May 2014 Issue of Wines & Vines

Cluster Thinning in Late-Harvest Riesling: Does it Pay?

by Justine Vanden Heuvel and Trent Preszler
Cluster Thinning
Leaves were pulled in order to photograph cluster thinning to one cluster per shoot (left) and two-plus clusters per shoot (right). No leaf pulling occurred on the actual vines studied.

The production of late-harvest wines provides an avenue for wineries to reach a segment of consumers who may prefer medium-dry or sweet wines. In cool-climate regions, late-harvest wines are typically made from aromatic white Vitis vinifera cultivars such as Riesling or Gewürztraminer that are harvested later in the growing season, when soluble solids accumulation or infection by Botrytis cinerea allow an end product containing natural residual sugar and lower alcohol by volume. The widespread popular belief that low-yielding grapevines produce higher quality wines is entrenched in the late-harvest wine segment because late-harvest wine can be marketed to consumers using scarcity as one of its quality signals. But does this ideology translate to the vineyard?

There are two primary costs associated with cluster thinning: skilled human labor and lower revenue from reduced yield. Commercial growers generally cluster thin assuming it will result in quality improvements desired by the buyer so that lost revenues may be recouped through higher prices. To assess whether cluster thinning achieves targeted wine characteristics and business objectives, a quantitative and holistic approach to all aspects of yield management including determining whether consumers will have a greater or lesser willingness to pay for the resulting wines was necessary.


  • A field experiment with cluster thinning was conducted in a Finger Lakes commercial vineyard from 2008 to 2010.
  • The authors investigated whether lower yields would increase the wine trade’s willingness to pay.
  • An expert panel of wine professionals did not prefer wines made from cluster-thinned vines when blind tested against wines from non-thinned vines.

The New York City metropolitan area is the largest wine market in the United States and consumes approximately 30% of America’s total imported wines (Wine Market Council 2009), making it a highly contested market for many quality wine regions. Consumers’ wine-drinking habits in New York City are influenced heavily by a specialized niche of restaurant sommeliers, wine writers and wine shop owners who act as market gatekeepers and have specific awareness of New York wines (Preszler and Schmit 2009). Such industry professionals make desirable sensory panelists because they have a robust vocabulary, expert understanding of important wine attributes and more finely honed sensory acumen than general consumers.

To give grapegrowers the best advice regarding optimal cluster thinning and yield-management decisions, it is important to quantify the additional value relationship assumed by buyers conditional to the level of cluster thinning. If a buyer’s willingness to pay for wines is differentiated for quality and parallels a grower’s cluster-thinning practices, then the grower’s minimum price requirements for grapes can be expressed as a function of crop level. A model published by this research group (Preszler et al. 2010) was the first to position grapevine yield within a quantitative economic decision-making framework.

In this study, the model was applied to viticultural and sensory data from a field trial of varying crop levels in a mature commercial Riesling vineyard, and the resulting late-harvest wines were panel tested for sensory qualities and willingness to pay among New York City wine professionals. The goal was to elucidate the effects of cluster thinning on yield components, fruit composition, late-harvest wine quality, financial net returns and willingness to pay among New York wine professionals—and by doing so, to enhance decision-making acuity among wine grape growers using or considering cluster thinning.

Vineyard site and experimental design
This field experiment was conducted from 2008 to 2010 at a commercial vineyard on the east side of Cayuga Lake in King Ferry, N.Y., in the Finger Lakes AVA, on a soil type classified as Cazenovia series with a silt loam structure (USDA-NRCS soil maps). Vines were Vitis vinifera L. cv. Riesling cl. 239, grafted on 3309C rootstock, planted in 1984 in a north-south row orientation on a west-facing slope, with 4.5-foot spacing between vines and 9 feet between rows. Vines were cane-pruned and trained to the Pendelbogen vertically shoot-positioned system. The cooperating commercial grower managed the vineyard.

Three cluster-thinning treatments were applied when berries were pea-sized: low crop (one cluster per shoot), medium crop (1.5 clusters per shoot), and high crop (two clusters per shoot). An additional control treatment (i.e., unthinned) was maintained (two-plus clusters per shoot). Treatments were arranged in a randomized complete block with four replicates. Each cluster-thinning treatment was nine vines, with the inner seven used for data collection. In 2008 primary shoots were thinned to 27 per vine, which was the lowest density counted before thinning. In 2009 and 2010, primary shoots were thinned to 36 per vine. Clusters located distally nearest the shoot apex were removed first, and the lowest clusters were left intact.

Harvest and yield components
Vegetative and yield component data were collected on a per-vine basis. Vines were harvested by hand in consort with the late harvest schedule of the cooperating commercial grower: Oct. 29, 2008, Oct. 28, 2009, and Nov. 3, 2010. Yield per vine, total yield and average cluster weights were recorded. In the spring, vines were pruned to three remaining canes with 40 nodes per vine, and the prunings were weighed.

