significantly (p � 0.001) affected by the interaction of variety with ripeners (Table 2). e highest sucrose content (11.02%) was obtained from the variety SP70-1284 sprayed with Moddus ™ + Fusilade Forte ™ , while the lowest sucrose content was obtained from the variety B52-298 (8.27%) with no spray (Figure 4). Except for variety C86-56, where none of the ripener treatments outperformed the control treatment, all the ripener treatments outperformed the control treatment in the case of varieties B52-298 and SP70-1284 ( Figure 4). However, in the case of variety NCo334, all the treatments, except for the Ethephon ™ treatment, outperformed the control (Figure 4). e difference among the varieties in sucrose content response to the chemical ripeners could be due to their innate genetic variation. Earlier works also demonstrated the variation in response to chemical ripeners and that all varieties do not necessarily respond equally well to chemical ripeners [28,39]. e effectiveness of Ethephon ™ in enhancing sucrose content is accredited to ethylene release [12]. e mechanism through which 2-chlorethylphosphonic acid increased sucrose content might be due to its potential to regulate sink strength due to its synergistic action with abscisic acid [40], which decreases the demand of sucrose for vegetative growth [24].
Fusilade Forte ™ increased sucrose content through the suppression of new tissue formation due to the interference with long-chain fatty acid synthesis in the stalk apical meristem [12]. On the other hand, Moddus ™ increases sucrose content through the inhibition of internode elongation resulting from the reduction in the gibberellic acid (GA 1 ) levels [24].
Previous studies also demonstrated the effectiveness of 2-chloroethylphosphonic acid, fluazifop-p-butyl [33,35], and trinexapac-ethyl [24] in enhancing the sucrose content of sugarcane. e effectiveness of the combination of 2chloroethylphosphonic acid + fluazifop-p-butyl in increasing sucrose content of sugarcane was also confirmed in previous studies [34,37]. Similarly, van Heerden [37] confirmed the effectiveness of trinexapac-ethyl + fluazifopp-butyl in improving sucrose content.

Sucrose Yield.
e ripener main effect was significant (p � 0.001) on sucrose yield (t ha − 1 ); however, none of the interactions with ripeners were significant (Table 2). e ripener treatments Moddus ™ + Fusilade Forte ™ combination and Ethephon ™ + Fusilade Forte ™ combination resulted in the highest sucrose yields of 12.26 and 12.18 t ha − 1 , respectively, and were the only treatments that differed significantly from the control (unsprayed) treatment ( Figure 5). e significantly higher sucrose yield obtained from the Moddus ™ + Fusilade Forte ™ combination and Ethephon ™ + Fusilade Forte ™ combination treatments could be attributed to the significant increase in sucrose content that exceeded the other treatments ( Figure 5) and the absence of any negative effect of these treatments on cane yield (Figure 4). In line with the current results, other studies [33,35] (Table 3).     (Table 3). Since net field benefit and marginal rate of return are not a final criterion for recommendation of the best ripener treatment as they do not account for returns on investment (residuals), returns on investment were also calculated (Table 3). e maximum returns on investment was obtained from the Moddus ™ + Fusilade Forte ™ combination treatment (USD 6704.90 ha − 1 ). erefore, the most economical option was derived from the Moddus ™ + Fusilade Forte ™ combination treatment with a marginal rate of return of 1244% (Table 3). In general, the marginal rate of returns obtained in all the ripener treatments compared with the control (unsprayed) treatment were greater than 1 (Table 3), which is above the minimum value (50%) assumed.

Conclusions
e results of this study showed that the two combination ripener treatments led to a considerable increase in sucrose yield of the sugarcane varieties B52-298, NCo334, and SP70-1284 at Wonji-Shoa and Metahara sugarcane plantations. e study also illustrated the influence of ripeners on stalk height and weight in some treatments and varieties, although these effects did not translate in significant effects on cane yield. It was also noted that ripener treatments did not influence stalk diameter, number of internodes, and stalk population. e superior increase in sucrose yield in the two combinations of ripener treatments was primarily attributed to the significant increase in sucrose content. However, in economic terms, the Moddus ™ + Fusilade Forte ™ combination treatment was found to be the best option. erefore, it is recommended that these sugarcane plantations should verify this experimental result at a commercial scale in the early crushing season on immature crops.

Data Availability
e data used to support the study were obtained from Ethiopian Sugar Research Main Center.

Conflicts of Interest
e authors declare that they have no conflicts of interest. International Journal of Agronomy 7