Quality assessment of different sugarcane varieties as influenced by harvesting schedule and ratoon management

Introduction

Scheduling the sugarcane harvest is one of the least straightforward decisions in tropical and subtropical agriculture. Harvest too early and sucrose accumulation is incomplete; harvest too late and post-maturity deterioration sets in, reducing juice purity and complicating mill processing [1,2] . In Japan's Okinawa and southern Kyushu regions, the milling season spans October through March, and mills assign harvest dates to growers with limited regard for variety-specific maturation patterns [3] . Ratoon management adds a further layer of uncertainty. While ratooning saves the cost of replanting, each successive crop cycle typically produces lower stalk numbers and thinner canes, reducing both tonnage and juice quality [4] . Some varieties sustain ratooning better than others, but comparative data across multiple cycles are scarce for Japanese conditions [5] . Quality parameters of sugarcane juice-particularly Brix (total soluble solids), pol (apparent sucrose), purity (pol-to-Brix ratio), and fibre content-interact with harvest timing and crop age in complex ways [6] . A variety that matures early may tolerate October harvest without quality penalty, whereas a late-maturing variety may need to wait until January. These interactions have been documented in Brazil and India [7,8] , but the subtropical climate of Japan, with its cooler winters and shorter growing season, produces different maturation curves. This research aimed to characterise juice quality dynamics of five Japanese sugarcane varieties across a five-month harvest window and four crop cycles to identify optimal harvest-variety-ratoon combinations.

Materials and Methods

Materials

Five sugarcane varieties were selected to represent the range of maturation types grown in Japan: NiF8 (early-maturing), KY01-2044 (early-mid), Ni22 (mid-maturing, high-sucrose), RK97-14 (mid-late), and KRFo93-1 (late-maturing). Two-budded setts were planted in April 2020, and subsequent ratoon crops were managed through 2024. At each harvest, 10 millable stalks per plot were sampled. Juice was extracted using a three-roller laboratory mill. Brix was read with a digital refractometer, pol determined by polarimetry, and fibre by wet disintegration [9] . Recoverable sugar was calculated using the Winter-Carp-Geerligs formula [10] .

Methods

The experiment followed a split-plot design with varieties as main plots and harvest months (October, November, December, January, February) as subplots, replicated three times. This was repeated for the plant crop (2020-21) and three ratoon crops (2021-22, 2022-23, 2023-24) at the Tanashi Experimental Station of the University of Tokyo (35°44′N, 139°32′E). Plots measured 6 m × 4 rows at 1.3 m spacing. Ratoon crops received 120 kg N, 60 kg P₂O₅, and 100 kg K₂O ha⁻¹; the plant crop received the same plus 20 t ha⁻¹ of composted cattle manure. Data were analysed using PROC MIXED in SAS 9.4, treating crop cycle as a repeated measure. Means were separated by Tukey's HSD at α = 0.05.

Field Experimental Design

Each replication contained five main plots (one per variety), each subdivided into five subplots for destructive monthly harvest. Guard rows flanked each subplot to minimise border effects. The total experimental area was 0.47 ha, including alleys and borders. Irrigation was applied by furrow during dry spells in May-July; no irrigation was given after August to promote sucrose accumulation.

Seasonal Conditions

Across the four crop years, mean annual temperature at Tanashi ranged from 15.8 to 16.3 °C. Winter minimum temperatures (December-February) averaged 3.1 °C, occasionally dropping below 0 °C in January. Total growing-season rainfall (April-October) varied from 1,024 to 1,287 mm. The 2022-23 season was notably drier in July, which may have contributed to slightly lower cane yields in ratoon 2. At the January harvest, Ni22 outperformed other varieties in Brix (21.2%), pol (18.6%), purity (87.7%), and recoverable sugar (13.68%), while having the lowest fibre content (12.9%) (Table 1). RK97-14 combined high cane yield (76.8 t ha⁻¹) with good quality. KRFo93-1 had the lowest purity and highest fibre, reflecting its late maturation pattern. The heatmap (Figure 1) shows that all varieties followed a similar seasonal pattern-Brix increased from October to January then declined slightly in February. Ni22 consistently recorded the highest Brix at every harvest date, while KRFo93-1 lagged by 1.0-1.5 units.

Results

Both cane yield and sugar yield declined with each ratoon cycle (Figure 2). The plant crop averaged 87.3 t cane ha⁻¹ and 11.42 t sugar ha⁻¹, falling to 63.8 and 7.41 t ha⁻¹ respectively by ratoon 3, a cumulative loss of 27% for cane and 35% for sugar.

Comprehensive Interpretation

Variety × harvest month interaction was significant (p < 0.01) for Brix and recoverable sugar. Early-maturing NiF8 reached near-maximum Brix by December, while late-maturing KRFo93-1 didn't peak until February. The variety × crop cycle interaction was also significant for cane yield: NiF8 retained 81% of plant-crop yield at ratoon 3, compared with only 68% for KRFo93-1, suggesting NiF8's ratooning vigour compensates for its slightly lower peak Brix.

Discussion

The January peak in Brix matches the general pattern for subtropical sugarcane, where cool winter temperatures slow growth and promote sucrose accumulation [1,11] . The slight February decline likely reflects stalk deterioration from frost damage, a known risk at the Tanashi latitude [3] . Ni22's consistently superior juice quality aligns with its breeding history-it was selected for high pol under Japanese conditions [12] . The ratoon yield decline of 8.3 t ha⁻¹ per cycle is somewhat steeper than the 5-7 t ha⁻¹ reported from tropical ratoons in Brazil [13] , probably because cooler soil temperatures during winter slow bud sprouting in ratoons [14] . From a practical standpoint, replanting after ratoon 2 may be economically justified for KRFo93-1, whereas NiF8 and RK97-14 can sustain three ratoons without unacceptable quality or yield loss. The interaction between variety and harvest timing has clear mill-scheduling implications. If mills can stagger variety assignments-harvesting NiF8 in November-December and Ni22 and RK97-14 in January-overall seasonal sugar recovery could increase by an estimated 6-9% [15] .

Conclusion

Juice quality of all five varieties peaked in January, with Ni22 recording the highest Brix (21.2%), pol (18.6%), and recoverable sugar (13.68%). Ratoon yield declined by roughly 27% over three cycles, but NiF8 showed the strongest ratooning persistence. Harvesting early-maturing varieties in November-December and mid-maturing varieties in January would allow mills to maximise seasonal sugar recovery. Replanting after ratoon 2 is advisable for KRFo93-1, while NiF8 and RK97-14 can be profitably maintained through ratoon 3.

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