The harvesting time of stevia is generally controlled by growth behavior and accumulation pattern of the steviol glycosides. The exact harvest schedule thus depends on the specific Stevia cultivar used and the growing season. The first harvest is generally done after 4 months of planting and the next harvests are done after every three months. This harvest schedule is generally followed in tropical and subtropical regions. In colder climates, the number of harvests in a year is generally less. There are some places, particularly in Mainland China, where only one harvest is done in a year.
Harvest is generally done just before flowering, because after flowering, the steviol glycoside content of the leaves always goes down.
In India, highest dry leaf yield was recorded with the perennial stevia under three harvests in a year management system but remained statistically at par with two harvests in a year management system. However, two harvests in a year produced maximum amount of steviol glycoside (10.29 g plant−1) (Pal et al: Harvesting regimes to optimize yield and quality in annual and perennial Stevia rebaudiana under sub-temperate conditions: Industrial Crops and Products, Volume 65, March 2015, Pages 556–564).
For harvesting, stevia plants are cut at approximately 4 inches (10 cm) height from ground. Some experts suggest cutting at 6 inches (15 cm) height. A variety of manual and mechanized harvesting system can be adopted for stevia harvesting. After harvesting, often a prophylactic fungicide is applied to control fungal infection at the cut end of the stem. Some harvesting methods are described below:
Manual harvesting is often the preferred method for small plantations. It is often practiced in the first harvesting in large plantations also in the initial phases of growth. In the first harvest, the plant heights are often not same, hence mechanical pruning may not be effective in this stage. Moreover, mechanical harvesting, specially the harvesting processes in which the leaves are plucked from the plants with rotating rubber rollers, may uproot young plants without pervasive root system. In this system of harvesting, plants are cut with a sickle or a scythe. The harvested parts of the plants are manually gathered together for post harvest processing.
Sickle Bar Mower
The cheapest mechanized option is small walk behind sickle bar mower/harvester.
Sickle mowers, also called reciprocating mowers, bar mowers, sickle-bar mowers, or finger-bar mowers, is a small engine driven equipment. These machines have long horizontal bars (from 16”/40 cm to 60”/150 cm) and the bar have one set of stationary and another set of sideways moving teeth on it. All the teeth are triangular in shape and the stationary and the moving teeth act like a set of scissors. The plant stems are cut cleanly by this scissors. The bar rides on the ground, supported on a skid at the inner end, and it can be tilted to adjust the height of the cut. The action of the machine can be likened to an electric hair clipper.
This machine is suitable for small plantations and especially where the terrain is undulating. The cut part of the plants remains on the ground, which can be dried there, or can be collected by manual or mechanized rakes and transported to the post harvest facility.
Modified Rice Reaper in Chinese Plantations
In Chinese plantations, a form of modified dry-land rice reaper is used for stevia harvesting. The reaper consists of engine, power transmission box, pneumatic wheels, cutter bar, crop row dividers, conveyor belts with lugs, star wheels, operating controls and a sturdy frame. The engine power is transmitted to cutter bar and conveyor belts through belt pulleys. During forward motion of the reaper, crop row dividers divide the crop, which come in contact with cutter bar, where shearing of crop stems takes place.
The cut crop is conveyed to one side of the machine by the conveyor belt fitted with lugs and is windrowed in the field. The crop is bundled manually and transported to drying yard. This type of machines can reap 3 hectares in one hour.
Disc or drum mowers are often recommended for stevia harvest, but they have some serious draw backs especially for stevia harvest. The sideways sheer force of the rotary blades often crushes the leafs and stems and thus there may considerable leaf damage if these machines are used. Both the mowers throw rocks and clods and thus impose serious safety issues to the user. They need tractor with strong hydraulics for proper operation and may not be very suitable for undulating terrain.
Eurostevia launched the stevia harvester designed by them, which also picks the leaves and does not cut the plants. This harvester is of considerably smaller dimensions and is pulled by a tractor. The original machine was designed to be used on a bed 1.5 meters wide. In this machine, the leaf is plucked by meshing rollers with speed differential. Though this machine is expected to perform well in older plantations, it may create the same uprooting issue in early stage plantations.
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Fully Mechanized Oxbo Stevia Harvester
Oxbo International Corp, field tested their stevia harvester in 2010 – 11. This is a modified bean harvester. Oxbo’s development work on stevia harvesting was focused on leaf stripping with their VPCII (Variable Position Concave II) picking head. The header uses special, pneumatically controlled helix-shaped strippers to remove the pepper from the plant. That picking head can harvest multiple crops, delivers row independent operations and 3 different head widths. The VPCII head can efficiently and effectively strip stevia leafs. Some of their systems have continuous elevator discharge and are capable of plant cutting as well as leaf stripping. The systems are generally configured to harvest 2.0 meters wide raised beds.
Information received through personal communication revealed that, this harvester may not be very suitable for early stage plantations specially in leaf stripping mode, since the plants are susceptible to uprooting in absence of well developed root systems. In later staged, when the root system becomes much more pervasive and the plants get a very good anchoring in the soil, this harvester system can be very useful.
Harvesters with different designs are used in plantations in South America, but no specific technical details are available for them. Most of them are developed from forage harvesters.
Emergence of new shoots after harvesting
Selection of optimal harvest method depends on the plantation system, the scale of farming, variety, local agro-climate, soil condition and local man power economics. No single harvesting system is suitable for all locations and plantation economy heavily depends on selection of proper system. There is no guarantee that the most modern mechanization will get translated into most favorable farm economics. In some cases, even the most primitive system may be the most optimal.