Significant research on and development of automated thinners was conducted in the 1960s and 1970s due to a shortage of low-cost labor (Lillywhite et al., 2004; Siemens et al., 2012). During that time, nonselective and selective automated thinners were developed not only for specialty crops but also for field crops such as cotton (Gossypium hirsutum) and sugar beet (Beta vulgaris). An oscillating hoe or a rotating blade was used in nonselected thinners to remove “blocks” of plants at fixed intervals without considering the actual location of plants. When the stand before thinning was not uniform and plants were removed between set spacing, this resulted in large gaps in the rows (Siemens et al., 2012). For selective thinners, various sensors were used to detect plant locations, and excess plants were removed by hydraulically actuated knife arms (Siemens et al., 2012). The development of this equipment was halted due to availability of precision planters in cotton and sugar beet, and the higher cost of automated thinners compared with hand thinning at the time (Lillywhite et al., 2004).
In recent years, lack of skilled farm laborers and their high cost have become significant issues for the vegetable industry, forcing growers to search for mechanical means to replace manual management practices (Taylor et al., 2012). According to vegetable producers in Imperial Valley, CA, the time per person to thin 1 acre and hand thinning quality in lettuce have been declining in recent years. Development of automated thinners enables the vegetable industry to cope with the rising cost of hand labor and to move hand labor to other tasks that are not automated, such as harvesting.
Currently available automated thinners for lettuce have been developed by four companies: Agmechtronix (Silver City, NM), Blue River (Sunnyvale, CA), Foothill Packing (Somerton, AZ), and Vision Robotics (San Diego, CA). These machines employ digital cameras to identify each lettuce seedling, analyze the spacings, and remove unwanted plants by spraying them with salt, acid-based fertilizer, or herbicide. The automated thinners have been made economically possible through the development of powerful and cost-effective computers, lower-cost sensors, and small, powerful electric motors. Compared with hand thinning, these automated thinners can lower the cost and increase the speed of the thinning progress. While thousands of hectares of lettuce have been thinned using automated thinners in California and Arizona in the last 2 to 3 years, little research has been conducted on the performance of these automated thinners.
Plant spacing affects lettuce yield and quality significantly (Maboko and Du Plooy, 2009; Ririe, 1972). In both iceberg and romaine lettuce, seeds are typically sown 2.5 inches apart. Conventionally, plants are thinned at about four-leaf stage by hand hoeing to 10- to 12-inch in-row spacing, depending on lettuce type and growing season. Lettuce crops with in-row spacing larger than 12 inches result in a small number of harvestable plants. In contrast, lettuce crops with in-row spacing smaller than 10 inches result in large number of small hearts that often do not meet market standards (Ririe, 1972). A nonuniform stand also reduces the number of harvestable plants due to different maturity times among plants (Ririe, 1972). Maintaining lettuce uniformity is more challenging when the crop is planted during a warm period because high temperature increases plant variability in crop emergence and plant size in lettuce (Guzman, 1986). This is especially important in southern California and southwestern Arizona, where the average maximum temperature in September is 102 °F and where over 90% of U.S. wintertime lettuce is produced.
Given the rapid increase in the use of automated thinners in the lettuce industry, it is important to examine the effects of this technology on in-row spacing, plant size, and crop yield and to determine if there are ways to enhance its benefits. Therefore, the objective of this study was to compare the effects of automated thinning and hand thinning on the uniformity of in-row plant spacing, plant size after thinning and at harvest, and crop yield.
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