Winemaking and basic juice chemistry
In 2009 and 2010 grapes from all field replicates were combined with like cluster-thinning treatments and processed on the day of harvest. Grapes from each separate cluster-thinning treatment were whole-cluster pressed in a 40-liter stainless steel hydraulic bladder press. Duplicate 200mL juice samples were collected from each separate cluster-thinning treatment field replicate and frozen at -20°C for later analysis. Pressed juice from the field replicates were then combined with their like cluster-thinning treatments, treated with 50mg per liter sulfur dioxide added as potassium metabisulfite, and allowed to settle for 12 hours at 4°C. After settling, the juice was racked according to cluster-thinning treatment into duplicate 19-liter glass carboys. Carboys were chaptalized to 22° Brix if necessary, and juice was inoculated with 0.25g per liter Saccharomyces cerevisiae strain R-HST yeast previously rehydrated in GoFerm. Carboys were fitted with airlocks, moved to a 16° C room and stirred daily. FermAid K was added (0.15g per liter) at inoculation and again when wines reached 10° Brix. Wines fermented until residual sugar was measured in the medium-sweet category between 6% and 10% using Clinitest tablets (Bayer, West Haven, Conn.), and then fermentation was stopped by adding 40mg per liter free sulfur dioxide and moving carboys into a 2° C storage room. Wines did not undergo any acid adjustments or malolactic fermentation and were screened for faults by an expert panel prior to being bottled and stored at 16° C.

    Results in reproductive growth and fruit composition


     As expected, cluster thinning reduced yield in late-harvest Riesling in 2008 and 2009 (see “Production Costs and Pricing Parameters” on page 93). In 2010 cluster thinning predictably reduced yield in low- and medium-crop vines, however high crop and control yield was lower than in previous years and thus yield was equal among treatments.

    In 2008 soluble solids differed between low and medium crop (average 19.5° Brix) and high crop and control (average 17.3° Brix), and likewise in 2009 soluble solids differed between low and medium crop (average 19.7° Brix) and high crop and control (average 18° Brix).

    In 2010 Brix of low crop was highest, but all other treatments were equivalent. Juice acid levels at harvest were highest in 2009, but cluster thinning had little or no significant effect on pH and TA in any year. There was no cluster thinning effect on wine pH, wine TA or percent alcohol by volume in any year.
    —J.V.H. & T.P.

Wine sensory willingness to pay panel
Approximately two years after bottling 2009 wines, and one year after bottling 2010 wines, sensory attribute preference information was collected through controlled tastings with a judgment sample of 27 highly skilled sommeliers and professionals in the New York City wine, restaurant and hospitality industries. Panelists were recruited and assembled at the International Wine Center in New York City, the American headquarters of the Wine & Spirit Education Trust U.K. (WSET), which administers the Master of Wine (MW) certification program. At the time of the tasting, 14 of the panelists worked in wine sales and the other 13 were writers, bloggers, critics, educators or media-relations specialists. All panelists completed some stages of wine training through WSET, and 21 of them graduated at the highest levels with the WSET Level 4 Diploma (or MW). Nineteen of 27 panelists were female; all panelists were between 25 and 55 years old; 14 panelists were between the ages of 31 and 40. No predetermination of attributes was made to discriminate among wines, no advance training of panelists was conducted, and no specific information was provided about the wines or viticultural practices used to make them.

Panelists were served wines in 30mL aliquots at room temperature in clear tulip-shaped (ISO) 220mL wine glasses. Wines made in 2009 from all three cluster-thinning treatments and non-thinned control wines were poured and presented simultaneously to panelists. Each glass was coded with a random identification number, and the order of wines presented was randomized among panelists. Panelists were asked to smell and taste each wine, expectorate between samples and use their own experience to record their impressions of three important sensory attributes that have been previously identified (Cliff et al. 2002) for evaluating late-harvest Riesling wines: the fruity or floral intensity of the aroma, the petrol or fusel intensity of the flavor and the structure or mouthfeel. In addition, panelists were asked to rate the overall likability and their likelihood of purchase for each wine. The intensity of each attribute was measured on a 10cm line scale, with anchors at zero for “not at all” and 10 for “extremely,” and panelists were asked to mark a dot along the scale to indicate their response, which was later measured and converted to a numerical rating. After completing the attribute response exercise, panelists were asked to record their typical willingness to pay for an average 750mL bottle of late-harvest Riesling in a New York City retail store. Then they were asked to record their maximum willingness to pay for each of the experimental wines, assuming a hypothetical situation where they were buying a 750mL bottle of the wine at a New York City retail store. After completing the entire tasting exercise for 2009 wines, the same procedure was repeated for 2010 wines.

