Automation, combined methods rid fields of weeds

Weeds in lettuce are a continual challenge for California growers. There is no silver-bullet solution, but they may be controlled with a combined approach.

Richard Smith, University of California Cooperative Extension vegetable crop weed science advisor for Monterey, Santa Cruz and San Benito counties, offers several techniques for ridding lettuce of weeds.

Pre-irrigation is a key practice during the fallow period to germinate weeds before planting. Water is applied to the bed and, once the weeds germinate, they are removed with shallow cultivation. This practice reduces weed pressure in the subsequent crops, Smith said.

"Weeds are funny. Not all of them are ready to germinate," he said. "There's a certain percent of their seed that are dormant and so what you do is germinate up the ones that are ready to germinate."

Pregermination will greatly reduce the weed seed population when the crop is planted.

"Typically, pregermination eliminates about 50% of the weeds," Smith said.

Chemical control starts with pre-emergent treatment. There are three pre-emergent herbicides used in lettuce production: benefin, bensulide and pronamide. Benefin and bensulide control warm-season weeds. Benefin is mostly mechanically incorporated into the soil, and bensulide and pronamide are commonly applied at planting with germination water, Smith said.

In the Salinas Valley, lettuce is irrigated by sprinkler, single-use drip tape or a combination of both. Where both irrigation systems are used, sprinkler irrigation is employed until lettuce seedlings are thinned to promote mature growth. Then single-use drip irrigation is used for the remainder of the crop cycle, Smith said.

The pre-emergent applications were designed to be incorporated by sprinklers, but that has changed with the surge in use of single-use drip tape.

Single-use drip tape has good uniformity. It is typically planted 2 to 3 inches deep, and the water moves upward, opposite of the downward movement of sprinklers.

"We've done research on trying to optimize activation of the herbicides with drip, and it can be done," Smith said.

Based on the chemistry of a pre-emergent, some move more readily while others don't. That creates some challenges for activating and getting them down into the soil where they need to be, Smith said.

"Activating the pre-emergent herbicides with the drip is a little trickier, but it can be done and it is done," he said. "The (pest control advisors) and the growers are doing it—and so it works. But there's some nuances to it, because of the way the drip water moves."

Because drip water moves up, sometimes the drip may not effectively activate the pre-emergent herbicides on the soil surface. That's a problem, because it misses the weeds it is trying to reach.

A relatively new and fast-expanding weed control management tool is automated weeders. Automated thinning has been in use for about eight years, but auto weeders, in comparison, are a fairly new technology.

Smith says automation is a response to current labor shortages. When farm employees are plentiful, he said, it's difficult to justify investing in automation. But with workforce shortages, an investment into an automated weeder can pencil out, Smith said.

"We're kind of on the cusp of that technology kicking off," he said.

A typical grower raises multiple crops. An automated weeder can be used on lettuce as well as broccoli, cauliflower and celery. With multiple uses, it can bring down costs per acre.

"If a grower can buy the machine, then the cost can be spread over many, many acres," Smith said.

However, he said, "At this point, it's really more of the bigger growers that can afford this kind of technology." For others, the economics may be trickier, he said.

For the machines to work effectively, a grower has to have a low to moderate weed population to begin with, Smith said.

While fully automated machines are on the market, their use is restricted under Occupational Safety and Health Administration regulations. Under the rules, an attendant is required to be in the field running the equipment, Smith said.

Even using pregermination, chemical pre-emergents and auto weeders, some hand weeding may still be necessary, Smith said. But it is dependent on the weed population in the field.

If growers are able to keep the weed population moderate, they may get by with very little hand weeding. But usually there's always some hand weeding required, Smith said.

For instance, if a weed has germinated right at the base of a lettuce plant, the automated weeder will never reach that weed. The blade can't get that close to the lettuce plant without damaging it, Smith said.

The goal for managing weeds is to get the populations as low as possible using all of these practices: pregermination; pre-emergent herbicide; regular cultivation; and cultivation with an automated weeder.

"Every one of those steps reduces the amount of subsequent hand weeding that might be necessary," Smith said.

In some cases, the growers with low weed populations and who've also done a good job weeding may need very little or no hand weeding, Smith said. But usually, some hand weeding is necessary to prevent weeds from surviving and setting seed, which would make the next crop weedier, he added.

"That's why hand weeding is really necessary still at this point," Smith said.

But, he added, "If the grower does all the practices as well as they can, they can have really, really low weed populations."

(Kathy Coatney is a reporter in Bend, Oregon. She may be contacted at kacoatney@gmail.com)