Use of automated thinners shows benefits, research says

Automated lettuce thinners proved as accurate in thinning the crop as manual methods "and produced comparable standards and yield," researchers report in a University of California publication.

Researchers from California State University, Fresno, and UC Cooperative Extension tested automated thinners from four manufacturers in Salinas Valley lettuce fields, comparing the speed and accuracy of the machines to crews of farm employees using hoes to thin fields and remove weeds. The researchers reported their results in the April-June issue of the UC research publication California Agriculture.

An automated thinner, typically operated by one person, removes plants to ensure accurate final plant spacing and can provide "a measure of weed control," the study said. Machine vision technology allows the thinner to distinguish plants from soil, though the machines used in the study could not tell crop plants from weeds. The machine chooses keeper plants based on plant-spacing settings, calculates the spacing, selects the next keeper plant and eliminates unwanted plants between by treating them with a registered herbicide or topical applications of fertilizer.

Manual thinning involves people using hoes. Two to three weeks after thinning, a second manual operation removes weeds and doubles—lettuce plants spaced too closely together and missed in the thinning operation. The manual method typically involved a crew of 15 to 25 people, the researchers said.

After timing and otherwise evaluating automated and manual thinning, the researchers determined that automated thinning required less than one-third the labor of manual thinning.

"Automated thinners took an average of 2 person-hours per acre to thin the lettuce plots; manual thinning took more than 7 person-hours per acre," the study said. "Labor costs were estimated at $43.40 per acre for automated thinning and $112.70 per hour for manual thinning."

The researchers based their employment-cost estimates on an equipment-operator wage rate of $21.70 an hour and a field labor rate of $16.10 an hour, including 40 percent benefits. The cost estimates did not include capital costs, depreciation or overhead costs for the automated thinner, and the research did not consider net profits to farmers.

"The automated thinners evaluated in this study traveled at speeds greater than 2 miles per hour and thinned as many as eight seedlines per pass," the researchers wrote. "Hand crews thin at less than 1 mph and thin one seedline at a time."

However, the study noted, automated thinners can be slowed by wet soil and windy weather, so the machines are often operated in the morning when wind speeds are lower.

Final crop stands were similar in automated and manual thinning treatments, the study said, though automated thinning left five times more doubles. The researchers also concluded that the automated thinners were as efficient as the manual crews in removing weeds during thinning.

Employment costs for doubles and weed removal proved to be higher with automated thinning—$111.09 per acre vs. $86.94 for manual thinning—and the researchers noted the costs appeared to be influenced by differences in planting accuracy.

"The comparable results of automated thinning and manual thinning suggest that growers could direct labor from thinning to other jobs such as irrigation and harvest," the study said.

"We did not evaluate the economics of automated thinning," the researchers wrote, "but a cursory comparison of labor costs shows that automated thinners can be more cost effective, and given the adoption of automated thinning already, it is evident that growers see the benefits of it on their bottom line."

The article was co-authored by Elizabeth Mosqueda, a former graduate student at Fresno State; UCCE farm advisor Richard Smith; Dave Goorahoo, an associate professor of vegetable science at Fresno State; and Anil Shrestha, Fresno State weed science professor. The full article may be found online at calag.ucanr.edu/.