spp. hybrid) is a globally important crop for sugar and biofuel production. China is one of the largest sugarcane producers in the world, which produced 105 million tons of millable canes in 2017 [1
]. In China, sugarcane is grown mostly in the hilly areas of Guangxi, Yunnan, and northern Guangdong provinces. A survey of 38 counties in Yunnan Province indicated that 71% of sugarcane was grown on slopes >6°, including 16% on slopes >25° [2
]. Further, most sugarcane is grown by small land-holders averaging 0.6 ha per grower [3
]. The geographical complexity and small land-holders make conventional combine harvesting impracticable. Only 2% of sugarcane in China was mechanically harvested during the 2017–2018 milling season [4
Small-scale mechanical harvesting is an alternative for the sugarcane growing areas with a labor shortage. A single worker can manually harvest 1–2 t day−1
, compared to 200–300 t day−1
for a combine harvester [5
]. A small-scale harvester 4GZ-9 (total weight 1140 kg) was designed for harvesting sugarcane in a hilly area in Guangxi province. Its capacity was chopping down 7.5–10 t of whole stalk per hr (in the sugarcane field yielding 75 t ha−1
), aided with a small-scale leaf and cane top removing machine; the clean stalk harvesting capacity was approximately 4 t hr−1
. To harvest 1000 t of sugarcane, the costs for manual harvesting (MH) and small-scale mechanical harvesting using a 4GZ-9 harvester were 150,000 and 31,250 RMB, respectively. Thus, the cost of small-scale harvesting was 21% of MH [6
]. While MH creates more jobs for laborers [7
], the labor pool for sugarcane harvest is shrinking, and farm workers receive much lower wages than they may earn in other industries or in cities [6
]. It is imperative that the sugarcane industry develop small-scale harvester technology to ensure the sustainability of sugarcane production.
The harvesting process is an external disturbance to sugarcane growth in consequent ratoon crop (RC), and the strength of disturbance differs with harvesting methods, so the strength is a key factor to determine whether harvesting is done by hand or by machine. Combine harvesting increases soil compaction (SC) [8
] and stool damage for consequent RC [10
]. Tractor-caused compaction significantly increases in a soil depth of 10–20 cm, causing decreased sprouting by 11%–28%, and decreased plant height by 9%–12% [11
]. Thus, sugarcane yield can be negatively affected by mechanical harvesting [12
]. As the number of mechanical harvests and tractor passages increases, the cane yield tends to decrease compared to treatment without tractor passages [14
Sugarcane is a vegetative crop that allows several ratoon crops (RCs) after the plant crop (PC) is harvested. Post-harvest buds that survive underground on the stool sprout to produce stalks in the consequent RC. According to the concept of underground bud bank (UBB) of other vegetative grasses [15
], the total amount of buds that remain in a certain area after harvesting a previous crop becomes the UBB in sugarcane. Mechanical harvesting and tractor passages can have adverse effects on soil structure and may also negatively affect the constitution of the UBB by damaging ratoon stools compared to MH. SC significantly decreases the concentration and plant uptake of N, P, and K in wheat [16
], reduces leaf area and dry matter accumulation in sugar beet and field bean [17
], and significantly decreases sugarcane root growth [12
]. The compacted soil may also induce stress on the sprouting and early growth of the subsequent RCs in sugarcane. Added passages of field equipment during transportation of millable cane also contribute to SC.
There have been limited studies on the UBB of sugarcane, even though this is an essential source for sprouting and early crop establishment. There has also been limited information on the impact of small-scale harvesting equipment on early growth and cane yield in the RC of sugarcane; if the UBB and ratoon sprouting is related to the harvest method, changing from MH to small-scale harvesting will result in changes to the UBB and ratoon sprouting; if SC is related to early sugarcane growth, harvest methods that compact soil should affect early sugarcane growth. There is no information on the genotype × treatment (GT) interaction if MH and small-scale mechanical harvesting followed by tractor passages (SMH) are considered as two treatments.
