Deficit irrigation is not just for controlling canopy size. A Washington State wine company sees potential in using deficit irrigation to "stylize" grapes in the field, providing its winemakers with different blending options from the same vineyard.
Washington wine grape growers already use deficit irrigation to save labor costs, but Dr. Russell Smithyman believes the irrigation strategy can be taken a step further.
Taking water deficit to the next level, growers can go beyond saving money by eliminating the task of removing basal leaves to open up the fruiting zone, he said. "By turning water off—something that’s easy to do—you can create something different for your winemakers."
As viticulture director for Ste. Michelle, Smithyman has studied deficit irrigation for years, but only recently has looked at it as a tool to stylize grapes. He conducted research in 2006 and 2007 in a five-acre block of Cabernet Sauvignon, comparing three irrigation treatments: the grower’s standard irrigation practice (the control), deficit irrigation without leaf removal, and deficit irrigation with leaf removal. The Cabernet vineyard was on a vigorous site, and the grower usually did shoot removal and hedging to keep the canopy in check.
At the end of the growing season, the control treatment received 31 irrigation applications totaling 286 hours. The deficit treatments averaged 7 applications totaling 64 hours.
"The big difference between treatments was withholding water for a longer period of time until we really got control of the canopy," he said. On July 19, 2007, there were no shoot tendrils in the deficit treatments, although tendrils were still growing in the control. A comparison of the treatments on August 27 showed a long canopy shadow in the control, and the canopy blocked sunlight from reaching the fruiting zone. However, the deficit treatments had gaps in the canopy, lots of light flecks, fewer leaves, and more dappled sunlight.
Addressing grower worries about needing leaves for sunburn protection, Smithyman said that most winemakers would rather have a little sunburn on the fruit than herbaceous and vegetal flavors. "We’re after more fruit exposure, not less."
The no-leafing deficit-treatment canopy was left to sprawl over the canopy wires. "With this treatment, we didn’t have to pay to leaf and we didn’t pay to put the shoots up on the catch wires," he said. "I like the sprawl system on deficit irrigation because it gives more dappled shade and utilizes more leaf area to trap the sunlight."
He noted that the deficit treatments resulted in a smaller berry size and produced different flavors from fruit grown with more irrigation.
Berry weight from the standard treatment averaged 1.28 grams; deficit weight averaged 1.01 grams. Brix from the treatments were similar, at around 28°. The target yield for the block was five tons per acre. In the first year, tonnage in all treatments was more than five tons, but in the second, the control averaged 5.2 tons per acre, while the two deficits averaged 4.6 tons. It took two years for the deficit vines to show the full effects of receiving less water, he explained.
"You could use common viticultural practices to easily bring the tonnage average up to five tons per acre."
The main difference he found between the deficit and control treatments was in wine quality. Deficit fruit had more berry, cherry, and dried fruit flavors and less herbaceous, mushroom, and spice flavors than the control. Also, anthocyanin and tannin levels were higher in the deficit treatment grapes than in the control. The deficit grapes without leafing had the highest phenolic compounds.
Dr. James Harbertson, Washington State University Extension enologist, is looking at differences in berry and wine composition from deficit and standard irrigation. Results from some of his early work are showing that there are tannin differences due to exposure and deficit irrigation.
Ste. Michelle made wine from each of the three treatments and had a trained sensory panel taste them. The deficit-leafed wine was the most preferred, while the control was the least preferred.
As Smithyman gave his talk during a session at the Washington Association of Wine Grape Growers’s annual meeting, the audience tasted wines made from the different treatments of the trial. A hand vote showed that there were distinct preferences of the crowd.
It’s not which wine is preferred, he said. "The point is that these wines are different. It’s all about stylizing wines in the vineyard and creating something different for the winemakers and letting them blend as they see fit."
Though the research is still preliminary, it may provide another tool for growers to use in creating different styles of wine in their vineyards. He envisions growers creating different wine styles by managing vineyards based on terroir, using aerial imagery to manage canopy variations within a vineyard and harvesting fruit from high vigor and low vigor separately, and now, managing irrigation to create differences.