Recently, large-scale PPSLs have been constructed and started operations. This type of system is generally a closed structure and thermally insulated, so that the inside environmental conditions [e.g., light intensity, air temperature, and carbon dioxide (CO2) concentration] are hardly affected by the outside weather. Moreover, because the environmental conditions can be controlled as desired, the appropriate plant size and quality can be continuously harvested all year round. In Japan, the number of PPSLs was reported to be 197 as of Mar. 2017 (Japan Greenhouse Horticulture Association, 2017). Currently, leafy vegetables (e.g., lettuce) are the main products in PPSLs.
Intensive research has been conducted on PPSLs (Kozai et al., 2016), resulting in more PPSLs being completed; however, significant problems have arisen in terms of management (e.g., labor management), especially for large-scale systems, and so the cost of labor has become a major factor affecting operating expenses. Ohyama (2015) reported a case study on a simple cost analysis of a commercial PPSL (maximum production rate: ≈7000 plants/d). The report showed that labor was a major expense, accounting for 27% of the total expenses. To increase the revenue, the labor cost must be reduced by efficiently using human resources and/or by using mechanization or automation.
In plant production systems with natural lighting (e.g., fields and greenhouses), efficient use of human resources has been investigated through work analyses. For example, Bechar et al. (2007) conducted a time study in a tomato (Solanum lycopersicum) greenhouse based on the direct measurement of trellising and harvesting. Vitner and Bechar (2008) used the time study method to measure harvesting in screen houses where ornamental plants were cultivated. Luxhoj and Giacomelli (1990) compared the time-study method and two tabular methods: Element Times for Agriculture tables and the Maynard Operation Sequence Technique tables (Zandin, 2003) for evaluating the operations in a tomato greenhouse. To increase the accuracy and reliability of measuring the processing time, several handheld devices have been developed (Ampatzidis et al., 2012, 2013; Bechar and Eben-Chaime, 2014; Riemer and Bechar, 2016). However, for PPSLs, there is little information based on a work analysis including the evaluation of labor productivity.
The objective of this study was to evaluate labor productivity in terms of material value for a PPSL, which is defined in this study as the weight of plants harvested per hour per worker. Labor data were collected for 6 months from the start of operations in a research facility at Osaka Prefecture University (Osaka, Japan), where the maximum possible rate of plant production was ≈5000 plants/d.
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