To our understanding, there is no literature reporting the GT interaction dealing with this specific interaction between genotype and treatments of MH and SMH. If the sugarcane production changes to SMH, will the genotype selected from trials under MH management suit the SMH? However, to serve a specific production system, for example, breeding for rain-fed production conditions had been expected to be conducted under the targeted condition even complicated by irregular weather conditions. Reported findings of litter or no interaction between genotype and treatments of water regimes, i.e., irrigated and rain-fed conditions for cane yield and components suggested that a genotype selection trial conducted under irrigated conditions might also be suited for rain-fed conditions [19
]. Current sugarcane breeding programs are conducted under MH in China with the aim of supporting sugarcane production under MH. So, understanding the genotype and harvesting method (MH and SMH) interaction may help improve the breeding program for production with SMH.
The objectives of this study were to determine if the SMH impacts SC, UBB, early growth, and cane yield in RCs, and to determine the GT interactions for traits describing early growth and final cane yield in sugarcane.
As compared with MH, the SMH significantly increased the stool damage for the consequent RCs and caused significantly greater SC. Under the condition of more gaps and greater SC, the increased stool damage led to more gaps in the RCs, and the greater SC restricted the germination of the UBB, and early crop growth in sugarcane. The SMH had adverse effects on early growth, cane yield, and most of the yield components. Currently, most sugarcane breeding trials are still conducted under MH in China. The important findings in this study related to significant GT interaction for stool damage, gaps, and seedlings in an earlier period (April), and height uniformity at the maturing stage and final millable stalks suggested that the genotype selection trial done under the MH system may not suit the production system with SMH well. As compared with the number of buds for establishing the plant cane, buds in the UBB were more than enough for establishing an RC. More focus for better RC establishment should be on improving the soil condition for germination of the UBB.
4.1. Soil Compaction, Stool Damage, and Gaps
Sugarcane is a perennial C4 grass, and after the PC is harvested, the underground buds regenerate the RCs. RCs are extremely important in the economics of sugarcane production, and the proper management of RC is fundamental to sustainable production. In Australia, Cuba, Philippines, and Mauritius, six to eight RCs are commonly harvested from a single sugarcane planting [30
]. However, as the number of RCs increases, gaps increase, which tends to reduce sugarcane yield and, ultimately, necessitates replanting.
In this study, we found that the SMH tended to increase SC sharply compared to the MH. The subplots subjected to SMH showed significantly greater SC at all the soil depths from 5 to 30 cm (Figure 2
). As the sugarcane planting depth was approximately 10 cm, the vertical distribution of the UBB in RCs was within the depth of 0–10 cm, where the SC would be greater due to the SMH. An increase in SC would physically constrain bud sprouting and restrict water absorbance by the buds, and SC is a major limitation to root elongation and root dry weight [31
]. On sandy loam, the SC at 10 cm depth was 1.98 MPa in MH plots and increased to 2.95 MPa in the plots harvested by a combine harvester (CASE7000) [32
]. In this study, at the same depth of 10 cm, the SC was 1.70 and 2.10 MPa for the SMH in the first RC and second RC, respectively. Even with a much lower SC by SMH than the combine harvester, root elongation rate ceased at root penetration resistances of about 1.0 MPa [33
]. The SC was close to or greater than 1.0 MPa in the MH plots 10 cm or below in this study (Figure 2
). This is probably why farmers tend to relieve the ratoon stools after the previous crop is manually harvested in Yunnan. In this study, the tractor rolled directly on the row for the specific purpose of maximizing potential SC and crop damage. In sugarcane production, reducing traffic over the row mitigates the adverse effects of compaction on RCs in the mechanical harvesting [14
Significant genotype differences were found under mechanical harvesting conducted by a combine harvester [10
], which implied the possibility of selecting genotype for less stool damage under the harvesting conditions. In this study, significant genotype differences for stool damage in two RCs and for gaps in the second RC suggested the possibility for selecting genotype for less stool damage and gaps under the SMH. However, the significant GT interaction for stool damage in both RCs and for gaps in the second RC (Table 2
) suggested that genotypes selected in a manual harvesting system might not be suited for SMH with respect to stool damage and gaps.
