Can the venerable herbicide Treflan fill a role for weed control in lettuce?

Steve Fennimore, University of California weed specialist at Salinas, says it may, when teamed with Kerb, based on the results of trials in Salinas and Gonzales earlier this year.

Detailing his Salinas Valley trials during the recent meeting of the California Lettuce Research Board at Seaside to hear mid-year reports, Fennimore said Treflan has several potential advantages, even though it is not labeled presently for lettuce.

“The strongest reason is it is a mature product registered for many crops, including beans, carrots, celery, all cole crops, tomatoes, and others that are grown where we crop lettuce.”

He cited the background on the typically soil-incorporated, pre-emergence, Treflan dating back to the 1960s, when it and its relative, Balan, were evaluated on a host of vegetable crops.

“Balan was chosen because it was judged to be the safer on lettuce. Even so, Treflan still has a food tolerance for lettuce and other leafy vegetables of 0.5 parts per million.”

He went on to say that Dow AgroSciences, the manufacturer, has an agreement with EPA to relinquish the tolerance unless evidence of a need for Treflan can be produced.

“We would need to convince Dow AgroSciences to keep the tolerance so they could re-negotiate with EPA. The point of this research was to evaluate whether it would be worth pursuing a label for Treflan on lettuce.”

Pursue new label

He urged the board and the industry to consider “vigorously pursuing” the possibilities of a new label for Treflan on lettuce. “Otherwise, Dow AgroSciences will follow through with their previous agreement with EPA and drop the tolerance.”

Fennimore said the first question he was asked by company officials was: why not just use Balan, which will continue to be available, on lettuce?

The main thing Treflan has going for it, he replied, is it stays put in the soil. “You can apply all the water you want on it and it still moves very little. So it might be very useful around Yuma, perhaps in combination with Kerb.”

Kerb has been found to leach below the weed germination zone in soils of the desert and other locations when high levels of irrigation are applied. The combination, Fennimore reasons, could bring out the best in both products.

Treflan is cheap, he said, considering the low rates at which it could be used compared to hand-labor costs, and it is an “old” and established product. What's more, it can be applied on the surface and watered in, post-plant.

Fennimore said although he does not know the relative effectiveness of Treflan and Kerb on all the various weed species, he suspects that Treflan is basically the superior herbicide. Among the weeds Treflan does control well are purslane, pigweed, nettleleaf goosefoot, and — important for lettuce grown in the desert — grasses.

“Another big advantage is that Treflan is registered for so many other crops, and Balan is registered for just a handful of them, lettuce, alfalfa, and birdsfoot trefoil. And the Balan label requires mechanical incorporation.”

Low-rate trials

In trials at Salinas and Gonzales, he and Richard Smith, Monterey County farm advisor, used low rates of Treflan tank-mixed with Kerb, since the customary rates for Treflan are too low for it to be used as a stand-alone product.

Essentially, they found no significant difference between yields in plots treated with Kerb or with the Treflan-Kerb tank-mix. They did report, however, some stunting of lettuce plants with the higher rate of the combination.

Meanwhile, Fennimore is continuing a probe of the Kerb leaching problem. Yuma area growers have reported that intermittent initial irrigation following application confines the herbicide where it is needed. These, however, have not been verified.

In a lab setting, he has been using columns of untreated soils, Chualar sandy loam from Salinas and Indio silty clay loam from Yuma. Kerb treated soils are placed on the top of the columns, ryegrass is planted, and various irrigation routines are metered in. The object is to manage the irrigations to keep the herbicide in the critical grass germination zone.

“The continuous irrigation treatment readily moves Kerb through the Yuma soils,” he said, “but the Chualar series soils impede Kerb movement. We are currently evaluating whether intermittent irrigation can be used to reduce Kerb movement in the soil.” He expects to report results to the board at final reports of the research year at Sacramento in March.

Prickly lettuce

In yet another project, Fennimore has been making progress with germplasm of a weed, prickly lettuce, obtained from the University of Idaho and tolerant of sulfonylurea (SU) herbicides.

The experimental butterhead lettuce plant material, designated IDBR1 and further developed with conventional methods by Beiquan Mou, USDA plant breeder at Salinas, was found in pre-emergence and postemergence trials this past spring to have good tolerance of the SU herbicides Mavrick and Upbeet and marginal tolerance of Accent.

The objective is to refine SU-tolerant germplasm, which could be turned over to commercial lettuce breeders for new, tolerant varieties. The process, however, would take several years to combine all the necessary traits for a new cultivar acceptable to the industry and consumers.

Prickly lettuce, a weed common to wheat fields in the northwest, naturally carries a gene tolerant of SU compounds. University of Idaho plant breeders, using conventional methods, crossed it into Bibb-type lettuce.

Fennimore said earlier that it is a significant advancement toward herbicide-tolerant lettuce varieties, of high importance to the industry, considering the objections of consumer groups and others to genetically modified plant material.

Cover crop work

In a separate project funded by the board, Richard Smith and other UC specialists worked with mustard and other Brassica species cover crops in 2003 and 2004 as rotations for lettuce.

While some weed reduction was observed after the cover crops where weed pressure was high, Smith said no meaningful difference was seen in the number of infections of Sclerotinia minor, the cause of lettuce drop, between mustard plots and fallowed ground.

With the cooperation of growers, short-term plots of lettuce were planted immediately following a mustard cover crop.

The studies were funded to learn whether use of white mustards and Indian mustards could optimize rotations with lettuce crops by reducing some soilborne diseases, nematodes, and weeds. Mustard species cover crops have reportedly reduced these pests in other parts of the world, said Smith.

While effective rotations are essential in lettuce production, he said, “Unfortunately, given economic pressures, high land rents, and lower returns for rotational crops, growers are often not able to rotate away from lettuce as often as they would like.”