University of California researchers say Kerb herbicide applied through drip irrigation lines has potential to control of tough weed pests like shepherd's purse and nettle in lettuce, although it performs better on certain soil types than others.

In reporting on the continuing trials to the California Lettuce Research Board (CLRB) recently at Seaside, Richard Smith, Monterey County farm advisor, said a soil's clay and organic content govern whether the herbicide leaches too deeply in the soil to be effective on weeds.

In the project, led by Steve Fennimore, UC Davis weed specialist based at Salinas, columns of soil samples are being evaluated in the laboratory with a gas chromatograph. The object is to gauge how Kerb binds to soils of the state's major lettuce producing areas.

“We have surveyed four sites in the Salinas Valley, five in Fresno County, one in Ventura County, two in the Coachella Valley, and two at Yuma, Arizona,” Smith said.

Preliminary results of the work, which is scheduled to continue next year, indicate that Kerb is mobile in some soils in the King City area and Huron, sandy-loam soils in the Thermal area and Indio silty-clay soils around Yuma.

The herbicide appears to be less mobile in Chualar loam, Salinas clay-loam, Guadalupe clay-loam and Cropley series soils.

In field trials this season at Salinas comparing sprinkler and drip applied Kerb on the iceberg lettuce variety Sniper, the researchers found that switching from sprinkler to drip irrigation reduced shepherd's purse and total weed densities more than tenfold.

Kerb applied by chemigation through surface or buried drip tape controlled weeds as well as spray applied Kerb irrigated by surface or buried tape.

They saw no visual damage to the crop and planned to record lettuce yields at crop maturity.

The team has also been screening lettuce germplasm for greater tolerance to herbicides. They reasoned that if they could find lettuce that tolerates Caparol, a very effective herbicide in celery but lethal to existing lettuce varieties, lettuce weed control would be easier.

But after screening 3,000 lines of lettuce they found none tolerant of Caparol. They now plan to examine several lettuce lines found by UC Davis geneticists to have some tolerance to glyphosate herbicide.

Smith also detailed the project he is leading to evaluate control of lettuce drop (Sclerotinia minor) and weeds with BQ Mulch and other cover crops, including Merced rye and Caliente 99 (a combination of two varieties of Brassica juncea), between lettuce crops.

BQ Mulch, a New Zealand product, is a blend of canola-type and turnip-type Brassicas, which reportedly reduced infection by S. minor on lettuce in Australia. One theory is that roots of the blend, when incorporated in the soil, produce glucosinolate compounds antagonistic to the pathogen.

Results of this and previous studies, Smith said, have been mixed with respect to the effectiveness of the biofumigation with Brassicas.

The latest research, he added, suggests that a cover crop itself has some beneficial effects. “The first lettuce crop following a cover crop did give a significant boost in yield over bare fallow.”

He and his colleagues found the glucosinolate content of the cover crops they tested to be “not very much” in comparison with commercial application rates of Vapam fumigant. The compound does occur in the roots, but, Smith explained, the roots make up only a small part of the total biomass.

“One of the problems we have seen in both the laboratory and field trials with mustard cover crops, especially the Brassicas, is they are very susceptible to S. minor themselves, although not to the extent of lettuce,” Smith said.

In weed control evaluations, he said BQ Mulch and Caliente 99 both reduced the amount of weeds at thinning of the crop in a cooperating grower's field.

He also learned that BQ Mulch has a strong tendency to resprout from roots following incorporation and can become a nuisance in the subsequent lettuce planting.

The leader of another CLRB-funded project, Michael Cahn, Monterey County farm advisor, reported his observations on drip irrigation used to germinate lettuce.

Noting that drip irrigation of lettuce has expanded in the last 10 years in the Salinas Valley, he said the general practice is still to establish stands with overhead sprinklers.

“Although some growers are using drip irrigation to germinate lettuce, germination rates can be low at some locations within a field if moisture does not adequately move from the drip tape to the seed,” Cahn said.

If drip could be used through the entire growing season, he added, growers could save costs by using only one type of irrigation system.

“There's potential to save water use if drip works correctly and perhaps there are food safety benefits because we would not be applying water directly to the crop during the entire season.”

It could also be a positive as growers comply with regulations on irrigation run-off from certain soil types.

The project evaluated depth of drip tape, spacing of emitters, discharge rate of emitters and compaction of bed tops by rolling.

Cahn said only the depth of tape significantly affected gravimetric moisture in the seedline during subsequent irrigations and no differences were found in the cultural practices evaluated.

An additional trial, however, showed higher germination under both buried and surface drip compared to sprinklers when a similar amount (3 inches) of water was applied.

The lower germination with sprinklers was because of crusting of the surface of the soil, and Cahn said decomposed granite soils are conducive to drip irrigation because they tend to better move water laterally.

Nitrate leaching losses were comparable among sprinkler and drip irrigated fields.