1. Introduction
Sugarcane is a perennial grass species that is native to the warm temperate regions of Asia. In Thailand, the sugarcane crop has a large impact on the Thai economy. In 2016, Thailand was the world’s fourth largest sugar exporter after Brazil, India, and China [
1]. Sugarcane is one of the primary agricultural sources of sugar. After harvesting, the sugarcane stalks are shredded, and the sugarcane juice is extracted. Raw sugar is produced from the juice, and is later refined into white sugar [
2,
3].
The Thailand Office of Agricultural Economics reported that over 103 million tons of sugarcane was produced in 2015 [
4]. Sugarcane is grown in central, northern, and northeastern Thailand. The northeastern region has the largest growing production area. The main cultivation areas are in the Kanchanaburi, Suphanburi, Udonthani, Chaiyaphum, Nakhonratchasima, Khonkaen, and Nakornsawan provinces [
4]. In 2015, the northeastern region produced 41.5 million tons of the 103 million tons of sugarcane produced in Thailand, while the northern and central regions generated 29.6 and 31.8 million tons, respectively.
The annual sugarcane plantation area of Thailand is 0.48 million ha. An area of 1 ha produces approximately 11 tons of sugarcane leaves and residues per year (
Figure 1). This is reflected by the 11 ton·ha
−1 of waste leaves burned each year that are not returned to the soil [
5]. Burnt cane is easy to handle. Sugarcane milling should be performed as soon as possible to maximize sugar recovery. Green cane harvesting is not considered ideal, because it results in a large amount of sugarcane residue. Farmers usually burn the residues to avoid impacting (clogging) the implement used for the cultivation of the next ratoon cane. Most of the farmers prefer not to burn residues, as they can contribute a large amount of organic matter to the soil [
6]. Burning sugarcane destroys soil organic matter, and thus frequent sugarcane farming results in decreased soil organic matter, poor soil physical properties, and low water infiltration and retention. Poor soil affects plant nutrients, particularly nitrogen, and returning leaves and other residues can reduce nitrogen fertilizer use. However, some farmers continue to burn residues before preparing the land [
5]. Burning sugarcane debris, especially leaves and stems, produces carbon dioxide gas, which is one of the major greenhouse gases (GHGs) that is released to the environment [
7,
8].
There are two prototypes of inter-row cultivators used in sugarcane fields in Thailand. The first prototype, designed by Chainarong [
6], is a rotary tiller attached to a 21-kW tractor with a tilling width of 80 cm, and is used for mixing sugarcane residues. The rotary tiller with 18 European L-shaped blades is powered by the power take-off (PTO) shaft of the tractor. The performance of the rotary tiller was evaluated in a sugarcane field with a clay loam soil at a moisture content of 17.2% on a dry basis (db). The experimental field measured 45 × 50 m, and consisted of a crop that was ratooned two weeks earlier. A large amount of sugarcane residue was observed, which remained following the harvest of the earlier sugarcane crop. Tests were conducted with the tractor's forward speed at 1.0, 1.5 and 2 km·h
−1. It was found that mixing up to 95% of the residue was possible with this rotavator. The forward speed of the tractor did not affect the performance of the rotary tiller. During the field performance of the rotary tiller, it was observed that the average field capacity was 0.18 ha·h
−1, and the field efficiency was 78%. The lower field capacity in terms of working hours may be due to the limitations in the specified speeds used in the study.
The second inter-row cultivator prototype was designed by Sngiamphongse [
9]. They developed an inter-row cultivator that can be mounted on a tractor (
Figure 2). This implement is offset at the rear of the tractor and is attached by three-point hitches. The working width is 90 cm. The PTO of the tractor is transmitted via a 67.14-kW gearbox, drive shaft, and driving chain system to the rotor shaft, which has a speed of 500 rpm. The rotor shaft has four flanges; each flange is fitted with six European C-shaped blades in a spiral arrangement. Test results from Khon Kaen Province at a soil moisture content of 7.88% (db) indicated that using European C-shaped blades with thicknesses of 8 mm and 6 mm resulted in an average fuel consumption of 25.67 L·ha
−1. The capacity of this arrangement was 0.3 ha·h
−1 with 89.64% field efficiency. Sugarcane leaves covered 96% of the soil surface. However, an inter-row cultivator attached to a 63.4-kW tractor is rather expensive, and has high fuel consumption [
9].
Before the rainy season, a limited amount of time is available for land preparation, cane planting, and the cultivation of ratoon cane. Within one to two weeks after harvesting, sugarcane residues need to be chopped and mixed into the soil. Performing this activity using a small tractor (21 kW) will take more time, but a larger tractor (63.4 kW) will be expensive. With this in mind, the objectives of this research were to develop an inter-row cultivator to be mounted on a medium-sized tractor (25.3 or 37.3 kW) for sugarcane fields after harvesting by hand, and to assess its use for mixing sugarcane residues in the field after harvesting using a sugarcane harvester.
3. Results and Discussion
A field test was conducted in a sugarcane field in Bo Phloi, Kanchanaburi Province, Thailand. The soil consisted of 30% sand, 32% silt, and 38% clay (i.e., a clay loam), and supported a ratoon crop. Before the test run, the field was found to contain a very large amount of widely spread sugarcane residues.
Figure 6 shows the sugarcane residues after harvesting by hand.
Figure 7 shows the sugarcane residues after harvesting using a sugarcane harvester. The working inter-row cultivator is shown in
Figure 8. After the test run, the sugarcane residues were mixed into the soil using the inter-row cultivator (
Figure 9).