4.2. Underground Bud Banks and Sprouting
The sprouting of the UBB is the initial stage in establishing an RC. The UBB of sugarcane consisted of the number of remaining stools in the soil and the number of buds on the underground part of stools (buds per stool). In this study, the numbers of buds per stool were lower with the SMH in two RCs, and between the first and second RCs, the buds per stool tended to decline regardless of the harvesting method. There was no significant treatment effect on the number of buds (×1000) ha−1
in the first RC because no treatment applied to the PC, i.e., the amount of remaining stalks in soil had not been impacted by harvesting treatment. The significant difference in the second RC was mainly due to the significant effects on millable stalks in the first RC. With MH, the UBB increased by 58,000 buds ha−1
from the first to the second RC, but it decreased by 37,700 buds ha−1
with the SMH. The planting density for the PC was approximately 120,000 buds ha−1
, and the UBB ranged from 1.09 million to 1.16 million in the two RCs (Table 3
). This was nearly nine times the buds used for establishing the PC. To maintain a suitable number of seedlings for the RC, it is more important to relieve SC for promoting the germination of the buds in UBB. A significant genotype effect was observed in the second RC for buds per stool (Table 3
), and a significant genotype effect was observed in two RCs for UBB. Lack of significant GT interaction for bud per stool and UBB suggested the genotype with higher buds per stool or UBB in MH may perform relatively high in SMH.
The seedling counts, a trait describing the sprouting of RC, were significantly impacted by treatment and genotype (Table 4
). The SMH significantly increased SC (Figure 2
), stool damage, and gaps compared to the MH (Table 2
). A significantly negative correlation (r2
= 0.91) between SC and ratoon sprouting [32
] indicates that increased SC poses a great negative impact on the sprouting in RC. In addition, a lower saturated hydraulic conductivity in compacted soil [14
] may have affected the sprouting of the dormant underground buds. As compared to MH, the number of seedlings was reduced by 5537 seedlings ha−1
in the mechanical harvesting conducted by combine harvester [34
]. Seedling counts in April and May were significantly lower in the SMH and significantly affected by genotype. An interesting finding was that significant GT interaction for seedling counts was observed in April, and it became not significant in May for both RCs. This is similar to the GT interaction for ratoon sprouting, which was in a declining trend from one month to three months after the harvest of the previous crop [26
]. In sugarcane, the number of seedlings consisted of the shoots from the UBB and the tillers generated later. Typically, in sugarcane production, late tillers are prone to senesce and contribute less to the final amount of millable stalks than early-produced tillers [35
]. The impact of GT interaction on tillers was significant for the earlier observation (seedling counts in April) after the harvest of the previous crop, and it was not significant for the later observation (seedling counts in May). The observation on earlier sprouting (April) reflected the interaction better than the observation made later (May).
Larger numbers in the UBB generate more seedlings for the RC. Correlations across all subplots were significant for all seedlings counts in April and May for two RCs (Table 5
). In most cases, the trait correlations in April were lower than those in May because the late sprouting generated seedlings as well. As compared with the MH, the correlations were much higher in the SMH. Although the bud amount of UBB was more than adequate for the establishment of the RC in the SMH, the SC adversely affected the spouting, which may have led to a lower emergence rate. Therefore, the bud amount is more important, and the correlations were much higher under SMH.
4.3. Plant Height and Height Uniformity
Stalk elongation is a sensitive trait reflecting the environmental stress in sugarcane; a decrease in plant height under water stress conditions is used as a parameter for evaluating drought tolerance in sugarcane [36
]. Regardless of water or nutrient availability, water and nutrient uptake by plant roots are impacted by SC, which resulted in a significantly reduced plant height for most of the measurements (Table 6
). As compared with MH, the plant height in the combine harvesting treatment decreased by 6.21% and 6.84% during June and July, respectively, and no obvious differences were observed at the maturing stage during November and December [34
]. In this study, the plant height in SMH decreased by 14.87%–29.54% from May to July, and the decrease was merely 2.70% and 9.27% in November for first RC and second RC, respectively. This might mean that SMH impacts more on the early growth than final height. The plant height was mostly impacted by the treatment and genotype; merely one measurement at the grand growth stage was significantly affected by the interaction in this study.
