Can cover crops and conservation tillage work in Salinas Valley vegetables? Growers likely have their opinions, but University of California researchers hope to harvest details about the concept from current trials with broccoli at USDA's Spence Research Farm south of Salinas.

“Bed preparation probably isn't done anywhere with more intensity than in the Salinas Valley,” said Richard Smith, Monterey County farm advisor, during a recent field day at the site.

He is evaluating a mustard cover crop, Caliente 119, against conventional practices on listed and shaped beds.

The cover was planted April 11 to plots and 88 pounds of nitrogen as ammonium nitrate was applied to the growing mustard, which was flail chopped June 16.

Four tillage treatments were made the following day to separate plots: no cover crop and conventional tillage with three passes with a Sundance minimum-till system, the cover crop with no tillage, the cover crop with partial tillage of a single Sundance pass, and the cover crop with complete incorporation by three Sundance passes.

Sixty pounds of nitrogen as ammonium nitrate was applied and Goal herbicide was applied to selected portions. Heritage variety broccoli was transplanted in the plots June 18.

Collaborating with Smith on the project is Michael Cahn, irrigation farm advisor for Monterey County. Before coming to Salinas, Cahn worked with conservation tillage in Yuba County as a means of reducing costs, both in tillage itself and fuel.

Support reasons

He gave three reasons in support of conservation tillage:

First, considerable thought is being given by governmental agencies across the U.S. to providing farmers environmental credits for storing carbon in the soil with conservation tillage practices.

Another is to improve water quality by preventing run-off of nutrients and soil erosion, factors of high interest in Monterey County.

A third reason is reduction of dust, a mounting issue for agriculture throughout the state.

Use of a mustard cover crop, Smith said, is also of interest as a way to suppress disease in lettuce. He and others last year began a separate, multi-year investigation of mustard covers that may help to control lettuce head-drop caused by Sclerotinia minor. The project also aims at learning what impact a cover has on weed populations.

“There doesn't appear to be a significant amount of dormancy in the mustard cover crop seeds, and therefore, they do not appear to pose a problem as an aggressive, long-term weed in production fields,” Smith said.

He added, however, that summer plantings (those after June) of mustards set seed in as few as 55 days, while spring and fall plantings can take more than 70 days to produce seed.

That's why, he explained, “summer plantings of mustard cover crops should be incorporated quickly after the onset of flowering to avoid them setting seed.”

Breeding efforts

Steve Fennimore, Monterey County weed science farm advisor, told field day attendees that efforts to develop a sulfonylurea herbicide-tolerant lettuce by conventional breeding means are moving forward.

It is a significant breakthrough, he noted, because of consumer groups' and other others' objections have left little likelihood of acceptance of genetically modified lettuce having tolerance to herbicides.

University of Idaho researchers found a naturally occurring, resistant gene in prickly lettuce, a common weed in wheat fields in the northwest, and crossed it into Bibb-type lettuce.

Fennimore said the University of California obtained the plant material, and Beiquan Mou, lettuce breeder with USDA-ARS in Salinas, is collaborating by crossing the resistant material with lettuce types commercially grown in the Salinas Valley.

Conventional breeding typically takes several years to achieve cultivars having all desired traits.

“The Roundup Ready lettuce idea,” Fennimore said, “would have been an excellent weed control system, but it is biotechnology and it is now at a dead-end. We will be screening the new plant material, which is not genetically modified, with different chemicals in the field and hopefully we will get some useful leads.”

Collaboration

Fennimore also cooperated with Shachar Shem-Tov, a visiting scientist from Israel, in a trial at the Spence site to quantify the advantages of pre-irrigating lettuce fields as a weed control practice.

By stimulating early weed emergence, local growers have found that much of the growth can be destroyed by tillage prior to seeding and the amount of herbicide needed can be reduced. In the trial, Kerb was used at rates of 0.6 and 1.2 pounds per acre.

The two researchers confirmed that weed densities and hand weeding time were reduced by preirrigation vs. no preirrigation. They measured weed densities 21 days after planting and the time required to thin the stand.

“Where a one-week preplant interval was used, sprinkler irrigation was the most effective method to deplete weed emergence, while furrow irrigation resulted in no reduction in weed densities,” Shem-Tov reported.

Where a two-week preplant interval was used, he added, weed densities in the control plots were twice those in the preirrigated plots regardless of irrigation method.

“Thinning times in the furrow- and sprinkler-irrigated plot were reduced by 37 percent to 49 percent compared with the control,” Shem-Tov said.

Differences between the preirrigation treatments and the control were significant, regardless of whether the low or high rate of Kerb was applied.

No meaningful difference was observed between the two rates, suggesting that the low rate could be used.