The Mechanical Harvesting of Hemp Using In-Field Stand-Retting: A Simpler Approach Converted to the Production of Fibers for Industrial Use

Abstract

The mechanical harvesting of hemp is a key step toward a profitable use of the product. Various fractions (fiber, seeds, residual biomass) may be recovered, and their correct management is fundamental for complying with the requirements of processors/end users. In the light of the renewed interest for its industrial use (panels and insulators), this work proposes the use of modified commercial machines to implement a field separation of the fibrous fraction of stand-retted hemp, a practice that would be profitable if realized with the systems adopted for textile use. The present work was conducted to test the efficiency of harvesting partially macerated plants by using a modified self-propelled forage harvester (SPFH). In Northern Italy, a hemp crop was stand-retted for four months. Then, an SPFH—with rotor knives reduced in number from 24 to 12—was used. Stand-retting made it possible to separate cortical fibers from the inner stem cylinder during harvesting; 53.3% of the material (fibers and shives) was separated automatically by the SPFH together with the chopped bast fiber, while the remaining 46.7% was separated on exiting the launch tube. More than 50% of the fibers were shorter than 5 cm in length, while almost 15% were longer than 10 cm. The SPFH had an effective operating speed of 3.48 km h−1, and no clogging occurred during the test. Therefore, the combination of stand-retting with harvesting using a modified SPFH could be helpful in obtaining an early separation of fibers from shives, thus facilitating the product treatment during its subsequent processing, e.g., by enhancing the defibration.

Hemp: From Field to Fiber—A Review

“…The harvesting process is reported as one of the main factors contributing to the profitable development of the hemp supply chain, due to a lack of developed machines suitable for the morphological characteristics of hemp, leading to a high level of specialization and thus increasing the overall cost of harvesting [28]. Therefore, one of the main problems with hemp harvesting for fiber applications is the clogging of improper machines due to tough fibers [29].…”

“…Therefore, one of the main problems with hemp harvesting for fiber applications is the clogging of improper machines due to tough fibers [29]. Hence, some modifications or device couplings to the current machines used for harvesting hemp stems are necessary [28]. Therefore, the first step in harvesting involves cutting the stems and air-drying them in the field, and this can be achieved using a sickle bar mower to lay the hemp [17,29].…”

“…During the retting process, or the drying process in the field, which will be discussed further in greater detail, the stems need to be turned using a wheel rake before baling either in big round bales or big square ones for transportation. Now, there are developments in this area to join cutting and bailing [17,[28][29][30]. In harvesting hemp for seeds, combine harvesters are normally used for corn production.…”

“…Also, hemp grain must be harvested with more moisture for less shattering to occur than crops like corn. Therefore, alternatives have been suggested to help overcome these problems, such as the introduction of other equipment like draper heads [17,28]. An important breakthrough in harvesting industrial hemp is to harvest the hemp seeds and leaves and simultaneously cut the stems for fiber extraction.…”

Extraction of cellulose fibers from flax and hemp: a review

“…The advantage is that stand retting does not require swathing and turning, and the plants after retting may directly be harvested and transported for further processing. Alternatives to the spraying of herbicides include the use of open flames to terminate plant growth or allow the plants to die naturally (Assirelli et al 2020;Ramesh 2018).…”