Mechnical Thinning versus Hand Thinning in Lettuce

The availability and the high cost of hand labor in California have been forcing vegetable growers to search for mechanical ways to replace manual management practices. Mechanical thinning in lettuce has been developed in recent years by a few companies. Mechanical thinners employ digital cameras to identify each lettuce seedling, analyze the spacing, and to remove unwanted plants by spraying a material (salt, acid based fertilizer, or herbicide) over to kill them. Three field experiments were conducted on romaine heart lettuce at Holtville, California during September 2013 and April 2014 to test if mechanical thinners can increase crop uniformity and crop yield. A mechanical thinner from Vision Robotics Corporation (San Diego, California, USA) was used to thin the lettuce mechanically. Manual thinning was used as the control. The treatments in each experiment were replicated twice with a plot size of six 2-m bed wide and 400 m long. In-row spacing was measured after the thinning and overhead images were analyzed to measure plant size and uniformity during growing seasons. The impacts of mechanical and manual thinning on lettuce yield and uniformity were determined at each harvest by measuring plant size in two areas of 6 m² in each plot and counting harvested boxes in the stripes. Mechanical thinning increased uniformity of in-row spacing and increased percentage of plants with desired in-row spacing of 9-12 inches. Mechanical thinning also increased plant size at about 2 weeks after mechanical thinning (1 week after hand thinning) and plant uniformity at harvest. However, crop yield was not significantly affected by mechanical thinning. This is due to more plants were left in the hand thinning treatments. Our study suggests that mechanical thinning can increase plant uniformity, maintain crop yield, and reduce labor cost in romaine heart lettuce.