The vineyard isn't all that pretty, and growers will want more proof, but University of California researchers see potential with a new wrinkle to reduce costs of raisin production: vine-dried Thompson Seedless on a conventional trellis system and harvested mechanically after about six weeks of drying.
Dried-on-the-vine (DOV) — seen as a future direction of the raisin industry — is under development with several vineyard designs.
However, the costs of conversion to them with new varieties, plus more complicated trellising ranging in cost from $2,000 to $4,000 per acre, are far more than the depressed industry of some 170,000 to 200,000 acres of Thompsons for drying can manage.
The heart of the conventional-trellis-DOV concept is creating alternating fruiting and renewal sections. Canes of fruiting sections are cut in mid-August to initiate drying. Those canes are removed at pruning time from the fruiting section, making a renewal section for the following year, and canes of the opposite side are left and tied, making a new fruiting section.
Just before bloom, six to eight shoots are selected from each renewal section and positioned on trellis wires. Four to six of them become the next year's canes and the rest become spurs.
Shoots in a renewal section have ample light exposure and develop into canes with high yield potential. Twenty percent more flowers have been observed on the DOV system than on conventional vines.
The practice leaves about half the canopy active after cane severance to support the vines and optimize yields. By harvesttime the vineyard is a checkerboard of green renewal sections and brown fruiting sections.
Bill Peacock, Tulare County farm advisor, was at the Kearney Agricultural Center at Parlier recently to host a demonstration of harvesting the three-acre plot of established Thompson vines with cover-crop middles and a conventional trellis with 24-inch cross-arms and two wires. About 175 growers and interested others came to watch the demonstration.
Peacock, who collaborated in the two years of trials with Fred Swanson, superintendent of the center, Stephen Vasquez, Fresno County farm advisor, and Peter Christensen, Fresno County farm advisor emeritus, cited the economies and other advantages of DOV.
Peacock estimates the system will cost about $350 per acre and said the cost of traditional tray drying is about $350 per ton. “So if you have a one-ton yield of raisins, this system would cost about the same as a tray vineyard. If you have a two-ton vineyard, your costs with this system would be about half. If you have a three-ton vineyard, your savings are even greater.”
Among the advantages, he said, are reduction of cultivation and elimination of terracing for trays. Without terracing, vines can be irrigated later in the season to reduce stress and dust that attract spider mites. DOV also eliminates the need for paper trays and their disposal by burning in the field.
As in all DOV systems, in mid- to late-August when fruit develops to about 19 or 20 degrees sugar, fruiting canes are cut. In this system vines are also summer pruned: old canes are cut close to the trunk, two or three spurs are selected for renewal in the following year, and all remaining shoots are cut close to the trunk.
Thus the process, Peacock pointed out, reduces winter pruning to only the removal of fruiting canes, since spur selection has already been done.
“If we have a year when maturity is late and we don't make sugar in time to leave enough time for the fruit to dry on the vine, we can still go traditional and disk, terrace out, and lay trays,” he said. This year canes were cut in mid-August and the fruit was harvested the first week in October.
A standard mechanical harvester with the picking head adjusted to clear the cross arms can be used. Commercial harvesters can operate on the raisin vines at 2-1/2 to 3 miles per hour, vs. about 1-1/2 miles per hour for wine grapes. Peacock said a shortage of harvesters is unlikely in October when the wine grape harvest is winding down.
The DOV raisins also can be easily picked by hand, with workers using citrus harvesting bags or tubs emptied into tractor-drawn bins.
Peacock conceded that green clusters or individual berries may be occasionally picked up with the raisins by the mechanical harvesters. “This is one of our biggest problems with this system, and they have to be removed. However, we are working on ways to separate the green fruit from the raisins during harvesting.”
“This year, conditions were ideal for our project, but it may not work every year,” he said, pointing to the need to sever fruiting canes in time to allow the fruit to dry below 16 percent moisture.
Adverse weather may prevent maturity, timely cane cutting, and sufficient drying time in some seasons, and growers will have to be prepared those years to do complete or partial drying at a dehydrator, at additional cost.
Or, Peacock added, if all else fails, a grower can terrace the vineyard and harvested the crop conventionally for tray drying.
Three mechanical harvesters: a tractor-towed URM unit, a self-propelled AGH Quantum, and a self-propelled Korvan 3000 picked a couple of rows each.
Fred Swanson, superintendent of the KAC and a grape grower himself, said the system could be a transitional program to help the industry move toward better economic times.
“With little prospect of reducing other costs, we know the only way to survive is to increase production with DOV systems. This system faces its challenges, but maybe it will enable to cut some costs in half.”
Peacock said he was very optimistic after the two years of trials, but he cautioned that research on the idea is only in its beginning stages.
Other DOV systems, using extensive trellising, have been in development during the past decade in the search to avoid having raisins drying on the ground when rains come in the fall.
Those systems are integrated with varieties that ripen earlier and yield more than Thompson Seedless. The Fiesta variety has been most popular for DOV, but more recent varieties, DOVine (released in 1995), Summer Muscat (released in 1999), Diamond Muscat (released in 2000), and Selma Pete (released in 2001), have since become alternatives.
Among the current trials at KAC are the SunMaid South Side system with DOV for comparison of head and bilateral cordon training; the open gable with all five DOV varieties for comparison of head, bilateral cordon, and quadrilateral cordon training; the overhead with alternating middles, comparing Thompson, Fiesta, DOVine, and Selma Pete; and the open gable and Shaw swing-arm, comparing north-south and east-west row direction with Selma Pete.
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