The U.S. commercial lettuce industry is estimated to be worth $2 billion. Mechanizing production throughout the fruit and vegetable industry continues to be a high priority among industry leaders, growers, researchers and equipment manufacturers. Labor costs and availability are driving the move toward increased mechanization in the lettuce industry to help growers remain profitable as they compete with imports from countries with ample cheap labor. Lettuce thinning machines are one development that’s helping to increase efficiency for lettuce growers.

Mark Siemens discusses progress on the University of Arizona’s lettuce thinning machine.

The University of Arizona (UA) has developed a prototype lettuce thinning machine at its Yuma Agricultural Center. The project is led by Dr. Mark Siemens, associate professor and agricultural mechanization specialist, with the goal of improving labor resource use efficiency.

Research conducted in 2011 compared hand thinning of lettuce seedlings to machine thinning. The results indicated that lettuce yields are not negatively affected when seedlings are thinned using herbicidal sprays delivered by the automated machine.

“The research role is part of our mission,” Siemens said. About 80 percent of total U.S. lettuce production during spring, summer and fall comes from the Salinas Valley in California’s Central Coast. As for the winter lettuce crop, about 90 percent of that is grown in the Yuma, Ariz., area and neighboring Imperial County, Calif. About 220,000 acres of romaine and iceberg lettuce are grown annually in the two states, according to USDA statistics.

Research and mechanization

Traditionally, seedbeds of numerous crops have been heavily seeded to help assure establishment of a full stand. Throughout much of the 1900s, cheap labor for hand thinning was readily available. Labor for thinning, harvesting and other tasks saw some changes with the ending of the Bracero program. Initially instituted to supplement labor needs during World War II, the Bracero program allowed workers to cross the U.S.-Mexico border to work and was continued for agriculture until 1964.

Research has historically led the way to mechanization in crop production. Land-grant university researchers have partnered with private industry and growers to conduct the initial research and field trials that lead to successful development of agricultural equipment. Mechanization has replaced hand thinning and harvesting of numerous crops over the years. Row crops such as corn and cotton benefited from mechanization during the first half of the 1900s, and specialty crops are following a similar pattern.

Mechanized production is taking on increased significance as growers struggle to maintain profitability with increased labor costs and competition from imports. Although labor costs eased somewhat with the economic downturn that reduced construction and other jobs, labor availability is a concern for growers. And increased immigration enforcement has reduced the number of available workers. As a result, the processing of tomatoes, red chiles and a range of other fruits and vegetables is moving toward more mechanized production.

Thinning machine development

Lettuce is usually seeded about 2 inches apart. About two weeks after germination, the crop is thinned to a spacing of 10 to 12 inches to allow sufficient space for plant development. This thinning has most frequently been done by crews using hoes. Several lettuce thinning machines are now commercially available and more are expected.

The UA thinning machine has undergone a number of modifications throughout its development. “We are now developing a generation two machine,” Siemens said. A digital camera is used to photograph images of the plants, and a mini PC processes the images. A laptop computer allows the user to establish machine settings. Before operation, the machine vision system is calibrated to correctly identify plants.

Modifications to the spray nozzle assembly include ensuring that the main operating components ride at a consistent height. The spray nozzle assembly is attached to a tandem. This and the camera are attached to a linkage assembly, which has wheels that float over the soil. Siemens said, “The machine has been exhibited at field days and is in commercial field trials. The machine’s mini PC and microcontroller are enclosed inside a large steel box. The box is used not only to support the floating bed followers, but also to provide controlled lighting conditions so that images of good and consistent quality can be captured.”

The machine was demonstrated at the May 2011 University of California (UC) Monterey County Field Day. Dr. Richard Smith, a UC Extension farm adviser, said, “There was tremendous interest in the UA machine. It’s simple and some growers wanted to purchase it right there. The machine is currently in field trials here in Monterey County.”

Siemens noted that large-scale field testing and an economic analysis are needed to determine the system’s commercial viability. He is quick to point out the collaboration involved in developing the UA’s prototype thinning machine. Team contributions were made by Kurt Nolte, UA Extension director, Yuma County, Ariz.; Smith and Dr. Steve Fennimore, UC Extension specialists; Davie Brooks, Pasquinelli Produce Co.; Ron Gayler, UA staff technician; and Ryan Herbon, engineering consultant.

Increased interest in thinning machines

Lettuce thinning machines must be able to locate and identify plants, determine which plants to remove and which to leave, and remove unwanted plants. Various approaches to developing these machines have been implemented, from rotating or spinning steel cutting discs to mechanisms that spray herbicides directly onto plants.

Smith and Fennimore work extensively with lettuce growers in Monterey County. They first worked with Tillett & Hague (, a research and development company that provides automation technology. The company’s technology was used in the Robocrop weeder/thinner, initially manufactured by Garford Farm Machinery ( for weed removal. The machine used a steel cutting disc to remove weeds or unwanted plants from the seedling row. Fennimore said, “The Tillett machine did what it was designed to do in removing weeds, but it didn’t work well for what we were trying to do with it to thin lettuce.” While commercial lettuce growers exhibited little interest in the Tillett machine, it was purchased by some California organic growers.

Smith expects increased development in thinning machines as labor availability concerns continue throughout the vegetable production industry. He said, “Blue River here in Sunnyvale, Calif., is getting into lettuce thinning.” Blue River Technology ( is a start-up technology, integration and systems development company. Several other companies have developed or are in the process of developing thinning machines or multiuse weeder/thinners.

CEMCO, based in Belen, N.M., ( manufactures and sells a chile thinning machine, used in New Mexico chile fields, and subsequently developed a lettuce thinning machine in response to interest expressed by California lettuce growers. “We had the machine at agricultural shows in 2006, 2007 and 2008,” said Jerry Jackson, CEMCO special projects manager. “At that time, housing was booming and labor was very hard to get. Growers were very interested.” As labor availability changed and cost eased somewhat with the 2008 economic downturn, growers exhibited less interest in purchasing the machine. CEMCO continues to offer the lettuce thinning machine for sale.

Agmechtronix (, formerly Mule Deer Automation, Silver City, N.M., started offering a lettuce thinning machine for commercial sale in October 2012. The machine was used in field trials in Las Cruces, N.M., and Yuma, Ariz.

The technology works by sensing the location of lettuce seedlings with a digital camera. An onboard computer system identifies plants versus weeds. The system decides which plants to remove based on settings entered by the user, and herbicide is sprayed on the plants to be removed. The mechanically adjustable row spacing can accommodate typical head and romaine lettuce spacing. In-row settings are electronically adjustable via a touch screen.

Nancy Riggs is a freelance writer. She resides in Mount Zion, Ill.