The response of grapevines to crop load management varies substantially from year to year and between varieties and sites, which makes following viticulture "recipes" meaningless, says a cool climate viticulture and enology expert from Ontario, Canada. He believes there’s no substitution for knowing your vineyard and adjusting crop load based on knowledge and seasonal weather issues.
"There is some science into proving that crop load management and crop load reduction can have a positive impact on wine quality," said Dr. Andrew Reynolds of Brock University’s Cool Climate Oenology and Viticulture Institute in Ontario, Canada. "But there’s also the belief system out there that dropping half the crop will get them [winemakers] better wine. The controversy is that low yield is going to guarantee you high wine quality. This is often not the case."
Reynolds shared data with Washington’s wine grape industry during a recent state wine convention in Kennewick that showed high yields, in some circumstances, gave better wine quality than grapes with low yields.
"Crop load management is not about reducing crop so much as it is about maintaining balance in your vineyard between vine vigor and crop size," he said, adding that balance may occur at two to three tons or twelve tons per acre, depending on site, variety, and the given year.
"All vineyards have some sort of ‘sweet’ spot where they are in balance."
While some growers try to adjust crop load by severe pruning, he explained that the vine then becomes invigorated and responds by growing bigger leaves, more laterals, and creates shading problems that lead to more vegetal characters in the wine. But not pruning can also result in high shoot densities and too much shade.
What is balance?
Reynold’s concept of vine balance is a triangle or pyramid, with berry maturity at the apex and crop and vine size on the bottom angles. "It’s not a teeter-totter where you must balance between crop size and vine size," he said, adding that maintaining vine balance can involve compromises from environmental factors like winter injury or cool growing seasons. He offered the following guidelines to help growers find balance in the vineyard:
• Cane pruning weights—.3 pounds per foot of canopy length
• Ravez Index (crop yield to vine size ratio)—between 5 and 12
• Shoot density—5 to 8 shoots per foot per row of vine
• Use pruning first to manage crop load, followed by cluster thinning adjustments if needed.
Responses to crop load reduction vary substantially:
• From site to site
• Between varieties—Research showed Pinot Noir is very responsive to cluster thinning; Riesling showed little response.
• Vintage to vintage—Three-year trial showed grape quality differences from cluster thinning treatments in some years, but not all years.
• Timing—Data analyzing timing of cluster thinning treatments showed great variability. Early treatment was best in some years; in other years, late treatments were positive.
Reducing crop load
In some years, there is good justification to reduce your crop load, he said. There are years when growers know they have too large a crop on the vines or they need to maintain consistent yields year after year to have a sustainable system. He notes that some varieties, like Zinfandel, Grenache, and Syrah, produce large clusters that need to be thinned to avoid overcropping. And, in some difficult weather years, clusters may need thinning to get the crop to ripen on time.
"But don’t take crop management recipes from Europe and try to apply them here," he emphasized. "The more you know your vineyard, the more you will know whether you need to drop crop."
He reminds growers that often the cropping level effects are really shade effects caused by poor pruning practices.
"Very often, once you get the shoot density right, you’ll have the crop level right, and the vine will be in balance," he concluded.