Automated Thinners in California Lettuce Production

Thinning crops is a labor intensive activity and growers and researchers have sought a means of mechanizing this crop production practice.  Efforts by Land Grant Universities and private companies to develop automated thinners for sugar beets extend back over 100 years. Early designs incorporated various swinging or spinning blades, but no plant detection technology. In the 1960’s, John Deere Corp. developed a beet thinner that utilized a plant detection mechanism (moisture sensitive metal plate) which greatly improved the resulting stand. Computer processing of digital images crop plantings is now available and has made it possible to make precise decisions on which plants to remove and leave. In 2011, the first automated thinners with computer processing were introduced to the lettuce industry. They used a spray kill mechanism to remove unwanted lettuce plants and associated weeds. There are now four companies with commercially available machines for thinning lettuce. These machines remove unwanted lettuce plants and associated weeds in a 10 cm wide strip around the seedline and within 9 to 12 mm to either side of the keeper plants. Materials used to remove the unwanted lettuce plants include fertilizer and the herbicide carfentrazone. Evaluations in 2014 showed that automated thinners took 2.25 hours/ha (range 0.59 to 4.92 hours/ha) and hand thinning 16.21 hours/ha (range 11.56 to 22.53 hours/ha) to thin lettuce. However, the double removal/weeding operation 7-14 days following thinning took 16.23 hours/ha in the automated thinner treatment and 13.12 hours/ha in the hand thinned treatment; the increase in time for double removal and weeding in the automated thinner area was due to the greater number of doubles in the automated thinner treatment (2,530 double/ha) than hand thinning (368 doubles/ha). The total time for thinning and double removal/weeding operations was 18.48 hours/ha in the automated thinner treatment and 29.33 hours/ha in the hand thinned treatment. The desired spacing for all fields in the study was 25.4 cm. The mean plant spacing in the automated thinner treatment was 26.1 cm (range 25.4 to 27.6 cm) and 26.6 cm in the hand thinned treatment (range 24.4 to 29.5 cm). Automated thinners have some key advantages over hand thinning and this technology is expected to continue to improve and find applications to other crop production practices.