Some growers using the open-gable dried-on-the-vine (DOV) raisin production system have been disappointed to find their yields falling short of those reported in research trials conducted at the University of California Kearney Agricultural Center, Parlier
The DOV systems there, typically, produce about 4 tons of raisins per acre. That compares to the 3.5 to 3.75 tons-per- acre yields reported by many DOV growers, notes Matthew Fidelibus, a UC Cooperative Extension viticulture specialist, based at the Kearney facility.
He’s in the middle of a two-year study to see if he can find the reason for this difference. Fidelibus is conducting field trials to study the effects of various canopy management systems on grape production. Also, he’s visiting DOV vineyards using open gable trellises to compare their features with those of the open gable trellises used in the Kearney research vineyards.
He’s waiting until the end of trials this season to draw any firm conclusions. But, Fidelibus suspects the variation in production levels reflects differences in the way growers and researchers manage exposure of the vines to sunlight.
The Kearney DOV system was developed in the 1990s by L. Peter Christensen. It features an open-gable design with a Y-shaped trellis, which can support cordon or head-trained vines.
“It’s important to provide renewal shoots with the best possible exposure to sunlight in order to optimize bud fruitfulness and minimize bud necrosis.” Fidelibus says. Christensen intended this exposure to be achieved through the use of rake wires, which could be used to gather renewal shoots toward the center of the trellis where a foliage catch wire was mounted.
To save the cost of wires and metal stakes, he’s observed that many growers don’t include rake wires or a center foliage catch wire in their open gable DOV systems. These omissions could reduce exposure of the renewal shoots to sunlight and, thus, account for some of the yield reductions growers are reporting, he notes.
Some other growers separate renewal from fruiting shoots by pruning the vines so that all the fruiting canes are trained to grow on one side of the vine and all the spurs on the other.
“This way, every other space between the vines is either all fruiting canes or all spurs,” Fidelibus explains. “The idea is that, since there are no fruiting canes next to the spurs, the renewal shoots will receive more sunlight.”
Difference in yields
He’s testing how this and other methods of training vines affect both the sunlight environment and fruitfulness.
Difference in yields between various types of DOV systems may also be due to difference in pruning practices, Fidelibus notes. The number of buds on a cane depends on cane length and the length of the internodes. For example, the length of a 15-node cane varies from about 3.5 feet for Dovine and Thompson Seedless vines, to nearly 4 feet for Fiesta and to about 4.25 feet for Selma Pete.
“If you cut all canes 3-feet long, regardless of variety, you’ll end up with fewer buds on the canes with the longer internodes, since those buds are farther apart than those on canes with shorter internodes,” he says. “Fewer buds means a potentially smaller crop.”
The length of the trellis cross arms can also affect DOV yields, Fidelibus notes. Shorter cross arms result in smaller canopies and lower grape production. The cross arms used in the Kearney DOV vineyards are much longer and more expansive than those used in many commercial vineyards. These longer cross arms require a much larger harvester than many growers use.
“From what growers tell me, our DOV trellis system at Kearney is at the high edge in terms of size that they can manage commercially,” Fidelibus says. “But you have to realize that when you make the cross arms narrower, yields will be somewhat lower.”
In many of the vineyards he has visited, growers have established cordons as far as 1.5 feet below the Y. Those in the Kearney vineyards are near the bottom of the Y, The lower location makes for easier pruning and other management practices, Fidelibus says. But, it also means much more shading on all the shoots below the Y.
Some growers, who are familiar with the results of his DOV trials and observations, have begun making changes in the DOV trellis systems, without waiting for the findings from this season, Fidelibus reports. For these and others considering a change, he advises proceeding cautiously.
For example, he points out, lowering the cross arms to reduce the distance between the cordons and the Y of the trellis could limit cane length and reduce the size of the canopy. This would restrict yields.
“Changing a DOV system that is already established is expensive and may introduce unintended consequences,” he says. “Altering one feature can affect another, either in a good or a bad way.”