The height uniformity was considered as a trait presenting individual uniformity among the whole crop with respect to plant height and was used in the evaluation of 11 maize cultivars developed from 1950 to 2010 for yield stability [25
]. Sugarcane stalk is the part that accumulates sugar and the right part to be harvested. Higher uniformity in plant height indicates relatively higher uniformity in tillers growth and contributes to higher harvest quality. Under SMH, more gaps cause uneven competition among stools, and the increased SC may also stress more on the tiller plants whose roots developed later and shorter than the main shoots. Moreover, the soil was compacted more in topsoil where the roots of later tillers may distribute in. In the SMH, the height uniformities across all measurements that started from grand growth to maturing were significantly lower than the MH, and significant GT interaction was observed at the maturing stage in two RCs (Table 6
), which suggested that selecting genotypes under MH for SMH could be difficult for height uniformity.
4.4. Cane Yield and Its Components Responses to Small-Scale Mechanical Harvest
Cane yield tends to decline with an increased number of RCs and declines faster in poor soils [37
]. As compared with favorable growth conditions in irrigated fields, rain-fed fields showed sharper declines in cane yield (6–7 t ha−1
for each crop) [38
]. In this study, as compared with the MH treatment, more compacted soil in SMH could be a less favorable growth condition. Cane yield increased slightly from the plant to the first RC and decreased by 11.36% from the first to second RC with the MH, while all decline trends were observed from the plant to the first and second RCs with the SMH, which particularly decreased 20.59% from the first to the second RC. The SMH had an adverse effect on the cane yield in two RCs, and a significant effect was found on the second RC. No significant GT interaction for the final cane yield in two RCs was in accordance with the findings by Jackson et al. [26
]. However, the previous study [26
] aimed at determining the GT interaction for final cane yield under combine harvesting in dry conditions and wet conditions.
The stalk diameter was significantly larger with SMH in the second RC (Table 7
), which was possibly caused by less competition for light under more gaps conditions, and plants beside the gaps received more light, particularly when the gaps increased from 463 m ha−1
in the first RC to 1070.30 m ha−1
in the second RC (Table 2
). Purity was significantly lower with SMH in two RCs, which may be caused by poorer height uniformity; shorter plants may not reach maturing and resulted in a dilution of the sucrose content [39
] and purity. The sugar content was not significantly affected by the treatment because sugar content is a trait that varies little across environments [37
In this study, genotypes had significant effects on cane yield and yield components in the plant and two RCs. The GT interaction had a significant effect on merely one trait, i.e., millable stalks in the second RC. However, millable stalk is an essential component for cane yield and contributes directly to the UBB. The existing significant GT interaction for millable stalks suggested that conducting genotype selection trials under MH may not be suited for SMH. In addition, under MH conditions, stool damage and gaps of genotype may perform differently from those under SMH.
As compared with MH, the treatment with SMH significantly increased SC and caused significantly greater gaps and stool damage, which negatively impacted the sprouting of the buds in underground bud bank (UBB) and plant growth, and therefore, decreased the final cane yield. Even small-scale harvesting equipment should avoid rolling over the stool and bed directly for minimizing the damage to the RC stool and soil compaction. Significant differences among genotypes were detected for all traits in this study, and particularly, the significant differences for stool damages suggested the possibility for selecting genotypes for SMH. Moreover, the existing significant GT interactions for stool damage, gaps, early seedling counts, millable stalks, and height uniformity suggested that conducting sugarcane genotype selection trials under traditional MH may not be suitable for sugarcane production under SMH. The UBB of RC for sprouting was nearly nine times the buds used for establishing PC, and therefore, relieving the compacted soil for better sprouting of the UBB may help establish better RC. In addition, further understanding of the constitution and regulation of UBB may help prolong ratoon years.