3.1. Inter-Row Cultivator Attached to the 25.3-kW Tractor
The specifications of the 25.3-kW tractor are listed in
Table 1. The results of the field test are listed in
Table 2. The initial length of the sugarcane leaves collected after harvesting by hand was, on average, 9.62% longer than that after harvesting using the sugarcane harvester. The weight of the residue left per unit area on the soil surface was not significantly different among the fields. The forward speed of the machinery, the theoretical field capacity, and the effective field capacity in all field tests were not significantly different. The field efficiency of the machinery after harvesting using the sugarcane harvester was 3.23% higher than that after harvesting by hand, and 5.64% less fuel was consumed. The length of the sugarcane leaves after harvesting by hand was 40.80% longer compared with the sugarcane harvester. Soil inversion was greater (2.08%) after harvesting using the sugarcane harvester compared with harvesting by hand.
3.2. Inter-Row Cultivator Attached to the 37.3-kW Tractor
The specifications of the 37.30-kW tractor are listed in
Table 1. The results of the field test are listed in
Table 3. The initial length of the sugarcane leaves collected after harvesting by hand was, on average, 5.62% longer than that after harvesting using the sugarcane harvester. The weight of the residue left on the soil surface per unit area was not significantly different among the fields. The forward speed of the machinery, the theoretical field capacity, and the effective field capacity in all field tests were not significantly different. The field efficiency of the machinery after harvesting using the sugarcane harvester was 3.23% higher than that after harvesting by hand, and fuel consumption was 5.86% lower. The length of the sugarcane leaves after harvesting by hand was 44.04% longer compared with the sugarcane harvester. Soil inversion was greater (1.71%) after harvesting using the sugarcane harvester than after harvesting by hand.
3.3. Inter-Row Cultivator Attached to the 25.3-kW Tractor at Different Thicknesses of the Trash Blanket
The results of the field test are listed in
Table 4. The initial length of the sugarcane leaves was not significantly different among the fields. The weight of the residue left on the soil surface collected per unit area was 11.11% higher when the trash blanket thickness was 18 cm than when it was 10 cm. The forward speed of the machinery and the theoretical field capacity were not significantly different, whereas the effective field capacity differed by 3.23%. When the trash blanket thickness was 10 cm, the field efficiency of the machinery was 3.23% higher and the fuel consumption was 14.68% lower compared with a trash blanket thickness of 18 cm. When the trash blanket thickness was 18 cm, the length of the sugarcane leaves after harvesting was 39.68% longer than when the trash blanket thickness was 10 cm. The residue weight and soil inversion were not significantly different among the fields.
3.4. Inter-Row Cultivator Attached to the 37.3-kW Tractor at Different Thicknesses of the Trash Blanket
The results of the field test are listed in
Table 5. The initial length of the sugarcane leaves was not significantly different among the fields. When the trash blanket thickness was 18 cm, the weight of residue left on the soil surface collected per unit area was 14.85% higher than when it was only 10 cm thick. The forward speed of the machinery and the theoretical field capacity were not significantly different, whereas the effective field capacity differed by 6.25%. When the trash blanket thickness was 10 cm, the field efficiency of the machinery was 6.25% higher, and the fuel consumption was 10.51% lower compared with a trash blanket thickness of 18 cm. The length of the sugarcane leaves was 57.42% longer when the trash blanket thickness was 18 cm, compared with a thickness of 10 cm. The residue weight left on the soil surface and soil inversion were not significantly different among the fields.
From
Table 2,
Table 3,
Table 4 and
Table 5, we determined the performance of the developed inter-row cultivator, which was satisfactory in terms of residue burial. Soil inversion was higher than 90% in all tested fields. This indicates the effectiveness of the cultivator in chopping and mixing the sugarcane residue into the soil. The initial length of the sugarcane leaves collected after harvesting by hand was longer than that collected after using the sugarcane harvester. The weight of the residue left on the soil surface per unit area and the forward speed of the machinery were not significantly different among the fields. The fuel consumption of the machinery after harvesting by hand was higher than that after harvesting using the sugarcane harvester. However, when considered in relation to the thickness of the trash blanket, the initial length of the sugarcane leaves, the forward speed of the machinery, the theoretical field capacity, and soil inversion were not significantly different. Moreover, the effective field capacity and field efficiency when the trash blanket had a lower thickness were higher than those measured under a thicker trash blanket. In addition, fuel consumption at the lower thickness of the trash blanket was lower than that at the higher thickness of the trash blanket.
3.5. Economic Analysis of the Machinery
From Equations (5)–(7), along with
Table 6, we conducted a break-even analysis to quantify the benefits of purchasing a 25.3-kW tractor and an inter-row cultivator. It was determined that at least 48.15 ha would be needed to reach a break-even point (
Figure 10). Moreover, for a 37.3-kW tractor and an inter-row cultivator, cultivation of at least 101.81 ha would be needed to reach the break-even point (
Figure 10).
Eliminating sugarcane residues after harvest costs time and labor. Generally, farmers burn the residues, which causes environmental problems. To overcome this, an inter-row cultivator that incorporates residues into the soil was developed using European C-shaped blades. The inter-row cultivator was powered by a tractor PTO shaft. The performance of the developed inter-row cultivator was quantified in terms of residue burial. Field tests confirmed that the cultivator effectively chopped and mixed the residues into the soil.