Economic analysis
Fixed and variable production costs for managing 1 hectare of Riesling in the Finger Lakes, including additional labor costs of implementing cluster thinning, were derived from a published survey of vineyard management costs (White 2008) and entered into our cluster-thinning economic model (Preszler et al. 2010). Specific input parameter data were yield (t/ha) before and after cluster thinning, fixed and variable production costs ($/t) before and after cluster thinning, and the market price for late-harvest Riesling grapes ($/t). The model assumes net returns per tonne increase with higher per-tonne price requirements but are offset due to lower yields and higher production costs. Microsoft Excel was used for basic descriptive statistics and economic data were analyzed using a previously published model (Preszler et al. 2010).

Economic analysis and wine sensory willingness to pay trial
Cost, price, yield and revenue parameters obtained from data collected in the field trial or derived from published sources were entered in the cluster-thinning economic model (see “Production Costs and Pricing Parameters” on page 93). Yield of control vines was 8.4 tonnes per hectare in 2008, 7.8 tonnes per hectare in 2009 and 5.8 tonnes per hectare in 2010. Average market price for Finger Lakes late-harvest Riesling was $2,660 per tonne in 2008, $2,350 per tonne in 2009 and $2,400 per tonne in 2010. Since yield and price decreased over the course of the study, so did potential revenue before cluster thinning (based on yield per hectare of the control plots) from $22,344 per hectare in 2008 to $18,330 per hectare in 2009 and $13,920 per hectare in 2010. Taking into account the costs of implementing cluster thinning, the change in revenue after cluster thinning was -$7,714 per hectare in 2008, -$7,285 per hectare in 2009 and -$2,640 per hectare in 2010.

Grower net returns in 2008 ranged from $8,261 per hectare to $16,414 per hectare at low crop and control respectively, from $4,688 per hectare to $12,029 respectively in 2009, and from $4,936 per hectare to $7,613 respectively in 2010 (see “Grower Net Return” on page 94). Compared to the base market price of $2,660 per hectare (or $5.45 cost per bottle) the minimum price required to recoup costs for low crop grapes in 2008 was $4,142 per hectare ($8.49 per bottle), a 56% price increase. Compared to the base market price of $2,350 per tonne ($4.82 per bottle) in 2009, the minimum price for low crop grapes was $3,991 per tonne ($8.18 per bottle), a 70% price increase. And compared to the base market price of $2,400 per tonne ($4.92 per bottle) in 2010 the minimum price required for low crop grapes was $3,050 per tonne ($6.25 per bottle), a 27% price increase. If grapes were retained for winemaking by the grower rather than sold, based on the yields of these experimental conditions the additional cost per bottle of finished wine required to recoup costs of cluster thinning at the low crop level was $3.03 per bottle in 2008, $3.36 in 2009 and $1.33 in 2010. The elicited average willingness to pay by New York City wine professionals was $13.40 per bottle in 2009 and $14.99 per bottle in 2010, however there were no statistically significant differences in willingness to pay among crop levels in either year.

Sensory panel ratings for 2009 wines revealed reduced fruitiness in medium-crop wines and reduced structure or mouthfeel in low-crop wines but no differences in likability ratings or any other attributes (see graphic below). The same panel reported no differences among crop levels for any sensory or preference attributes in 2010 wines. Therefore, in both years of the study, no level of cluster thinning was advisable, as willingness to pay did not increase in the lower yield treatments.

Reproductive growth and fruit composition
The principal reason grapegrowers apply cluster thinning is to manage crop load for advanced ripening or increased soluble solids accumulation, which has benefits for regions with shortened growing seasons. In this experiment, cluster thinning predictably increased juice-soluble solids accumulation in the low and medium crop levels. Pressed juice soluble solids at the low crop level all three years did not reach the 22° Brix threshold as expected, based on observations of other comparably cropped vines in the same commercial vineyard. This could be the result of pests consuming the ripest berries late in the season, as the cooperating grower did not install bird netting, and pressure in the remaining late-harvest vines intensified as the rest of the surrounding vineyards had been harvested. Soluble solids may have been underestimated as a function of raisined fruit, which may have inhibited full sugar extraction from whole clusters in a small-scale (40-liter) bladder press. Nevertheless, in this experiment, as in many others, there seems to be little disputing that cluster thinning advances ripening. In some regions outside New York such as Ontario, Canada, growers are paid for bulk Riesling grapes based on a mandatory price schedule of soluble solids, and if such a regulated price structure were used in New York for this study, it would have resulted in a $244, $273 and $286 per tonne price premium in each respective year (Gedeon 2012), not enough to offset the reduced grower net return after cluster thinning.

In 2010 yield was equivalent among all treatments, ranging from 4.7 to 5.8 t/ha, which may have been an indicator of vine carbohydrate reserves declining following two previous years of relatively high crop level between 7.3 and 8.4 t/ha.

