Siemens says manual thinning costs about $100 or more per acre, with one worker thinning about one acre of lettuce per day. He believes mechanization can significantly reduce labor requirements and production costs for the thinning process. This would allow labor to be better utilized for other farm tasks.

“I estimate a four-bed thinning machine moving at 1 mph would thin about 1.4 acres per hour,” he says. “Fixed and variable operating costs would be about $33 per acre, for a savings of about $67 per acre.” That includes the machinery cost.

Mechanically thinning the 400,000 acres of iceberg and romaine lettuce in California and Arizona could save growers an estimated $26 million, Siemens says.

His prototype machine was developed with a $115,000 specialty crop block grant from the Arizona Department of Agriculture. It has three promising plant termination methods, including a mechanical knife, flame device, and a third method he doesn’t divulge because of potential patent opportunities.

The prototype employs all three devices, but a system developed for the commercial market would include the best single termination method, he says.

The guts of the machine-vision system are housed in a 15-inch metal box mounted on a toolbar. It includes a digital color video camera and electronics designed for a single lettuce bed to identify plant location. Two frame-mounted lights illuminate the plants for the camera.

“A computer algorithm determines which plants to keep and eliminate,” Siemens says. “The computer also controls the plant termination device, so unwanted plants are selectively thinned.”

A “walking beam” (tandem axle with a wheel on each end) maintains the system’s accurate height and depth control relative to the soil surface. The plant termination devices are located on the beam assembly.

Herbon, who is with Mule Deer Automation, Silver City, N.M., developed the machine vision and electronic control components. He has extensive experience in chile pepper mechanization, including current work on a mechanized destemmer. He co-invented a chile-thinning system later patented by New Mexico State University and manufactured by CEMCO.

Siemens and Herbon will test and calibrate the lettuce-thinner prototype through mid-May, with a goal of working out the bugs within a year.

“Our goal is to operate a four-bed machine, moving at a minimum 1 mph speed to accurately thin plants to the desired spacing,” Siemens says. “We want to develop a simple, robust machine that is easy for growers to utilize and maintain.”

Herbon believes commercial thinning equipment, regardless of the manufacturer, could be widely used in the lettuce industry within the next five to 10 years.

Siemens also has ideas for tweaking the machine for use as a mechanized weeder in lettuce. About 85 percent of fields are currently hand weeded. He believes,too, that the technology holds promise for thinning broccoli, and that it could also potentially be adapted for melon and chile production.

Pasquinelli Produce Co. at Yuma and Kurt Nolte, UA Cooperative Extension director, Yuma County, provided input for the thinner prototype.