Winter vegetable farmer Jesus Tovar embraces automation to increase efficiency plus reduce the dependence and costs of hand labor to thin his lettuce crop.

Tovar’s eyes were fixed on a University of Arizona (UA)-developed automated lettuce thinner prototype crawling at one-and-a-half miles per hour. The machine was demonstrated at a field day at the Yuma Agricultural Center (YAC) in Yuma, Ariz., in March held in conjunction with the Southwest Ag Summit.

Tovar, owner of T&P Farms, grows lettuce for Ocean Mist Farms on about 2,500 acres located from San Luis, Ariz., to Bard, Calif.   

“If we look at the future, we may not have enough people to do this (thinning) work,” Tovar said. “If a (thinning) machine like this is available, that’s where we are going to go.”

A mechanized thinner purchase would depend on costs, the speed of the machine, and other factors, Tovar noted.

Lettuce seed is sown at high rates due to poor germination rates in the field. Unneeded seedlings are thinned out two weeks after germination by farm workers armed with hoes to achieve the desired plant spacing in the final stand.

Mark Siemens, University of Arizona ag mechanization specialist, says thinning lettuce by hand cost growers $100-plus an acre, depending on the location and field conditions.

“One advantage of an automated thinner is to free up labor crews for other tasks,” Siemens said. “Another advantage is to reduce thinning costs. I estimate the tractor operating cost for an automated thinner at about $25/acre. The remaining $75/acre thinning cost reduction would be divided between the actual machine cost from a manufacturer and the savings to the grower.”

If one assumes cost savings to growers at $25/acre and a thinner device was implemented on the estimated 220,000 acres of lettuce grown in California and Arizona, Siemens projects growers could save about $5.5 million annually.

Siemens is at the helm of the UA automated thinner research. The project is funded by a three-year $115,000 specialty crop block grant from the Arizona Department of Agriculture.

The UA prototype is in its second year of development. An automated lettuce thinner must perform three necessary tasks, Siemens told a crowd during an Ag Summit advanced technology session.

“Required components include a plant sensing system to identify and locate crop plants, a control system to determine which plants to keep and eliminate, and a method for removing the unwanted plants,” Siemens said.

Siemens and ag engineer Ryan Herbon have worked closely on the sensing component. Herbon owns and operates Mule Deer Automation in Silver City, N.M. Also involved in the project is Ron Gayler, UA staff technician.

The thinner prototype is designed for traditional 40- or 42-inch wide, two-line iceberg lettuce beds and the same width three-line romaine lettuce beds. Machines could be hooked together to thin several beds at a time or larger 80- or 84-inch beds.

The mechanized thinner showcased at the field day is the second generation prototype developed by the research trio.

The original tested sensing system included photoelectric sensors to sense the lettuce plants. It performed unsatisfactorily in finding the small seedlings. The switch was made to a “machine vision” sensing system featuring a digital camera which captures images on the bed under controlled lighting.

Machine vision

“The machine vision system identifies each plant and makes decisions based on pre-set user control settings on which plants to keep and terminate,” Herbon said. “We catch the color image, process it, and identify whether the image is a plant or the soil.”

An optical shaft encoder mounted to a wheel tracks the machine’s location in the field. An interface in the tractor cab allows the operator to tweak the machine settings as needed.

The final piece of the puzzle is choosing a preferred method for plant termination. Initial methods tested by Siemens and Gayler included a mechanical knife – a modified ‘hula hoe’ - which moved vertically through a pneumatic cylinder to destroy unwanted plants.

“In some field conditions the mechanical knife works fairly well,” Siemens said. “This method has real potential in organic lettuce production and for those who prefer not to apply a spray solution.”

Other tested methods included a flamethrower, pressurized water, blasting, and other techniques.

“Based upon our overall tests, spraying a liquid form to kill the unwanted plants probably has the most potential for widespread commercial use,” Siemens said.

The first replicated trial of various spray products was conducted in January in a furrow-irrigated lettuce field at the YAC. Products tested included the fertilizers AN20 and UAN32, sulfuric acid, paraquat, and acetic acid – a herbicidal vinegar with 20 percent acid.

“The sulfuric acid and paraquat results were very encouraging,” Siemens said. “Nearly all of the targeted plants were killed and the system performance was as good as hand thinning. The herbicidal vinegar worked well and could be an additional good option to thin organic lettuce.”

Selecting a product to move forward with will depend on a variety of factors including effectiveness, costs per acre, environmental concerns, and other issues.

For now, Siemens and his project team members envision keeping the single-bed modular unit design which would accommodate beds up to 42 inches wide. Multi-bed machines comprised of multiple modular units would attach to the same tool bar.

“Growers in California and Arizona may have to slightly modify their growing practices for the machine to work optimally,” Siemens said. “The current machine design works best on well prepared, smooth, level beds with plants uniformly spaced approximately 2 or more inches apart. We could probably thin plants spaced closer together but we have not tried it. The minimum plant spacing requirement has yet to be determined.”

The UA is exploring patent opportunities for the proprietary technology. The long-term goal is to fine-tune the system to best meet the needs of commercial lettuce growers.

Siemens is seeking grower input on the preferred machine speed, the number of acres needed to thin per hour, the preferred machine bed width, and the maximum allowable machine weight to limit bed compaction.

Siemens wants to incorporate machine changes and test the upgraded prototype in commercial lettuce fields this fall.

“We want to make the machine as versatile and user friendly as possible, and possibly adapt the machine to other crops grown in the area,” Siemens said.

Others who have provided input into lettuce thinner development include Kurt Nolte, UA area Extension agent, Yuma County, and Davie Brooks of the Pasquinelli Produce Company.

The thinner research was launched in 2008. Siemens initially surveyed local growers asking for the top mechanized equipment needed in vegetable production. The top answer was an automated lettuce thinner.

For more information, contact Siemens at (928) 782-3836 or [email protected].

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