The case for replacing hand labor with machines to manage growth and production of wine grape vines in San Joaquin Valley vineyards gets stronger and stronger as researchers continue documenting the advantages of mechanization – advantages that extend beyond lower labor costs.
Currently, such use of machines by valley growers ranges, in varying degrees, from pruning, suckering and removing shoots and leaves to thinning clusters and harvesting the fruit.
Kaan Kurtural, a University of California Cooperative Extension viticulture specialist, reports mechanized practices account for about 45 percent of the pre-pruning and 17 percent of the precision pruning done in valley vineyards, while 45 percent of the leaf removal work and 7 percent of the shoot thinning is done mechanically.
In previous research, Kurtural documented the advantages of mechanizing shoot thinning and leaf removal practices compared to doing the work by hand.
His most recently completed study shows mechanical pruning of grape vines can produce more stable fruit yields from year to year and better fruit quality – even with limited precipitation and irrigation water – than the traditional, more costly practices of hand pruning spurs or canes.
The research trial, which ran from 2013 through 2015, was conducted in an 80-acre section of a commercial Zinfandel vineyard in Kern County.
The study was designed to identify ways for San Joaquin Valley growers to reduce the cost of producing a marketable crop using less water.
It was driven, in part, by the diminishing supply and increasing cost of labor and the relatively small profit margins associated with wine grape production in the San Joaquin Valley. Because of differences in growing conditions, typically, the market value of most wine grapes grown here are considerably less than those grown in California’s central and north coast regions. To help offset the lower prices for their grapes, most San Joaquin Valley growers strive to produce much higher yields on more acres that than those on the coast.
Meanwhile, the recent drought and dwindling ground water supplies only highlight the importance of irrigating vineyards with less water than has traditionally been used.
Kurtural compared three trellis systems:
- A machine-pruned high wire cordon trellis system, boxed pruned to a 4-inch hedge with 55 buds per meter;
- A traditional spur-pruned California Sprawl system with two buds per spur;
- A split-canopy system with vines separated by a 12-inch cross arm cane-pruned to leave six canes, each with eight buds.
None of these systems received any other type of canopy management.
Kurtural also looked at how these systems performed under each of two irrigation treatments:
- Sustained Deficit Irrigation, which provided the vines enough water to replace 80 percent of the crop evapotranspiration rate (Et) from fruit set to leaf fall, while keeping leaf water potential as close as possible to -12 bars during that time.
- Regulated Deficit Irrigation, applying water at 80 percent Et from bloom to fruit set with a leaf water potential of -12 bars, then reducing the water application to 50 percent Et from fruit set to veraison and -14 bars leaf water potential, and then increasing irrigation amounts to 80 percent Et from verasion to leaf fall and maintaining leaf water potential at -12 bars.
During the four-year trial, annual rainfall was highest during the first year of the study, totaling 87 percent of the previous 10-year average in 2013. Annual rainfall then declined to 30 percent of that 10-year average in 2014 and just 23 percent of that 10-year average in 2015.
“In these trials, the high-wire mechanically-boxed trellis system produced the most consistent yields and the best fruit quality, in terms of hitting our targets for ºBrix, anthocyanins and tannins,” Kurtural says. “Also, this system had the smallest water footprint. With a little bit of water, this system can do a lot, producing a canopy that’s not too big or too small but just right and providing the microclimate the vines and fruit needed for the conditions in this study.”
Among Kurtural’s specific findings:
The amount of exposed leaf area remained most stable from year to year with the mechanically-pruned vines and declined the most with the cane-pruned vines
The ratio of leaf area to fruit also remained most consistent from one year to the next with mechanical pruning.
“The leaf-area-to-yield ratio with mechanized pruning was fairly consistent through the trials and was near optimum for a hot climate region like the San Joaquin Valley,” Kurtural says. “The boxed canopy resulted in excellent exposure of the leaves to sun early in the season. Then, once the canopy was formed the grapes were able to ripen out well. With the majority of leaves on the outside of the canopy it was more efficient than the other two systems in terms of photosynthesis to support fruit production.”
Cluster numbers and yields per vine declined for all three trellis systems over the course of the trial. This may reflect impacts of the drought, Kurtural notes.
As the drought intensified during the trials and yields declined, the water footprint – the amount of water required to produce a ton of fruit – increased with both the cane-pruned and spur-pruned systems. At the same time, the water foot print with the mechanically-pruned system decreased.
Although Kurtural found some statistical differences in ºBrix, pH and total acid values of the fruit among the three types of trellis systems, they may not be significant in terms of wine quality, he reports.
However, he did find differences in flavonoids readings. Over the three-year trial, anthocyanin levels in fruit from the spur-pruned vines dropped by almost half. Anthocyanin, flavonol and tannin levels of grapes from the mechanically-pruned vines peaked during the second and third years of the study.
The effectiveness of the individual trellis systems varied with the amount of water the fields received. “With plentiful water, the spur-pruned and cane-pruned grape vines produced the highest fruit yields and greatest concentration of phenolic compounds,” Kurtural says.
“However, as precipitation events declined, which is what we expect in the future with continued climate change, clusters from the mechanically-pruned vines remained similar in size as those from spur- or cane-pruned vines, leading to the higher total yields of the mechanically-pruned vines over the three years.”
The study also shows that improved plant fitness, as measured by more exposed leaf surface area and higher yields, was frequently associated with improved berry skin composition, rather than lower yields or water stress, Kurtural adds.
Kurtural has expanded this study to include cabernet sauvignon grape vines in the North Coast region at the UC Oakville Experiment Station in Napa County.