Economic willingness to pay analysis and wine sensory panel
Commercial growers are motivated to recoup costs incurred from cluster thinning and maintain consistent net returns; the only way to do this is to charge above-market prices for grapes if buyers are willing to pay a premium. Without such a financial incentive, growers would likely not employ cluster thinning as a viticultural practice. In 2008 grower net returns were positive at the control crop level, but fruit from those vines only reached 17.1° Brix, and the low crop level (19.6° Brix) would have required a 56% price increase to recoup costs of cluster thinning. In 2009 the control fruit did not reach 18° Brix, and the grower experienced reduced net returns at the low crop level compared to the control, so the sustainable level of crop would need to balance considerations of ripeness and financial returns. In 2010 grower net returns were positive at all crop levels, and all fruit reached at least 19° Brix at harvest, so the most sustainable crop level was the one that maximized yields. Grower net returns were the lowest at the low crop level and required 56%, 70% and 27% grape price increases to maintain constant net return compared to the control each respective year, not including any possible increased risks associated with late harvest, such as grapevine pest and disease management. It is unlikely that such large price increases would be tolerated by the market or that improved grape quality would offset the financial costs of implementing cluster thinning. If the profit-motivated grower had kept the grapes for winemaking instead of selling them at inflated market prices, there would need to be assurances of higher willingness to pay for the late-harvest wines to justify the financial losses associated with cluster thinning. Nonetheless, a panel of New York City wine professionals reported no difference in willingness to pay or likability for any crop levels in 2009 and 2010, and the only perceived differences among wines either year were the 2009 low-crop wines, which exhibited reduced structure, and 2009 medium-crop wines, which exhibited reduced fruitiness.

Low- and medium-cropped vines led to substantially smaller grower financial net returns. Under these experimentally imposed yield and cost conditions, in order for the grapegrower to break even financially there would need to be an impetus to charge more for each bottle of wine to recoup the costs of cluster thinning. Such a price increase could only be warranted by higher willingness to pay for wines made from lower cropped vines.

The consumer preference elicitation technique applied here has been reported previously as a reliable tool for assessing willingness to pay for wine (Yang et al. 2009). Willingness to pay was relatively low for all wines in this study, likely due to the fact that the wines did not undergo typical commercial finishing techniques such as enzyme additions, acid and sugar adjustments or filtering.

The results
This three-year study examined the effects of cluster thinning on various fruit, wine, sensory and economic parameters of a commercial Riesling vineyard in the Finger Lakes region of New York, to understand how cluster thinning can be justified as part of a sustainable viticulture program for late-harvest winemaking. Despite having higher soluble solids at harvest, which are not valued economically in the Finger Lakes as they are in other regions, lower yields generally correlated to negative financial net returns for the grapegrower. According to surveyed New York City wine industry professionals, wines made from fruit grown at varying crop levels were neither preferred nor likely to garner higher bottle prices. Since cluster thinning did not result in enhanced quality ratings or higher elicited willingness to pay, the grower could not recoup costs associated with cluster thinning, and the practice in this vineyard would not be financially sustainable for the grower.

Justine Vanden Heuvel is an associate professor of viticulture at Cornell University. Trent Preszler is a former Ph.D. student in the Department of Horticulture at Cornell University. The authors wish to thank Linda Lawry, director of the International Wine Center in New York City, for coordinating the expert sensory panel participation by the sommeliers and wine industry professionals and allowing us to host the panel trials in their classroom. We also thank Todd Schmit for assistance with the economic analysis and Steven Lerch for technical field support.

Literature Cited
Cliff, M.A., D. Yuksel, B. Girard, and M.A. King. 2002. “Characterization of Canadian Ice Wines by Sensory and Compositional Analyses.” Am. J. Enol. Vitic. 53:46-53.

Gedeon, J. 2012. “Two-Year Deal Sets Bulk Prices for Most Popular Varietals in Ontario.” Wine Business Monthly, April 11.

Preszler, T., and T.M. Schmit. 2009. “Factors Affecting Wine Purchase Decisions and Presence of New York Wines in Upscale New York City Restaurants.” J. Food. Dist. Rsrch. 40:16-30.

Preszler, T., T.M. Schmit, and J.E. Vanden Heuvel. 2010. “A Model to Establish Economically Sustainable Cluster-Thinning Practices.” Am. J. Enol. Vitic. 61:140-146.

Wine Market Council. 2009. The U.S. Wine Market: Consumer Trends and Analysis. Wine Market Council, San Francisco.

White, G.B. 2008. Cost of establishment and production of vinifera grapes in the Finger Lakes region of New York–2007. Cornell University Department of Applied Economics and Management Extension Bulletin 2008-05.

Yang, N., J.J. McCluskey, and C. Ross. 2009. “Willingness to Pay for Sensory Properties in Washington State Red Wines.” J. Wine Econ. 4:81-93.

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