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DOV raisin trials yield new details

As the California raisin industry struggles in a vise-grip of high costs and low returns, experiments in dried-on-the-vine (DOV) practices are yielding new details and thoughts.

One DOV approach, designed to convert traditional Thompson Seedless vines without the expense of overhead trellising, was discussed at the Kearney Agricultural Center at Parlier by University of California researchers.

Bill Peacock, Tulare County farm advisor, and others continue a second season of trials at KAC, using alternating fruit and renewal sides of each vine in a three-acre block of 20-year-old Thompsons.

The vines are on a two-wire trellis with an 18-inch cross arm, and trials are being done with other wider cross arms and up to four wires.

The practice, becoming known as the “Peacock system,” is intended to reduce costs and eliminate risks of rain to fruit on drying trays on the ground. Several growers are experimenting with it on their own.

Essentially, the concept is based on cutting canes bearing fruit in mid-August and leaving the fruit in place to dry. Severed canes that had fruit are pulled out during winter pruning, creating the next renewal side. The severed canes reduce the amount of pruning in the winter and absorb some costs. Also during the winter, canes on the former renewal side are tied and become the new fruiting side.

April pruning

Another step in the process is mid-April removal of foot-long shoots from the head of the vine. Peacock said this both speeds up the cane cutting later and removes competition for renewal canes.

The vineyard becomes a checkerboard of dried and green foliage. The renewal sides of vines provide a canopy to support the vines while the fruit dries.

The dried crop, out of risk from fall rains, can be picked up to several weeks later by a conventional harvester with the head adjusted to clear the cross arms.

Peacock estimates the costs of the system to be about $350 an acre, or the same as for one ton dried on trays. For a two-ton yield of raisins the cost of the system would be about half that of the conventional.

One of his concerns is finding ways to reduce the costs of cane cutting, up to $100 per acre for a vigorous vineyard, and he is looking for ways to mechanize part of the operation.

Reporting on this year's trials to growers touring the trial block, Peacock said the sugar content is running two to three degrees Brix behind the same date last year. He made an application of Ethrel and was monitoring the effect.

“These Thompsons,” he said, “don't have the advanced maturity of newer varieties like DOVine, Fiesta, or Selma Pete, so we have to do everything we can, especially with a big crop year like this when maturity is slowed. And that means using Ethrel and good water management.

19 degrees magic

“We figure 19 degrees Brix will be the magic number this year. If we cut canes at 19 the amount of B-grade raisins will be about the same as if we put them on trays at 20 or 21 sugar.”

Another reason, he added, is at lower maturity the amount of substandard raisins would be higher than tray-dried fruit.

So, his guideline is to start cane cutting at 19 sugar and end it by about mid-August for an 80 to 90 percent chance of successfully drying on the vine.

Waiting another week trims the probability to 70 to 80 percent. Waiting two weeks until late August or early September means the chances are 3 to 5 out of 10 that the fruit won't have to be dried on the ground or taken to a dehydrator.

“You could think of Aug. 22 or 23 for DOV as about the same as waiting until Sept. 20 to put the crop on trays. You may not get them dry.”

Turning to cane tying, Peacock said they must be tied securely to the wires to prevent them from falling out of reach of the harvester head.

Among other efforts to speed drying, Steve Vasquez, Fresno County farm advisor, is conducting trials using flaming to remove leaves from bearing canes to promote sun exposure to drying fruit.

This year the KAC vines were flamed the day before cane severance, and leaf blowing with air-fan spray was done at various intervals afterward to measure effects.

Propane flaming

Fresno grower Ron Brase has converted 40 acres to the Peacock system and said he is using propane flaming to expose a “window” 18 to 24 inches wide on the fruiting portion of vines so workers can see canes better.

He estimates his cane-cutting cost was about $50 an acre, including about $10 per acre for flaming, and said the flaming decreases the chances of workers missing canes. He used a tractor towing a trailer with the propane tank and two workers.

Workers, however, he added, must be gentle with the flaming wands to keep from damaging fruit. “If berries start to amber, you are too close, too slow, or too hot.”

Fresno grower-harvester Earl Rocca said he starts cutting canes for his continuous-tray harvesting system at 20 degrees Brix. He said he has successfully harvested high quality raisins with it into October.

Rocca said the first year is the hardest in converting to alternatives to conventional tray drying. “We've been doing this for some time but more interest has been shown the last couple of seasons. We are learning every year and the more we know the easier it gets. Let's get this industry mechanized!”

Fred Swanson, KAC superintendent, said Thompson Seedless is not an ideal variety for DOV because of its later maturity and limited yield potential from fewer vines per acre on the traditional 8- by 12-foot vine spacing.

Transitional step

He said the Peacock system may not be the final solution, but it could be a transitional step for growers before they decide whether to spend the time and money to convert to earlier varieties and new trellis designs.

He said many ideas for solutions have surfaced, one being grafting existing Thompsons to an earlier-maturing variety such as Selma Pete, interplanting Selma Pete in the middles, and rotating the direction of the rows by 90 degrees to create an 8- by 6-foot spacing for a higher vine count. “It's no recommendation, just food for thought.”

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