Crop load and wine quality

Carrie McDonnell of J. Lohr Vineyards and Wines made wine from 20 different treatments for her study of crop load and extended ripening

Wines made from low-yielding vineyards don’t always produce the most preferred wines. New research investigating the effect of crop load and extended ripening on vine balance and wine quality showed that while wines with higher Brix were preferred in the study, the most acceptable wines came from grapes with higher Brix and higher crop loads.

The practice known as extended ripening—delaying the harvest date past traditional levels of ripeness to achieve flavor ripeness—has become a recent trend in harvest decision making, said Carrie McDonnell, viticulturist for California’s J. Lohr Vineyards and Wines. McDonnell shared her research results with winemakers and growers attending the annual meeting of the Washington Association of Wine Grape Growers.

"Extended ripening is perceived to improve wine composition," she said, explaining that it’s the idea of using flavor ripeness rather than sugar ripeness when making harvest decisions.

But delaying harvest comes with a cost. She highlighted problems associated with extended ripening, including significant yield and financial loss to the grower, negative consequences of high alcohol in wine, shortened harvest period, and tension between growers and winemakers.

McDonnell studied crop load and extended ripening on Cabernet Sauvignon vines and wines at J. Lohr in Paso Robles, California, for three years as part of her postdoctorate work at the University of Adelaide in Australia. One of the concepts she wanted to test was whether cluster thinning always improves quality. In recent years, cluster thinning has become a practice required of growers by many winemakers.

The trial was conducted from 2005–2007 in a Paso Robles, California, 14-acre vineyard of Cabernet Sauvignon. Crop load was adjusted from 20, 40, and 60 clusters per vine, with the control being unthinned vines. Grapes from the four crop load treatments were harvested at five different Brix levels: 22.5, 24, 25.5, 27, and 28.5°. (Brix is a measure of soluble solids.) Wine was made from each of the 20 treatments using a half ton of fruit; wine was fermented separately and stored in neutral barrels. Data was collected on yield components, berry development, canopy assessment, vine nutrition, effects on subsequent seasons, wine and fruit composition, wine sensory attributes, and economic viability.

Trends

As expected, yield was reduced as a result of both crop load adjustment and extended ripening, McDonnell stated. The higher crop loads lost less berry weight. Peak berry weight occurred at Brix between 24 and 25.5°. An inverse relationship was observed between cluster weight and yield. She also found that pruning weight increased in treatments thinned to lower crop levels in all three seasons, indicating changes in vegetative growth and the vine’s response to the crop thinning.

McDonnell found no detrimental effect in the nutrition or carbohydrate reserve of the vines from the crop load or extended ripening treatments, but believes that further research is needed, possibly looking at roots.

Trends in the wine analysis showed that extended ripening did increase wine color intensity. Moreover, wine color was improved in the higher crop load, and the highest average color density and anthocyanins were consistently found in wines made from the unthinned fruit.

In regards to wine descriptive analysis and scoring by trained and expert panels, wines from the higher Brix levels in all crop loads had more desirable flavor profiles and were preferred by the wine experts. Fruit from the higher crop loads achieved the desirable flavor profiles earlier than the lower crop loads.

"But the real cream of the crop, as scored by the expert panel, were the wines from the unthinned fruit in the 27 to 28.5° Brix range," she said. "This happened all three years."

McDonnell noted that the results suggest that wine quality can be improved with extended ripening, although significant yield is lost. Crop reduction may be a good remediation tool for unbalanced vines, but one can never achieve quality by starting out with an unbalanced vine.

"But yield, per se, is not a good indicator of wine quality," she said. "Rather, it’s the environment and management that affects yield and therefore influences wine quality. It’s important that you choose viticultural practices that are optimum for the site characteristics and desired flavor profile."

Overcoming yield loss

In a related trial, she studied the effects of late-season irrigation to see if she could mitigate the berry weight and yield loss from extended ripening. She compared two crop load treatments of 40 clusters per vine and unthinned vines with two irrigation treatments. Starting when the fruit reached 20° Brix, vines were irrigated at the standard practice or double the standard, plus a preharvest, 24-hour set irrigation. The preharvest irrigation was equivalent to 24 gallons of water per acre for 48 hours before harvest.

McDonnell found that the double irrigation maintained berry weight longer and resulted in higher yields than the standard irrigation treatment. She also observed lower Brix with the double irrigation. Pruning weights increased in the double irrigation in one of the years.

Although the wine sensory panelists found no differences in wines made from the two treatments, the trained expert panel rated the double-irrigation wines lower in terms of flavor intensity. However, the expert panel rated the two types of wines as similar, and agreed that they would use both wine types in the same wine program. Color intensity was lower in the double-irrigated vines in one of the two years.

"I think that irrigation could be a useful mitigation tool to overcome the yield loss of extended ripening, but it requires careful management," McDonnell concluded.

When questioned about the practice of doubling the amount of irrigation in eastern Washington vineyards, Washington State University viticulturist Dr. Markus Keller responded that unlike vineyardists in California, Pacific Northwest growers must be concerned about hardening off their vines before cold temperatures arrive. But he advocates applying small amounts of water during the harvest season to maintain berry weight and yield.